REVIEW Open Access
International Society of Sports Nutrition
Position Stand: protein and exercise
Ralf Jäger1
, Chad M. Kerksick2
, Bill I. Campbell3
, Paul J. Cribb4
, Shawn D. Wells5
, Tim M. Skwiat5
, Martin Purpura1
,
Tim N. Ziegenfuss6
, Arny A. Ferrando7
, Shawn M. Arent8
, Abbie E. Smith-Ryan9
, Jeffrey R. Stout10
, Paul J. Arciero11
,
Michael J. Ormsbee12,13
, Lem W. Taylor14
, Colin D. Wilborn14
, Doug S. Kalman15
, Richard B. Kreider16
,
Darryn S. Willoughby17
, Jay R. Hoffman10
, Jamie L. Krzykowski18
and Jose Antonio19*
Abstract
Position statement: The International Society of Sports Nutrition (ISSN) provides an objective and critical review
related to the intake of protein for healthy, exercising individuals. Based on the current available literature, the
position of the Society is as follows:
1) An acute exercise stimulus, particularly resistance exercise, and protein ingestion both stimulate muscle protein
synthesis (MPS) and are synergistic when protein consumption occurs before or after resistance exercise.
2) For building muscle mass and for maintaining muscle mass through a positive muscle protein balance, an
overall daily protein intake in the range of 1.4–2.0 g protein/kg body weight/day (g/kg/d) is sufficient for
most exercising individuals, a value that falls in line within the Acceptable Macronutrient Distribution Range
published by the Institute of Medicine for protein.
3) There is novel evidence that suggests higher protein intakes (>3.0 g/kg/d) may have positive effects on body
composition in resistance-trained individuals (i.e., promote loss of fat mass).
4) Recommendations regarding the optimal protein intake per serving for athletes to maximize MPS are mixed
and are dependent upon age and recent resistance exercise stimuli. General recommendations are 0.25 g of a
high-quality protein per kg of body weight, or an absolute dose of 20–40 g.
5) Acute protein doses should strive to contain 700–3000 mg of leucine and/or a higher relative leucine content,
in addition to a balanced array of the essential amino acids (EAAs).
6) These protein doses should ideally be evenly distributed, every 3–4 h, across the day.
7) The optimal time period during which to ingest protein is likely a matter of individual tolerance, since benefits
are derived from pre- or post-workout ingestion; however, the anabolic effect of exercise is long-lasting
(at least 24 h), but likely diminishes with increasing time post-exercise.
(Continued on next page)
* Correspondence: ja839@nova.edu
19
Department of Health and Human Performance, Nova Southeastern
University, Davie, FL, USA
Full list of author information is available at the end of the article
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20
DOI 10.1186/s12970-017-0177-8
Background
In 2007, the International Society of Sports Nutrition
(ISSN) published its first position stand devoted to the
science and application of dietary protein intake [1].
Subsequently, this paper has been accessed more than
200,000 times and continues to serve as a key reference
on the topic. In the past ten years, there have been continued
efforts to advance the science and application of dietary
protein intake for the benefit of athletes and fitnessminded
individuals. This updated position stand includes
new information and addresses the most important
dietary protein categories that affect physically active
individuals across domains such as exercise performance,
body composition, protein timing, recommended intakes,
protein sources and quality, and the preparation methods
of various proteins.
Benefits on exercise performance
Most of the scientific research investigating the effects
of protein intake on exercise performance has focused
on supplemental protein intake. From a broad perspective,
the dependent measures of these studies can be
categorized into two domains:
 Endurance exercise performance
 Resistance exercise performance (increases in
maximal strength)
Endurance exercise performance
Very few studies have investigated the effects of
prolonged periods (one week or more) of dietary protein
manipulation on endurance performance. Macdermid
and colleagues [2] compared the influence of an isoenergetic,
high-protein/moderate-carbohydrate diet (3.3 and
5.9 g of protein and carbohydrate/kg body weight per day,
respectively) with a diet that was more typical of an
endurance athlete (1.3 and 7.9 g of protein and carbohydrate/kg
body weight per day, respectively) in endurancetrained
cyclists. The trained cyclists ingested each diet for a
7-day period in a randomized, crossover fashion. Before
and following the 7-day diet intervention, a self-paced cycling
endurance time trial was conducted as the primary
measure of exercise performance. At the end of the treatment
period, it took cyclists on the higher protein diet 20%
more time to complete the self-paced time trial - significantly
longer than for those on the lower protein/higher
carbohydrate diet. This finding is not surprising given that
dietary protein is not a preferred energy source and the
dietary carbohydrate intakes in the higher protein treatment
were below recommended intakes for endurance athletes
(6–10 g of carbohydrate/kg/d) [3]. It should be noted
however that a 7-day treatment period is exceedingly brief.
It is unknown what the effect of a higher protein diet would
be over the course of several weeks or months.
In another study [4] utilizing highly trained cyclists during
a period of increased training intensity, it was observed
that 3 g of protein/kg/d offered no improvements
in a simulated time trial as compared to 1.5 g of protein/
kg body weight/day. Carbohydrate intake was kept constant
(6 g/kg/d) in both the moderate and high protein
treatments during this three-week intervention. Although
the number of investigations is limited, it appears as if increasing
protein intakes above recommended intakes does
not enhance endurance performance [2, 4, 5].
In addition to these studies that spanned one to three
weeks, several acute-response (single feeding and exercise
sessions) studies exist, during which protein was added to a
carbohydrate beverage prior to or during endurance exercise.
Similarly, most of these interventions also reported no added
improvements in endurance performance when protein was
added to a carbohydrate beverage as compared to carbohydrate
alone [6–9]. An important research design note, however,
is that those studies which reported improvements in
8) While it is possible for physically active individuals to obtain their daily protein requirements through the
consumption of whole foods, supplementation is a practical way of ensuring intake of adequate protein quality
and quantity, while minimizing caloric intake, particularly for athletes who typically complete high volumes of
training.
9) Rapidly digested proteins that contain high proportions of essential amino acids (EAAs) and adequate leucine,
are most effective in stimulating MPS.
10) Different types and quality of protein can affect amino acid bioavailability following protein
supplementation.
11) Athletes should consider focusing on whole food sources of protein that contain all of the EAAs (i.e., it is the
EAAs that are required to stimulate MPS).
12) Endurance athletes should focus on achieving adequate carbohydrate intake to promote optimal performance;
the addition of protein may help to offset muscle damage and promote recovery.
13) Pre-sleep casein protein intake (30–40 g) provides increases in overnight MPS and metabolic rate
without influencing lipolysis.
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 2 of 25
endurance performance when protein was added to a carbohydrate
beverage before and during exercise all used a timeto-exhaustion
test [10–12]. When specifically interested in
performance outcomes, a time trial is preferred as it better
mimics competition and pacing demands.
In conclusion, added protein does not appear to improve
endurance performance when given for several
days, weeks, or immediately prior to and during endurance
exercise. While no ergogenic outcomes may be evident,
the scientific literature is consistent in reporting that
adding protein to a carbohydrate beverage/gel during exhaustive
endurance exercise suppresses markers of muscle
damage (creatine kinase) 12 to 24 h post-exercise [8, 11–
13] and decreases the endurance athletes’ feelings of muscular
soreness [6–8, 13]. For these reasons, it seems prudent
to recommend for endurance athletes to ingest
approximately 0.25 g of protein/kg body weight per hour
of endurance exercise (in addition to the athlete’s regular
carbohydrate intake) to suppress markers of muscle damage
and improve subjective feelings of muscular soreness
[11, 12]. Another important consideration relates to the
impact of ingesting protein along with carbohydrate on
rates of protein synthesis and balance during prolonged
bouts of endurance exercise. Beelen and colleagues [14]
determined that adding protein to carbohydrate consumption
throughout a prolonged bout of endurance exercise
promotes a higher whole body net protein balance, but
the added protein does not exert any further impact on
rates of MPS. While performance outcomes were not
measured, these results shift the focus of nutrient ingestion
during prolonged bouts of endurance exercise to the
ingestion of carbohydrate.
Key points
 When adequate carbohydrate is delivered, adding
protein to carbohydrate does not appear to improve
endurance performance over the course of a few
days or weeks.
 Adding protein during or after an intensive bout of
endurance exercise may suppress the rise in plasma
proteins linked to myofibrillar damage and reduce
feelings of muscle soreness.
 There are relatively few investigations on the effects
of protein supplementation on endurance
performance.
Resistance exercise performance
The extent to which protein supplementation, in
conjunction with resistance training, enhances maximal
strength is contingent upon many factors, including:
 Resistance-training program variables
(such as intensity, volume, and progression)
 Length of the resistance-training program/
intervention
 Training status of the participants engaging in the
resistance-training program
 Energy intake in the diet
 Quality and quantity of protein intake (with an
emphasis on leucine content of the protein)
 Co-ingestion of additional dietary ingredients that
may favorably impact strength (e.g. creatine, HMB)
Taking each of these variables into consideration, the
effects of supplemental protein consumption has on
maximal strength enhancement are varied, with a majority
of the investigations reporting no benefit [15–25] and a
few reporting improvements in maximal strength [26–29].
With limited exceptions [16, 18, 23, 27], most of the studies
utilized young, healthy, untrained males as participants.
In one investigation examining college football
athletes supplementing with a proprietary milk protein
supplement (two servings of 42 g per day) for 12 weeks, a
14.5% increase in maximal squat strength was observed
compared to a 6.9% increase in the placebo group [28].
These differences were statistically significant. When
females were the only sex investigated, the outcomes
consistently indicated that supplemental protein does not
appear to enhance maximal strength at magnitudes that
reach statistical significance. Hida et al. [30] reported that
females supplementing with 15 g of egg white protein
(which raised daily protein intake to 1.23 g of protein/kg
body weight/day) experienced no improvements in maximal
upper and lower body strength as compared to a
carbohydrate placebo (ingesting one gram of protein/kg
body weight/day) over an 8-week period. An important
note for this study is that 15 g of egg protein is considered
by many to be a sub-optimal dose [31]. However, others
have advocated that the total daily intake of protein might
be as important or more important [32]. In another study,
Josse et al. [33] reported that non-resistance trained females
supplementing with one liter of skimmed bovine
milk (providing 36 g of protein) after resistance exercise
improved maximal strength in seven of nine measures as
compared to a carbohydrate placebo group, but only the
improvements to maximal bench press strength attained
statistical significance compared to the placebo. In
contrast, Taylor and colleagues [34] reported that pre- and
post-exercise whey protein ingestion significantly increased
maximal upper-body strength (+4.9 kg bench
press one repetition maximum) in comparison to changes
seen when a maltodextrin placebo (+2.3 kg) was ingested
in a group of female collegiate basketball players over an
8-week period.
In summary, while research investigating the addition
of supplemental protein to a diet with adequate energy
and nutrient intakes is inconclusive in regards to
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 3 of 25
stimulating strength gains in conjunction with a
resistance-training program to a statistically significant
degree, greater protein intakes that are achieved from
both dietary and supplemental sources do appear to have
some advantage. Hoffman and colleagues [29] reported
that in athletes consuming daily protein intakes above
2.0 g/kg/d which included protein intakes from both diet
and supplements, a 22% and 42% increase in strength
was noted in both the squat and bench press exercises
during off-season conditioning in college football players
compared to athletes that consumed only the recommended
levels (1.6–1.8 g/kg/d) for strength/power
athletes. Further, it is important to highlight that in most
studies cited, protein intervention resulted in greater but
non-statistically significant strength improvements as
compared to the placebo/control condition. Cermak and
colleagues [35] pooled the outcomes from 22 separate
clinical trials to yield 680 subjects in their statistical
analysis and found that protein supplementation with
resistance training resulted in a 13.5 kg increase (95%
Confidence Interval: 6.4–20.7 kg) in lower-body strength
when compared to changes seen when a placebo was
provided. A similar conclusion was also drawn by
Pasiakos et al. [36] in a meta-analysis where they
reported that in untrained participants, protein supplementation
might exert very little benefit on
strength during the initial weeks of a resistance training
program, but as duration, frequency and volume
of resistance training increased, protein supplementation
may favorably impact skeletal muscle hypertrophy
and strength.
Key points:
 Results from many single investigations indicate that
in both men and women protein supplementation
exerts a small to modest impact on strength
development.
 Pooled results of multiple studies using meta-analytic
and other systematic approaches consistently indicate
that protein supplementation (15 to 25 g over 4 to
21 weeks) exerts a positive impact on performance.
Body composition
Improving one’s body composition through the loss of
fat mass and increasing fat-free mass is often associated
with improvements in physical performance. In this respect,
many published investigations report that protein
supplementation results in significant improvements in
lean body weight/cross-sectional areas as compared to
placebo treatments [15, 17, 21–23, 26, 27, 33, 37].
Andersen et al. [15] examined 22 healthy men that
completed a 14-week resistance-training program
(3 days/week consisting of 3–4 sets of lower body
exercises) while supplementing with either 25 g of a
high-quality protein blend or 25 g of carbohydrate.
When the blend of milk proteins was provided, significantly
greater increases in fat-free mass, muscle crosssectional
area in both the Type I and Type II muscle
fibers occurred when compared to changes seen with
carbohydrate consumption. Collectively, a meta-analysis
by Cermak and colleagues [35] reported a mean increase
in fat-free mass of 0.69 kg (95% Confidence Interval:
0.47–0.91 kg) when protein supplementation was
provided versus a placebo during a resistance-training
program. Other reviews by Tipton, Phillips and Pasiakos,
respectively, [36, 38, 39] provide further support that
protein supplementation (15–25 g over 4–14 weeks)
augments lean mass accretion when combined with
completion of a resistance training program.
Beyond accretion of fat-free mass, increasing daily
protein intake through a combination of food and
supplementation to levels above the recommended daily
allowance (RDA) (RDA 0.8 g/kg/day, increasing to 1.2–
2.4 g/kg/day for the endurance and strength/power athletes)
while restricting energy intake (30–40% reduction
in energy intake) has been demonstrated to maximize
the loss of fat tissue while also promoting the maintenance
of fat-free mass [40–45]. The majority of this work
has been conducted using overweight and obese individuals
who were prescribed an energy-restricted diet that
delivered a greater ratio of protein relative to carbohydrate.
As a classic example, Layman and investigators
[40] randomized obese women to consume one of two
restricted energy diets (1600–1700 kcals/day) that were
either higher in carbohydrates (>3.5: carbohydrate-toprotein
ratio) or protein (<1.5: carbohydrate-to-protein ratio).
Groups were further divided into those that followed
a five-day per week exercise program (walking + resistance
training, 20–50 min/workout) and a control group that
performed light walking of less than 100 min per week.
Greater amounts of fat were lost when higher amounts of
protein were ingested, but even greater amounts of fat loss
occurred when the exercise program was added to the
high-protein diet group, resulting in significant decreases
in body fat. Using an active population that ranged from
normal weight to overweight (BMI: 22–29 kg/m2
),
Pasiakos and colleagues [42] examined the impact of progressively
increasing dietary protein over a 21-day study
period. An aggressive energy reduction model was
employed that resulted in each participant reducing their
caloric intake by 30% and increasing their energy expenditure
by 10%. Each person was randomly assigned to
consume a diet that contained either 1× (0.8 g/kg), 2×
(1.6 g/kg) or 3× (2.4 g/kg) the RDA for protein. Participants
were measured for changes in body weight and body
composition. While the greatest body weight loss occurred
in the 1× RDA group, this group also lost the highest
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 4 of 25
percentage of fat-free mass and lowest percentage of fat
mass. The 2× and 3× RDA groups lost significant amounts
of body weight that consisted of 70% and 64% fat mass,
respectively.
Collectively, these results indicate that increasing
dietary protein can promote favorable adaptations in
body composition through the promotion of fat-free
mass accretion when combined with a hyperenergetic
diet and a heavy resistance training program and can
also promote the loss of fat mass when higher intakes of
daily protein (2-3× the RDA) are combined with an
exercise program and a hypoenergetic diet.
Key points:
 When combined with a hyperenergetic diet and a
heavy resistance-training program, protein
supplementation may promote increases in skeletal
muscle cross-sectional area and lean body mass.
 When combined with a resistance-training program
and a hypoenergetic diet, an elevated daily intake of
protein (2 – 3× the RDA) can promote greater
losses of fat mass and greater overall improvements
in body composition.
Protein timing
Thanks to seminal work by pioneering research groups
[37, 46, 47], by the 1990’s it was clear that exercise and
macronutrient consumption interact synergistically to
provide a net anabolic effect far greater than either feeding
or exercise alone. In the absence of feeding, muscle
protein balance remains negative in response to an acute
bout of resistance exercise [48]. Tipton et al. [49] were
one of the first groups to illustrate that an acute feeding
of amino acids significantly increases rates of muscle
protein synthesis (MPS). Later, Burd et al. [50] indicated
that the combination of acute, exhaustive resistance
exercise increases the muscle’s anabolic responsiveness
to whey protein provision for up to 24 h. In addition to
heightened anabolic sensitivity that stems from the
combination of resistance exercise and protein/amino
acid feeding, the importance of the EAAs with respect
to muscle protein growth has also been elucidated.
Tipton et al. [51] first indicated that nonessential amino
acids were not necessary to stimulate MPS. Subsequently,
these conclusions were supported by Borsheim
[52] and Volpi [53]. The study by Borsheim also documented
a dose-response outcome characterized by a
near doubling of net protein balance in response to a
three to six gram dose of the EAAs [52]. Building on this
work, Tipton et al. [54] reported that EAAs (9–15 g
dose) before and after resistance exercise promoted
higher net protein accretion, not just 3 or 4 h post exercise
but also over a 24-h period [55]. These findings
formed the theoretical concept of protein timing for
resistance exercise that has since been transferred to not
only other short-duration, high-intensity activities [56]
but also endurance-based sports [57] and subsequent
performance outcomes [58]. The strategic consumption
of nutrition, namely protein or various forms of amino
acids, in the hours immediately before and during
exercise (i.e., peri-workout nutrition) has been shown to
maximize muscle repair and optimize strength- and
hypertrophy-related adaptations [59, 60]. While earlier
investigations reported positive effects from consumption
of amino acids [37, 46, 61], it is now clear that
intact protein supplements such as egg, whey, casein,
beef, soy and even whole milk can evoke an anabolic
response that can be similar or greater in magnitude
to free form amino acids, assuming ingestion of equal
EAA amounts [62–64].
For instance, whey protein ingested close to resistance
exercise, promotes a higher activation (phosphorylation)
of mTOR (a key signaling protein found in myocytes
that is linked to the synthesis of muscle proteins) and its
downstream mRNA translational signaling proteins (i.e.,
p70s6 kinase and eIF4BP) that further suggests timed ingestion
of protein may favorably promote heightened
muscle hypertrophy [21, 62]. Moreover, it was found that
the increased mTOR signaling corresponded with
significantly greater muscle hypertrophy after 10 weeks
of training [65]. However, the hypertrophic differences
between protein consumption and a non-caloric placebo
appeared to plateau by week 21, despite a persistently
greater activation of this molecular signaling pathway
from supplementation. Results from other research
groups [56–58, 66] show that timing of protein near
(± 2 h) aerobic and anaerobic exercise training appears
to provide a greater activation of the molecular
signalling pathways that regulate myofibrillar and
mitochondrial protein synthesis as well as glycogen
synthesis.
It is widely reported that protein consumption directly
after resistance exercise is an effective way to acutely
promote a positive muscle protein balance [31, 55, 67],
which if repeated over time should translate into a net
gain or hypertrophy of muscle [68]. Pennings and colleagues
[69] reported an increase in both the delivery
and incorporation of dietary proteins into the skeletal
muscle of young and older adults when protein was
ingested shortly after completion of exercise. These
findings and others add to the theoretical basis for
consumption of post-protein sooner rather than later
after exercise, since post workout MPS rates peak within
three hours and remain elevated for an additional 24–
72 h [50, 70]. This extended time frame also provides a
rationale for both immediate and sustained (i.e., every
3–4 h) feedings to optimize impact. These temporal
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 5 of 25
considerations would also capture the peak elevation in
signalling proteins shown to be pivotal for increasing the
initiation of translation of muscle proteins, which for the
most part appears to peak between 30 and 60 min after
exercise [71]. Finally, while some investigations have
shown that a rapid increase in amino acids (aminoacidemia)
from a protein dose immediately after or surrounding
exercise stimulates increased adaptations to resistance
training [72, 73], others examining competitive strength/
power athletes reported no advantage from pre/post supplement
feedings compared to similar feedings in morning
and evening hours [74]. However, these differences may
be related to the type of protein used between the studies.
The studies showing positive effects of protein timing
used milk proteins, whereas the latter study used a collagen
based protein supplement.
While a great deal of work has focused on postexercise
protein ingestion, other studies have suggested
that pre-exercise and even intra-exercise ingestion may
also support favorable changes in MPS and muscle
protein breakdown [14, 54, 75–78]. Initially, Tipton and
colleagues [54] directly compared immediate preexercise
and immediate post-exercise ingestion of a mixture
of carbohydrate (35 g) and EAAs (6 g) combination
on changes in MPS. They reported that pre-exercise
ingestion promoted higher rates of MPS while also
demonstrating that nutrient ingestion prior to exercise
increased nutrient delivery to a much greater extent
than other (immediate or one hour post-exercise)
time points. These results were later challenged by
Fujita in 2009 who employed an identical study design
with a different tracer incorporation approach
and concluded there was no difference between preor
post-exercise ingestion [75]. Subsequent work by
Tipton [79] also found that similar elevated rates of
MPS were achieved when ingesting 20 g of a whey
protein isolate immediately before or immediately
after resistance exercise.
At this point, whether any particular time of protein
ingestion confers any unique advantage over other time
points throughout a 24-h day to improve strength and
hypertrophy has yet to be adequately investigated. To
date, although a substantial amount of literature discusses
this concept [60, 80], a limited number of training
studies have assessed whether immediate pre- and postexercise
protein consumption provides unique advantages
compared to other time points [72, 73, 81]. Each
study differed in population, training program, environment
and nutrition utilized, with each reporting a
different result. What is becoming clear is that the
subject population, nutrition habits, dosing protocols on
both training and non-training days, energy and macronutrient
intake, as well as the exercise bout or training
program itself should be carefully considered alongside
the results. In particular, the daily amount of protein intake
seems to operate as a key consideration because the benefits
of protein timing in relation to the peri-workout period
seem to be lessened for people who are already ingesting
appropriate amounts of protein (e.g. ≥1.6 g/kg/day). This
observation can be seen when comparing the initial
results of Cribb [72], Hoffman [74] and most recently
with Schoenfeld [82]; however, one must also consider
that the participants in the Hoffman study may have
been hypocaloric as they reported consuming approximately
30 kcal/kg in all groups across the entire
study. A literature review by Aragon and Schoenfeld
[83] determined that while compelling evidence exists
showing muscle is sensitized to protein ingestion
following training, the increased sensitivity to protein
ingestion might be greatest in the first five to six
hours following exercise. Thus, the importance of
timing may be largely dependent on when a preworkout
meal was consumed, the size and composition
of that meal and the total daily protein in the
diet. In this respect, a pre-exercise meal will provide
amino acids during and after exercise and therefore it
stands to reason there is less need for immediate
post-exercise protein ingestion if a pre-exercise meal
is consumed less than five hours before the anticipated
completion of a workout. A meta-analysis by
Schoenfeld et al. [84] found that consuming protein
within one-hour post resistance exercise had a small
but significant effect on increasing muscle hypertrophy
compared to delaying consumption by at least
two hours. However, sub-analysis of these results revealed
the effect all but disappeared after controlling
for the total intake of protein, indicating that favorable
effects were due to unequal protein intake between
the experimental and control groups (∼1.7 g/kg
versus 1.3 g/kg, respectively) as opposed to temporal
aspects of feeding. The authors concluded that total
protein intake was the strongest predictor of muscular
hypertrophy and that protein timing likely influences
hypertrophy to a lesser degree. However, the conclusions
from this meta-analysis may be questioned
because the majority of the studies analyzed were not
protein timing studies but rather protein supplementation
studies. In that respect, the meta-analysis
provides evidence that protein supplementation (i.e.,
greater total daily protein intake) may indeed confer
an anabolic effect. While a strong rationale remains
to support the concept that the hours immediately
before or after resistance exercise represents an opportune
time to deliver key nutrients that will drive
the accretion of fat-free mass and possibly other favorable
adaptations, the majority of available literature
suggests that other factors may indeed be operating
to a similar degree that ultimately impact the
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 6 of 25
observed adaptations. In this respect, a key variable
that must be accounted for is the absolute need for
energy and protein required to appropriately set the
body up to accumulate fat-free mass.
A review by Bosse and Dixon [84] critically summarized
the available literature on protein supplementation
during resistance exercise and hypothesized that protein
intake may need to increase by as much as 59% above
baseline levels for significant changes in fat-free mass to
occur. Finally, it should be noted that for many athletes,
consuming a post- or pre-workout protein-containing
meal represents a feeding opportunity with little downside,
since there is no benefit from not consuming
protein pre- and/or post-exercise. In other words, not
consuming protein-containing foods/supplements postexercise
is a strategy that provides no benefit whatsoever.
Thus, the most practical recommendation is to
have athletes consume a meal during the post-workout
(or pre-workout) time period since it may either help or
have a neutral effect.
In younger subjects, the ingestion of 20–30 g of any
high biological value protein before or after resistance
exercise appears to be sufficient to maximally stimulate
MPS [21, 64]. More recently, Macnaughton and colleagues
[85] reported that 40 g of whey protein ingestion
significantly increased the MPS responses compared to a
20 g feeding after an acute bout of whole-body resistance
exercise, and that the absolute protein dose may
operate as a more important consideration than providing
a protein dose that is normalized to lean mass. Free
form EAAs, soy, milk, whey, caseinate, and other protein
hydrolysates are all capable of activating MPS [86].
However, maximal stimulation of MPS, which results in
higher net muscle protein accretion, is the product of
the total amount of EAA in circulation as well as the
pattern and appearance rate of aminoacidemia that modulates
the MPS response [86]. Recent work has clarified
that whey protein provides a distinct advantage over
other protein sources including soy (considered another
fast absorbing protein) and casein (a slower acting
protein source) on acute stimulation of MPS [86, 87].
Importantly, an elegant study by West and investigators
[87] sought to match the delivery of EAAs in feeding
patterns that replicated how whey and casein are
digested. The authors reported that a 25 g dose of whey
protein that promoted rapid aminoacidemia further
enhanced MPS and anabolic signaling when compared
to an identical total dose of whey protein when delivered
as ten separate 2.5 g doses intended to replicate a slower
digesting protein. The advantages of whey protein are
important to consider, particularly as all three sources
rank similarly in assessments of protein quality [88]. In
addition to soy, other plant sources (e.g., pea, rice, hemp,
etc.) have garnered interest as potential protein sources
to consider. Unfortunately, research that examines the
ability of these protein sources to modulate exercise
performance and training adaptations is limited at this
time. One study conducted by Joy and investigators [89]
compared the effect of supplementing a high-dose (48 g/
day) of whey or rice protein in experienced resistancetrained
subjects during an 8-week resistance training
program. The investigators concluded that gains in
strength, muscle thickness and body composition were
similar between the two protein groups, suggesting that
rice protein may be a suitable alternative to whey
protein at promoting resistance training adaptations.
Furthermore, differences in absorption kinetics, and the
subsequent impact on muscle protein metabolism
appear to extend beyond the degree of hydrolysis and
amino acid profiles [69, 86, 90–92]. For instance, unlike
soy more of the EAAs from whey proteins (hydrolysates
and isolates) survive splanchnic uptake and travel to the
periphery to activate a higher net gain in muscle [86].
Whey proteins (hydrolysates and isolates) appear to be
the most extensively researched for pre/post resistance
exercise supplementation, possibly because of their
higher EAA and leucine content [93, 94], solubility, and
optimal digestion kinetics [69]. These characteristics
yield a high concentration of amino acids in the blood
(aminoacidemia) [69, 87] that facilitates greater
activation of MPS and net muscle protein accretion, in
direct comparison to other protein choices [50, 69, 91].
The addition of creatine to whey protein supplementation
appears to further augment these adaptations
[27, 72, 95]; however, an optimal timing strategy for
this combination remains unclear.
The timing of protein-rich meals consumed throughout
a day has the potential to influence adaptations to
exercise. Using similar methods, other studies over recent
decades [53, 62, 87, 91, 96–100] have established
the following:
 MPS increases approximately 30–100% in response
to a protein-containing meal to promote a positive
net protein balance, and the major contributing
factor to this response is the EAA content.
 The anabolic response to feeding is pronounced but
transient. During the post-prandial phase (1–4 h
after a meal) MPS is elevated, resulting in a positive
muscle protein balance. In contrast, MPS rates are
lower in a fasted state and muscle protein balance is
negative. Protein accretion only occurs in the fed
state. The concentration of EAA in the blood
(plasma) regulates protein synthesis rates within
muscle at rest and post exercise. More recent work
has established that protein-carbohydrate supplementation
after strenuous endurance exercise
stimulates contractile MPS via similar signaling
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 7 of 25
pathways as resistance exercise [56, 57]. Most
importantly, and as mentioned initially in this
section, muscle appears to be “sensitized” to protein
feeding for at least 24 h after exercise [50]. That is,
the consumption of a protein-containing meal up to
24 h after a single bout of resistance exercise results
in a higher net stimulation of MPS and protein
accretion than the same meal consumed after 24 h
of inactivity [50].
 The effect of insulin on MPS is dependent on its
ability to increase amino acid availability, which does
not occur when insulin is systematically increased
(e.g., following feeding) [101]. In particular, insulin’s
impact on net protein balance seems to operate
most powerfully in an anti-catabolic manner on
muscle [102]. However, insulin-mediated effects that
reduce muscle protein breakdown peaks at low to
moderate levels of insulin (~15–30 μIU/mL)
[103, 104] that can be achieved by consumption of a
45-g dose of whey protein isolate alone [105]. Taken
together, these results seem to indicate that postworkout
carbohydrate supplementation offers very
little contribution from a muscle development
standpoint provided adequate protein is consumed.
For example, Staples and colleagues [106] compared
the impact of a carbohydrate + protein combination
on rates of MPS and reported no further increases in
MPS beyond what was seen with protein ingestion
alone. Importantly, these results are not to be
interpreted to mean that carbohydrate administration
offers no potential effect for an athlete engaging in
moderate to high volumes of training, but rather that
benefits derived from carbohydrate administration
appear to more favorably impact aspects of muscle
glycogen recovery as opposed to stimulating muscle
protein accretion.
Pre-sleep protein intake
Eating before sleep has long been controversial [107–109].
However, a methodological consideration in the original
studies such as the population used, time of feeding, and
size of the pre-sleep meal confounds firm conclusions
about benefits or drawbacks. Recent work using proteinrich
beverages 30-min prior to sleep and two hours after
the last meal (dinner) have identified pre-sleep protein
consumption/ingestion as advantageous to MPS, muscle
recovery, and overall metabolism in both acute and longterm
studies [110, 111]. Results from several investigations
indicate that 30–40 g of casein protein ingested 30-min
prior to sleep [112] or via nasogastric tubing [113]
increased overnight MPS in both young and old men,
respectively. Likewise, in an acute setting, 30 g of whey
protein, 30 g of casein protein, and 33 g of carbohydrate
consumed 30-min prior to sleep resulted in an elevated
morning resting metabolic rate in young fit men compared
to a non-caloric placebo [114]. Similarly, although
not statistically significant, morning increases in resting
metabolic rate were reported in young overweight and/or
obese women [115]. Interestingly, Madzima et al. [114]
reported that subjects’ respiratory quotient measured
during the morning after pre-sleep nutrient intake was
unchanged only for the placebo and casein protein trials,
while both carbohydrate and whey protein were increased
compared to placebo. This infers that casein protein
consumed pre-sleep maintains overnight lipolysis and fat
oxidation. This finding was further supported by Kinsey et
al. [116] using a microdialysis technique to measure interstitial
glycerol concentrations overnight from the subcutaneous
abdominal adipose tissue, reporting greater fat
oxidation following consumption of 30 g of casein compared
to a flavor and sensory-matched noncaloric placebo
in obese men. Similar to Madzima et al. [114], Kinsey et
al. [116] concluded that pre-sleep casein did not blunt
overnight lipolysis or fat oxidation. Interestingly, the presleep
protein and carbohydrate ingestion resulted in
elevated insulin concentrations the next morning and
decreased hunger in this overweight population. Of note,
it appears that exercise training completely ameliorates
any rise in insulin when eating at night before sleep [117],
while the combination of pre-sleep protein and exercise
has been shown to reduce blood pressure and arterial
stiffness in young obese women with prehypertension and
hypertension [118]. In athletes, evening chocolate milk
consumption has also been shown to influence carbohydrate
metabolism in the morning, but not running performance
[108]. In addition, data supports that exercise
performed in the evening augments the overnight MPS
response in both younger and older men [119–121].
To date, only a few studies involving nighttime protein
ingestion have been carried out for longer than four
weeks. Snijders et al. [122] randomly assigned young
men (average age of 22 years) to consume a proteincentric
supplement (27.5 g of casein protein, 15 g of
carbohydrate, and 0.1 g of fat) or a noncaloric placebo
every night before sleep while also completing a 12-week
progressive resistance exercise training program (3 times
per week). The group receiving the protein-centric supplement
each night before sleep had greater improvements
in muscle mass and strength over the 12-week
study. Of note, this study was non-nitrogen balanced
and the protein group received approximately 1.9 g/kg/
day of protein compared to 1.3 g/kg/day in the placebo
group. More recently, in a study in which total protein
intake was equal, Antonio et al. [123] studied young
healthy men and women that supplemented with casein
protein (54 g) for 8 weeks either in the morning (any
time before 12 pm) or the evening supplementation
(90 min or less prior to sleep). They examined the
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 8 of 25
effects on body composition and performance [123]. All
subjects maintained their usual exercise program. The
authors reported no differences in body composition or
performance between the morning and evening casein
supplementation groups. However, it is worth noting
that, although not statistically significant, the morning
group added 0.4 kg of fat free mass while the evening
protein group added 1.2 kg of fat free mass, even though
the habitual diet of the trained subjects in this study
consumed 1.7 to 1.9 g/kg/day of protein. Although this
finding was not statistically significant, it supports data
from Burk et al. [81] indicating that casein-based protein
consumed in the morning (10 am) and evening
(10:30 pm) was more beneficial for increasing fat-free
mass than consuming the protein supplement in the
morning (10 am) and afternoon (~3:50 pm). It should be
noted that the subjects in the Burk et al. study were
resistance training. A retrospective epidemiological study
by Buckner et al. [124] using NHANES data (1999–
2002) showed that participants consuming 20, 25, or
30 g of protein in the evening had greater leg lean mass
compared to subjects consuming protein in the afternoon.
Thus, it appears that protein consumption in the
evening before sleep might be an underutilized time to
take advantage of a protein feeding opportunity that can
potentially improve body composition and performance.
Protein ingestion and meal timing
In addition to direct assessments of timed administration of
nutrients, other studies have explored questions that center
upon the pattern of when certain protein-containing meals
are consumed. Paddon-Jones et al. [97] reported a correlation
between acute stimulation of MPS via protein consumption
and chronic changes in muscle mass. In this
study, participants were given an EAA supplement three
times a day for 28 days. Results indicated that acute stimulation
of MPS provided by the supplement on day 1 resulted
in a net gain of ~7.5 g of muscle over a 24-h period
[97]. When extrapolated over the entire 28-day study, the
predicted change in muscle mass corresponded to the actual
change in muscle mass (~210 g) measured by dualenergy
x-ray absorptiometry (DEXA) [97]. While these
findings are important, it is vital to highlight that this study
incorporated a bed rest model with no acute exercise
stimulus while other work by Mitchell et al. [125] reported
a lack of correlation between measures of acute MPS and
the accretion of skeletal muscle mass.
Interestingly, supplementation with 15 g of EAAs and
30 g of carbohydrate produced a greater anabolic effect (increase
in net phenylalanine balance) than the ingestion of a
mixed macronutrient meal, despite the fact that both interventions
contained a similar dose of EAAs [96]. Most importantly,
the consumption of the supplement did not
interfere with the normal anabolic response to the meal
consumed three hours later [96]. The results of these investigations
suggest that protein supplement timing between
the regular “three square meals a day” may provide an additive
effect on net protein accretion due to a more frequent
stimulation of MPS. Areta et al. [126] were the first to
examine the anabolic response in human skeletal muscle to
various protein feeding strategies for a day after a single
bout of resistance exercise. The researchers compared the
anabolic responses of three different patterns of ingestion
(a total of 80 g of protein) throughout a 12-h recovery
period after resistance exercise. Using a group of healthy
young adult males, the protein feeding strategies consisted
of small pulsed (8 × 10 g), intermediate (4 × 20 g), or bolus
(2 × 40 g) administration of whey protein over the 12-h
measurement window. Results showed that the intermediate
dosing (4 × 20 g) was superior for stimulating MPS for
the 12-h experimental period. Specifically, the rates of myofibrillar
protein synthesis were optimized throughout the
day of recovery by the consumption of 20 g protein every
three hours compared to large (2 × 40 g), less frequent
servings or smaller but more frequent (8 × 10 g) patterns of
protein intake [67]. Previously, the effect of various protein
feeding strategies on skeletal MPS during an entire day was
unknown. This study provided novel information demonstrating
that the regulation of MPS can be modulated by
the timing and distribution of protein over 12 h after a single
bout of resistance exercise. However, it should be noted
that an 80 g dose of protein over a 12-h period is quite low.
The logical next step for researchers is to extend these
findings into longitudinal training studies to see if these
patterns can significantly affect resistance-training
adaptations. Indeed, published studies by Arnal [127]
and Tinsley [128] have all made some attempt to examine
the impact of adjusting the pattern of protein consumption
across the day in combination with various
forms of exercise. Collective results from these studies
are mixed. Thus, future studies in young adults should
be designed to compare a balanced vs. skewed distribution
pattern of daily protein intake on the daytime
stimulation of MPS (under resting and post-exercise
conditions) and training-induced changes in muscle
mass, while taking into consideration the established
optimal dose of protein contained in a single serving for
young adults. Without more conclusive evidence
spanning several weeks, it seems pragmatic to recommend
the consumption of at least 20-25 g of protein
(~0.25 g/kg/meal) with each main meal with no more
than 3–4 h between meals [126].
Key points
 In the absence of feeding and in response to
resistance exercise, muscle protein balance
remains negative.
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 9 of 25
 Skeletal muscle is sensitized to the effects of protein
and amino acids for up to 24 h after completion of a
bout of resistance exercise.
 A protein dose of 20–40 g of protein (10–12 g of
EAAs, 1–3 g of leucine) stimulates MPS, which can
help to promote a positive nitrogen balance.
 The EAAs are critically needed for achieving
maximal rates of MPS making high-quality, protein
sources that are rich in EAAs and leucine the
preferred sources of protein.
 Studies have suggested that pre-exercise feedings of
amino acids in combination with carbohydrate can
achieve maximal rates of MPS, but protein and
amino acid feedings during this time are not clearly
documented to increase exercise performance.
 Ingestion of carbohydrate + protein or EAAs during
endurance and resistance exercise can help to
maintain a favorable anabolic hormone profile,
minimize increases in muscle damage, promote
increases in muscle cross-sectional area, and
increase time to exhaustion during prolonged
running and cycling.
 Post-exercise administration of protein when
combined with suboptimal intake of carbohydrates
(<1.2 g/kg/day) can heighten muscle glycogen
recovery, and may help mitigate changes in muscle
damage markers.
 Total protein and calorie intake appears to be the
most important consideration when it comes to
promoting positive adaptations to resistance
training, and the impact of timing strategies
(immediately before or immediately after) to
heighten these adaptations in non-athletic
populations appears to be minimal.
Recommended intake
Proteins provide the building blocks of all tissues via
their constituent amino acids. Athletes consume dietary
protein to repair and rebuild skeletal muscle and
connective tissues following intense training bouts or
athletic events. During in the 1980s and early 1990’s
Tarnopolsky [129], Phillips [130], and Lemon [131] first
demonstrated that total protein needs were 50 to 175%
greater in athletes than sedentary controls. A report in
2004 by Phillips [132] summarized the findings
surrounding protein requirements in resistance-trained
athletes. Using a regression approach, he concluded that
a protein intake of 1.2 g of protein per kg of body weight
per day (g/kg/day) should be recommended, and when
the upper limit of a 95% confidence interval was included
the amount approached 1.33 g/kg/day. A key
consideration regarding these recommended values is
that all generated data were obtained using the nitrogen
balance technique, which is known to underestimate
protein requirements. Interestingly, two of the included
papers had prescribed protein intakes of 2.4 and 2.5 g/kg/
day, respectively [129, 133]. All data points from these two
studies also had the highest levels of positive nitrogen
balance. For an athlete seeking to ensure an anabolic environment,
higher daily protein intakes might be needed.
Another challenge that underpins the ability to universally
and successfully recommend daily protein amounts are
factors related to the volume of the exercise program, age,
body composition and training status of the athlete; as
well as the total energy intake in the diet, particularly for
athletes who desire to lose fat and are restricting calories
to accomplish this goal [134]. For these reasons, and due
to an increase of published studies in areas related to optimal
protein dosing, timing and composition, protein
needs are being recommended within this position stand
on a per meal basis.
For example, Moore [31] found that muscle and albumin
protein synthesis was optimized at approximately
20 g of egg protein at rest. Witard et al. [135] provided
incremental doses of whey protein (0, 10, 20 and 40 g)
in conjunction with an acute bout of resistance exercise
and concluded that a minimum protein dose of 20 g
optimally promoted MPS rates. Finally, Yang and
colleagues [136] had 37 elderly men (average age of
71 years) consume incremental doses of whey protein
isolate (0, 10, 20 and 40 g/dose) in combination with a
single bout of lower body resistance exercise and concluded
that a 40 g dose of whey protein isolate is needed
in this population to maximize rates of MPS. Furthermore,
while results from these studies offer indications
of what optimal absolute dosing amounts may be,
Phillips [134] concluded that a relative dose of 0.25 g of
protein per kg of body weight per dose might operate as
an optimal supply of high-quality protein. Once a total
daily target protein intake has been achieved, the
frequency and pattern with which optimal doses are
ingested may serve as a key determinant of overall
changes in protein synthetic rates.
Research indicates that rates of MPS rapidly rise to
peak levels within 30 min of protein ingestion and are
maintained for up to three hours before rapidly beginning
to lower to basal rates of MPS even though amino
acids are still elevated in the blood [137]. Using an oral
ingestion model of 48 g of whey protein in healthy
young men, rates of myofibrillar protein synthesis
increased three-fold within 45–90 min before slowly
declining to basal rates of MPS all while plasma concentration
of EAAs remained significantly elevated [138].
While human models have not fully explored the mechanistic
basis of this ‘muscle-full’ phenomenon, an energy
deficit theory has been proposed which hypothesizes
that rates of MPS were blunted even though plasma
concentrations of amino acids remained elevated
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 10 of 25
because a relative lack of cellular ATP was available to
drive the synthetic process [139]. While largely unexplored
in a human model, these authors relied upon an
animal model and were able to reinstate increases in
MPS using the consumption of leucine and carbohydrate
135 min after ingestion of the first meal. As such, it is
suggested that individuals attempting to restrict caloric
intake should consume three to four whole meals
consisting of 20–40 g of protein per meal. While this
recommendation stems primarily from initial work that
indicated protein doses of 20–40 g favorably promote increased
rates of MPS [31, 135, 136], Kim and colleagues
[140] recently reported that a 70 g dose of protein promoted
a more favorable net balance of protein when
compared to a 40 g dose due to a stronger attenuation
of rates of muscle protein breakdown.
For those attempting to increase their calories, we suggest
consuming small snacks between meals consisting
of both a complete protein and a carbohydrate source.
This contention is supported by research from PaddonJones
et al. [97] that used a 28-day bed rest model.
These researchers compared three 850-cal mixed macronutrient
meals to three 850-cal meals combined with
three 180-cal amino acid-carbohydrate snacks between
meals. Results demonstrated that subjects, who also consumed
the small snacks, experienced a 23% increase in
muscle protein fractional synthesis and successful maintenance
of strength throughout the bed rest trial. Additionally,
using a protein distribution pattern of 20–25 g
doses every three hours in response to a single bout of
lower body resistance exercise appears to promote the
greatest increase in MPS rates and phosphorylation of
key intramuscular proteins linked to muscle hypertrophy
[126]. Finally, in a series of experiments, Arciero and
colleagues [116, 141] employed a protein pacing strategy
involving equitable distribution of effective doses of protein
(4–6 meals/day of 20–40 g per meal) alone and
combined with multicomponent exercise training. Using
this approach, their results consistently demonstrate
positive changes in body composition [116, 142] and
physical performance outcomes in both lean [143, 144]
and overweight/obese populations [142, 143, 145]. This
simple addition could provide benefits for individuals
looking to increase muscle mass and improve body
composition in general while also striving to maintain or
improve health and performance.
Key points
 The current RDA for protein is 0.8 g/kg/day with
multiple lines of evidence indicating this value is not
an appropriate amount for a training athlete to meet
their daily needs.
 While previous recommendations have suggested a
daily intake of 1.2–1.3 g/kg/day is an appropriate
amount, most of this work was completed using the
nitrogen balance technique, which is known to
systematically underestimate protein needs.
 Daily and per dose needs are combinations of many
factors including volume of exercise, age, body
composition, total energy intake and training status
of the athlete.
 Daily intakes of 1.4 to 2.0 g/kg/day operate as a
minimum recommended amount while greater
amounts may be needed for people attempting to
restrict energy intake while maintaining fat-free mass.
 Recommendations regarding the optimal protein
intake per serving for athletes to maximize MPS are
mixed and are dependent upon age and recent
resistance exercise stimuli. General
recommendations are 0.25 g of a high-quality
protein per kg of body weight, or an absolute
dose of 20–40 g.
 Higher doses (~40 g) are likely needed to maximize
MPS responses in elderly individuals.
 Even higher amounts (~70 g) appear to be
necessary to promote attenuation of muscle
protein breakdown.
 Pacing or spreading these feeding episodes
approximately three hours apart has been
consistently reported to promote sustained,
increased levels of MPS and performance benefits.
Protein quality
There are 20 total amino acids, comprised of 9 EAAs
and 11 non-essential amino acids (NEAAs). EAAs
cannot be produced in the body and therefore must be
consumed in the diet. Several methods exist to determine
protein quality such as Chemical Score, Protein
Efficiency Ratio, Biological Value, Protein DigestibilityCorrected
Amino Acid Score (PDCAAS) and most
recently, the Indicator Amino Acid Oxidation (IAAO)
technique. Ultimately, in vivo protein quality is typically
defined as how effective a protein is at stimulating MPS
and promoting muscle hypertrophy [146]. Overall, research
has shown that products containing animal and
dairy-based proteins contain the highest percentage of
EAAs and result in greater hypertrophy and protein synthesis
following resistance training when compared to a
vegetarian protein-matched control, which typically
lacks one or more EAAs [86, 93, 147].
Several studies, but not all, [148] have indicated that
EAAs alone stimulate protein synthesis in the same
magnitude as a whole protein with the same EAA content
[98]. For example, Borsheim et al. [52] found that
6 g of EAAs stimulated protein synthesis twice as much
as a mixture of 3 g of NEAAs combined with 3 g of
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 11 of 25
EAAs. Moreover, Paddon-Jones and colleagues [96]
found that a 180-cal supplement containing 15 g of
EAAs stimulated greater rates of protein synthesis than
an 850-cal meal with the same EAA content from a
whole protein source. While important, the impact of a
larger meal on changes in circulation and the subsequent
delivery of the relevant amino acids to the muscle
might operate as important considerations when interpreting
this data. In contrast, Katsanos and colleagues
[148] had 15 elderly subjects consume either 15 g of
whey protein or individual doses of the essential and
nonessential amino acids that were identical to what is
found in a 15-g whey protein dose on separate occasions.
Whey protein ingestion significantly increased leg phenylalanine
balance, an index of muscle protein accrual, while
EAA and NEAA ingestion exerted no significant impact
on leg phenylalanine balance. This study, and the results
reported by others [149] have led to the suggestion that an
approximate 10 g dose of EAAs might serve as an optimal
dose to maximally stimulate MPS and that intact protein
feedings of appropriate amounts (as opposed to free
amino acids) to elderly individuals may stimulate greater
improvements in leg muscle protein accrual.
Based on this research, scientists have also attempted to
determine which of the EAAs are primarily responsible
for modulating protein balance. The three branched-chain
amino acids (BCAAs), leucine, isoleucine, and valine are
unique among the EAAs for their roles in protein metabolism
[150], neural function [151–153], and blood glucose
and insulin regulation [154]. Additionally, enzymes responsible
for the degradation of BCAAs operate in a ratelimiting
fashion and are found in low levels in splanchnic
tissues [155]. Thus, orally ingested BCAAs appear rapidly
in the bloodstream and expose muscle to high concentrations
ultimately making them key components of skeletal
MPS [156]. Furthermore, Wilson and colleagues [157]
have recently demonstrated, in an animal model, that leucine
ingestion (alone and with carbohydrate) consumed
between meals (135 min post-consumption) extends
protein synthesis by increasing the energy status of the
muscle fiber. Multiple human studies have supported the
contention that leucine drives protein synthesis [158, 159].
Moreover, this response may occur in a dosedependent
fashion, plateauing at approximately two g
at rest [31, 157], and increasing up to 3.5 g when
ingestion occurs after completion of a 60-min bout of
moderate intensity cycling [159]. However, it is important
to realize that the duration of protein synthesis
after resistance exercise appears to be limited by
both the signal (leucine concentrations), ATP status,
as well as the availability of substrate (i.e., additional
EAAs found in a whole protein source) [160]. As
such, increasing leucine concentration may stimulate
increases in muscle protein, but a higher total dose of
all EAAs (as free form amino acids or intact protein
sources) seems to be most suited for sustaining the
increased rates of MPS [160].
It is well known that exercise improves net muscle
protein balance and in the absence of protein feeding,
this balance becomes more negative. When combined
with protein feeding, net muscle protein balance after
exercise becomes positive [161]. Norton and Layman
[150] proposed that consumption of leucine, could
turn a negative protein balance to a positive balance
following an intense exercise bout by prolonging the
MPS response to feeding. In support, the ingestion of
a protein or essential amino acid complex that
contains sufficient amounts of leucine has been
shown to shift protein balance to a net positive state
after intense exercise training [46, 150]. Even though
leucine has been demonstrated to independently
stimulate protein synthesis, it is important to
recognize that supplementation should not be with
just leucine alone. For instance, Wilson et al. [139]
demonstrated in an animal model that leucine
consumption resulted in a lower duration of protein
synthesis compared to a whole meal. In summary,
athletes should focus on consuming adequate leucine
content in each of their meals through selection of
high-quality protein sources [139].
Key points
 Protein sources containing higher levels of the EAAs
are considered to be higher quality sources of
protein.
 The body uses 20 amino acids to make proteins,
seven of which are essential (nine conditionally),
requiring their ingestion to meet daily needs.
 EAAs appear to be uniquely responsible for
increasing MPS with doses ranging from 6 to 15 g
all exerting stimulatory effects. In addition,
doses of approximately one to three g of
leucine per meal appear to be needed to
stimulate protein translation machinery.
 The BCAAs (i.e., isoleucine, leucine, and valine)
appear to exhibit individual and collective
abilities to stimulate protein translation. However,
the extent to which these changes are aligned with
changes in MPS remains to be fully explored.
 While greater doses of leucine have been
shown to independently stimulate increases
in protein synthesis, a balanced consumption
of the EAAs promotes the greatest increases.
 The prioritization of feedings of protein
with adequate levels of leucine/BCAAs
will best promote increases in MPS.
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 12 of 25
Protein sources
Milk proteins
Milk proteins have undergone extensive research related
to their potential roles in augmenting adaptations from
exercise training [86, 93]. For example, consuming milk
following exercise has been demonstrated to accelerate
recovery from muscle damaging exercise [162], increase
glycogen replenishment [163], improve hydration status
[162, 164], and improve protein balance to favor synthesis
[86, 93], ultimately resulting in increased gains in
both neuromuscular strength and skeletal muscle hypertrophy
[93]. Moreover, milk protein contains the highest
score on the PDCAAS rating system, and in general
contains the greatest density of leucine [156]. Milk can
be fractionated into two protein classes, casein and
whey.
Comparison of the quality of whey and casein reveal
that these two proteins routinely contain the highest
leucine content of all other protein sources at 11% and
9.3%, respectively. While both are high in quality, the
two differ in the rate at which they digest as well as the
impact they have on protein metabolism [165–167].
Whey protein is water soluble, mixes easily, and is rapidly
digested [168]. In contrast, casein is water insoluble,
coagulates in the gut and is digested more slowly than
whey protein [168]. Casein also has intrinsic properties
such as opioid peptides, which effectively slow gastric
motility [168]. Original research investigating the effects
of digestion rate was conducted by Boirie, Dangin and
colleagues [165–167]. These researchers gave a 30 g
bolus of whey protein and a 43 g bolus of casein protein
to subjects on separate occasions and measured amino
acid levels for several hours after ingestion. They
reported that the whey protein condition displayed robust
hyperaminoacidemia 100 min after administration.
However, by 300 min, amino acid concentrations had
returned to baseline. In contrast, the casein condition
resulted in a slow increase in amino acid concentrations,
which remained elevated above baseline after 300 min.
Over the study duration, casein produced a greater
whole body leucine balance than the whey protein condition,
leading the researcher to suggest that prolonged,
moderate hyperaminoacidemia is more effective at
stimulating increases in whole body protein anabolism
than a robust, short lasting hyperaminoacidemia.
While this research appears to support the efficacy of
slower digesting proteins, subsequent work has questioned
its validity in athletes. The first major criticism is
that Boire and colleagues investigated whole body (nonmuscle
and muscle) protein balance instead of skeletal
(myofibrillar) MPS. This is important considering that
skeletal muscle protein turnover occurs at a much
slower rate than protein turnover of both plasma and
gut proteins; as a result, MPS has been suggested to
contribute anywhere from 25 to 50% of total whole body
protein synthesis [169]. These findings suggest that
changes in whole body protein turnover may poorly reflect
the level of skeletal muscle protein metabolism that
may be taking place. Trommelen and investigators [121]
examined 24 young men ingesting 30 g of casein protein
with or without completion of a single bout of resistance
exercise, and concluded that rates of MPS were
increased, but whole-body protein synthesis rates were
not impacted.
More recently, Tang and colleagues [86] investigated
the effects of administering 22 g of hydrolyzed whey
isolate and micellar casein (10 g of EAAs) at both rest
and following a single bout of resistance training in
young males. The area under the curve calculations
demonstrated a 200% greater increase in leucine concentrations
in the blood following whey versus casein ingestion.
Moreover, these researchers reported that whey
protein ingestion stimulated greater MPS at both rest
and following exercise when compared to casein. Tipton
et al. [79] used an acute study design involving a single
bout of lower body resistance exercise and 20-g doses of
casein or whey after completing the exercise session. In
comparison to the control group, both whey and casein
significantly increased leucine balance, but no differences
were found between the two protein sources for
amino acid uptake and muscle protein balance.
Additional research has also demonstrated that 10 weeks
of whey protein supplementation in trained bodybuilders
resulted in greater gains in lean mass (5.0 vs. 0.8 kg) and
strength compared to casein [170]. These findings
suggest that the faster-digesting whey proteins may be
more beneficial for skeletal muscle adaptations than the
slower digesting casein.
Effects of milk proteins on glycogen replenishment and
skeletal muscle damage
Skeletal muscle glycogen stores are a critical element to
both prolonged and high-intensity exercise. In skeletal
muscle, glycogen synthase activity is considered one of
the key regulatory factors for glycogen synthesis.
Research has demonstrated that the addition of protein
in the form of milk and whey protein isolate (0.4 g/kg)
to a moderate (0.8 g/kg), but not high (1.2 g/kg)
carbohydrate-containing (dextrose-maltodextrin) beverage
promotes increased rates of muscle glycogen replenishment
following hard training [47]. Further, the
addition of protein facilitates repair and recovery of the
exercised muscle [12]. These effects are thought to be
related to a greater insulin response following the exercise
bout. Intriguingly, it has also been demonstrated
that whey protein enhances glycogen synthesis in the
liver and skeletal muscle more than casein in an insulinindependent
fashion that appears to be due to its
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 13 of 25
capacity to upregulate glycogen synthase activity [171].
Therefore, the addition of milk protein to a postworkout
meal may augment recovery, improve protein
balance, and speed glycogen replenishment.
Health benefits of milk-based proteins
While athletes tend to view whey as the ideal protein for
skeletal muscle repair and function it also has several
health benefits. In particular, whey protein contains an
array of biologically active peptides whose amino acids
sequences give them specific signaling effects when
liberated in the gut. Not only is whey protein high in βLactoglobulin
and α-lactalbumin (75% of total bovine
whey proteins), but it is also rich in EAAs (approximately
50% by weight). Furthermore, whey protein appears
to play a role in enhancing lymphatic and immune
system responses [106]. In addition, α-lactalbumin contains
an ample supply of tryptophan which increases
cognitive performance under stress [172], improves the
quality of sleep [172, 173], and may also speed wound
healing [172], properties which could be vital for recovery
from combat and contact sporting events. In
addition, lactoferrin is also found in both milk and in
whey protein, and has been demonstrated to have antibacterial,
antiviral, and antioxidant properties [174].
Moreover, there is some evidence that whey protein can
bind iron and therefore increase its absorption and
retention [175].
Egg proteins
Egg protein is often thought of as an ideal protein
because its amino acid profile has been used as the
standard for comparing other dietary proteins [168].
Due to their excellent digestibility and amino acid content,
eggs are an excellent source of protein for athletes.
While the consumption of eggs has been criticized due
to their cholesterol content, a growing body of evidence
demonstrates the lack of a relationship between egg consumption
and coronary heart disease, making egg-based
products more appealing [176]. One large egg has 75 kcal
and 6 g of protein, but only 1.5 g of saturated fat while
one large egg white has 16 kcal with 3.5 g of protein and
is fat-free. Research using eggs as the protein source for
athletic performance and body composition is lacking,
perhaps due to less funding opportunities relative to
funding for dairy. Egg protein may be particularly
important for athletes, as this protein source has been
demonstrated to significantly increase protein synthesis
of both skeletal muscle and plasma proteins after resistance
exercise at both 20 and 40 g doses. Leucine oxidation
rates were found to increase following the 40 g
dose, suggesting that this amount exceeds an optimal
dose [31]. In addition to providing a cost effective, highquality
source of protein rich in leucine (0.5 g of leucine
per serving), eggs have also been identified as a functional
food [177]. Functional foods are defined as foods
that, by the presence of physiologically active components,
provide a health benefit beyond basic nutrition
[178]. According to the Academy of Nutrition and
Dietetics, functional foods should be consumed as part
of a varied diet on a regular basis, at effective levels
[179]. Thus, it is essential that athletes select foods that
meet protein requirements and also optimize health and
prevent decrements in immune function following
intense training. Important nutrients provided by eggs
include riboflavin (15% RDA), selenium (17% RDA) and
vitamin K (31% RDA) [177]. Eggs are also rich in
choline, a nutrient which may have positive effects on
cognitive function [180]. Moreover, eggs provide an
excellent source of the carotenoid-based antioxidants
lutein and zeaxanthin [181]. Also, eggs can be prepared
with most meal choices, whether at breakfast, lunch, or
dinner. Such positive properties increase the probability
of the athletes adhering to a diet rich in egg protein.
Beef and other flesh proteins
Meat proteins are a major staple in the American diet
and, depending on the cut of meat, contain varying
amounts of fat and cholesterol. Meat proteins are well
known to be rich sources of the EAAs [182]. Beef is a
common source of dietary protein and is considered to
be of high biological value because it contains the full
balance of EAAs in a fraction similar to that found in
human skeletal muscle [182]. A standard serving of
113.4 g lean beef provides 10 g of the EAAs (3.5 g of
leucine) and 30 g of total amino acids. Moreover, this
30 g dose of beef protein has been shown to stimulate
protein synthesis in both young and elderly subjects
[182]. In addition to its rich content of amino acids, beef
and other flesh proteins can serve as important sources
of micronutrients such as iron, selenium, vitamins A,
B12 and folic acid. For the most part, these quality minerals
and micronutrients cannot be as easily obtained
through plant-based proteins and/or the bioavailability
of these macronutrients from plants is limited. This is a
particularly important consideration for pregnant and
breastfeeding women. Ultimately, as an essential part of
a mixed diet, meat helps to ensure adequate distribution
of essential micronutrients and amino acids to the body.
Research has shown that significant differences in
skeletal muscle mass and body composition between
older men who resistance train and either consume
meat-based or lactoovovegetarian diet [147]. Over a 12week
period, whole-body density, fat-free mass, and
whole-body muscle mass (as measured by urinary
creatinine excretion) increased in the meat-sourced diet
group but decreased in the lactoovovegetarian diet
group. These results indicate that not only do meatJäger
et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 14 of 25
based diets increase fat-free mass, but also they may
specifically increase muscle mass, thus supporting the
many benefits of meat-based diets. A diet high in meat
protein in older adults may provide an important
resource in reducing the risk of sarcopenia.
Positive results have also been seen in elite athletes
that consume meat-based proteins, as opposed to vegetarian
diets [183]. For example, carnitine is a molecule
that transports long-chain fatty acids into mitochondria
for oxidation and is found in high amounts in meat.
While evidence is lacking to support an increase in fat
oxidation with increased carnitine availability, carnitine
has been linked to the sparing of muscle glycogen, and
decreases in exercise-induced muscle damage [184].
Certainly, more research is needed to support these
assertions. Creatine is a naturally occurring compound
found mainly in muscle. The concentration of creatine
in uncooked chicken and beef is approximately
30 mmol/kg (4–5 g/kg), meaning that one serving of
beef contains approximately 0.4 g of creatine [185].
Vegetarians have lower total body creatine stores than
omnivores, which demonstrates that regular meat eating
has a significant effect on human creatine status [186].
Moreover, creatine supplementation studies with vegetarians
indicate that increased creatine uptake levels do
exist in people who practice various forms of vegetarianism
[187]. Sharp and investigators [188] published the
only study known to compare different supplemental
(powdered) forms of animal proteins on adaptations to
resistance training such as increases in strength and improvements
in body composition. Forty-one men and
women performed a standardized resistance-training
program over eight weeks and consumed a daily 46 g
dose of either hydrolyzed chicken protein, beef protein
isolate, or whey protein concentrate in comparison to a
control group. All groups experienced similar increases
in upper and lower-body strength, but all proteinsupplemented
groups reported significant increases in
lean mass and decreases in fat mass.
Meat-based diets have been shown to include additional
overall health benefits. Some studies have found
that meat, as a protein source, is associated with higher
serum levels of IGF-1 [189], which in turn is related to
increased bone mineralization and fewer fractures [190].
Meat vs. plant based proteins: Is one better than the other?
A highly debated topic in nutrition and epidemiology is
whether vegetarian diets are a healthier choice than
omnivorous diets. One key difference is the fact that
vegetarian diets often lack equivalent amounts of protein
when compared to omnivorous diets [147]. However,
with proper supplementation and careful nutritional
choices, it is possible to have complete proteins in a
vegetarian diet. Generally by consuming high-quality,
animal-based products (meat, milk, eggs, and cheese) an
individual will achieve optimal growth as compared to
ingesting only plant proteins [147]. Research has shown
that soy is considered a lower quality complete protein.
Hartman et al. [93] had participants consume a mixture
of sucrose and either 30 g of milk or soy proteins during
12-weeks of resistance training. They found that the
participants that consumed the milk protein increased
lean mass and decreased fat mass more than the control
and soy groups. Moreover, the soy group was not significantly
different from the control group. Similarly, a
study by Tang and colleagues [86] directly compared the
abilities of hydrolyzed whey isolate, soy isolate, and micellar
casein to stimulate rates of MPS both at rest and
in response to a single bout of lower body resistance
training. These authors reported that the ability of soy to
stimulate MPS was greater than casein, but less than
whey, at rest and in response to an acute resistance
exercise stimulus. While soy is considered a complete
protein, it contains lower amounts of BCAAs than
bovine milk [168]. Additionally, research has found that
dietary soy phytoestrogens inhibit mTOR expression in
skeletal muscle through activation of AMPK [191]. Thus,
not only does soy contain lower amounts of the EAAs
and leucine, but soy protein may also be responsible for
inhibiting growth factors and protein synthesis via its
negative regulation of mTOR. When considering the
multitude of plant sources of protein, soy overwhelmingly
has the most research. Limited evidence using
wheat protein in older men has suggested that wheat
protein stimulates significantly lower levels of MPS
when compared to an identical dose (35 g) of casein
protein, but when this dose is increased nearly two fold
(60 g) this protein source is able to significantly increase
rates of myofibrillar protein synthesis [192]. Rice protein
is a medium to slow absorbing protein, which is in line
with other non-meat/non-dairy proteins, however,
leucine from rice protein shows unique absorption kinetics,
peaking faster than leucine from whey protein [193].
As mentioned earlier, a study by Joy and colleagues [89]
in which participants participated in resistance training
program for eight weeks while taking identical, high
doses of either rice or whey protein, demonstrated that
rice protein stimulated similar increases in body composition
adaptations to whey protein.
Protein blends
The majority of available science has explored the efficacy
of ingesting single protein sources, but evidence
continues to mount that combining protein sources may
afford additional benefits [194]. For example, a 10-week
resistance training study by Kerksick and colleagues [22]
demonstrated that a combination of whey (40 g) and
casein (8 g) yielded the greatest increase in fat-free mass
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 15 of 25
(determined by DEXA) when compared to both a
combination of 40 g of whey, 5 g of glutamine, and 3 g
of BCAAs and a placebo consisting of 48 g of a maltodextrin
carbohydrate. Later, Kerksick et al. [95] demonstrated
various combinations of whey, casein, and
colostrum proteins with and without creatine can also
yield positive improvements in strength and body composition
over a 12-week resistance training and supplementation
regimen. Similarly, Hartman and investigators
[93] had 56 healthy young men train for 12 weeks while
either ingesting isocaloric and isonitrogenous doses of
fat-free milk (a blend of whey and casein), soy protein or
a carbohydrate placebo and concluded that fat-free milk
stimulated the greatest increases in Type I and II muscle
fiber area as well as fat-free mass; however, strength
outcomes were not affected. Moreover, Wilkinson and
colleagues [94] demonstrated that ingestion of fat-free
milk (vs. soy or carbohydrate) led to a greater area under
the curve for net balance of protein and that the fractional
synthesis rate of muscle protein was greatest after
milk ingestion. In 2013, Reidy et al. [195] indicated that
a mixture of whey and soy protein over a four-hour
measurement window similarly increased MPS rates
during the early (0–2 h) time-period versus whey
protein, but only the protein blend was able to stimulate
significantly increased MPS rates during the later (2–
4 h) measurement window. However, when the entire
four-hour measurement period was considered, no
difference in MPS rates were found. A follow-up publication
from the same clinical trial also reported that
ingestion of the protein blend resulted in a positive and
prolonged amino acid balance when compared to ingestion
of whey protein alone, while post-exercise rates of
myofibrillar protein synthesis were similar between the
two conditions [196]. Reidy et al. [197] reported that in
68 healthy young men who were participating in a
supervised resistance-training program over 12 weeks,
there were increases in whole body lean mass with either
whey protein or a whey protein and soy protein blend
compared to a maltodextrin placebo. No differences
were found between whey and the whey and soy blend.
Criteria for comparing protein sources
Some valid criteria exist to compare protein sources and
provide an objective method of how to include them in
a diet. As previously mentioned, common means of
assessing protein quality include Biological Value,
Protein Efficiency Ratio, PDCAAS and IAAO. The derivation
of each technique is different with all having
distinct advantages and disadvantages. For nearly all
populations, ideal methods should be linked to the
capacity of the protein to positively affect protein
balance in the short term, and facilitate increases and
decreases in lean and fat-mass, respectively, over the
long term. In addition, the protein’s ability to enhance
immune function and promote an anti-oxidative environment
should also be considered. To this point, dairy,
egg, meat, and plant-based proteins have been discussed.
Two critical variables exist that determine a protein’s
impact on overall protein accretion and protein turnover:
a) the protein’s leucine content and b) the rate at
which the protein is digested. In general, the proteins
with the greatest leucine content include dairy (9–11%),
egg (8.6%), and meat (8%), while sources low in leucine
include plant-based proteins. Faster digesting sources of
protein include whey and egg whites, soy, and very lean
cuts of meat (>95% lean). In contrast, casein and fatty
cuts of meat (<80% lean) act as slowly digested sources
of protein. As mentioned previously, initial research by
Boirie and Dangin has highlighted the impact of protein
digestion rate on net protein balance with the two milk
proteins: whey and casein [165–167]. Subsequent
follow-up work has used this premise as a reference
point for the digestion rates of other protein sources.
Using the criteria of leucine content, Norton and
Wilson et al. [198, 199] used animal models to compare
the potential to activate initiation factors and MPS
between four different protein sources: wheat (supplemented
with leucine), soy, egg, and whey, (containing
6.8, 8.0, 8.8, and 11% leucine, respectively) using a diet
consisting of three meals per day. Macronutrient intake
was 16/54/30% for protein, carbohydrates and fat, respectively.
Wheat and soy did not stimulate MPS above
fasted levels, whereas egg and whey proteins significantly
increased MPS rates, with MPS for whey protein being
greater than egg protein. MPS responses were closely related
to changes in plasma leucine and phosphorylation
of 4E–BP1 and S6 K protein signaling molecules. More
importantly, following 2- and 11-weeks of ingestion, it
was demonstrated that the leucine content of the meals
increased muscle mass and was inversely correlated with
body fat.
Tang et al. [86] compared high leucine/fast-digesting
(hydrolyzed whey isolate), lower leucine/intermediate
digesting (soy isolate) and high leucine/slow-digesting
(micellar casein) protein sources on MPS at rest and
following exercise. The researchers demonstrated that
MPS at rest was higher after ingestion of faster digesting
proteins compared to slower digesting proteins (whey
and soy > casein). Specifically, MPS after consumption
of whey was approximately 93% and 18% greater than
casein and soy, respectively. A similar pattern of results
was observed after resistance exercise (whey > soy > casein)
whereby protein synthesis following whey consumption
was approximately 122% and 31% greater than
casein and soy, respectively. MPS was also greater after
soy consumption at rest (64%) and following resistance
exercise (69%) compared with casein. These findings
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 16 of 25
lead us to conclude that athletes should seek protein
sources that are both fast-digesting and high in leucine
content to maximally stimulate rates of MPS at rest and
following training. Moreover, in consideration of the
various additional attributes that high-quality protein
sources deliver, it may be advantageous to consume a
combination of higher quality protein sources (dairy,
egg, and meat sources).
Key points
 Multiple protein sources are available for an athlete
to consider, and each has their own advantages and
disadvantages.
 Protein sources are commonly evaluated based upon
the content of amino acids, particularly the EAAs,
they provide. Beyond amino acid content, the fat,
calorie, and micronutrient content, and presence of
various bioactive peptides all contribute to a
protein’s quality.
 Leucine content and rate of digestion have also been
demonstrated in multiple scientific studies to play an
important role in an athlete’s ability to train,
compete, and recover.
 Blends of protein sources might afford a favorable
combination of key nutrients such as leucine, EAAs,
bioactive peptides, and antioxidants, but more
research is needed to determine their ideal
composition.
Preparation methods of various proteins
Nutrient density is defined as the amount of a particular
nutrient (carbohydrate, protein, fat, etc.) per unit of energy
in a given food. In many situations, the commercial
preparation method of foods can affect the actual nutrient
density of the resulting food. Using protein as an
example, full-fat milk is approximately 150 cal a serving,
and of this 8 g, or about 21% is from protein. Skim milk
on the other hand contains approximately 9 g of protein
in a 90-cal eight-ounce serving, making it approximately
40% protein. When producing milk protein supplements,
special preparations must be made to separate the
protein sources from the lactose and fat calories in milk.
For example, the addition of acid to milk causes the
casein to coagulate or collect at the bottom, while the
whey is left on the top [200]. These proteins are then
filtered to increase their purity. A concentrate is commonly
defined as any protein product that is 29–80%
protein by dry weight. Sport nutrition products generally
use concentrates that are 70–80% protein [200].
As extra filtering steps are added, the purity of the
final product increases and when a final protein product
yields greater than 90% protein, it is considered
an isolated protein [200].
Filtration processes
Filtration methods differ, and there are both benefits and
disadvantages to each. The two most popular methods
of filtration of a given protein are the use of ion exchange
and micro/ultrafiltration methods. Ion exchange
exposes a given protein source, such as whey, to hydrochloric
acid and sodium hydroxide, thereby producing
an electric charge on the proteins that can be used to
separate them from lactose and fat [200]. The advantage
of this method is that it is relatively cheap and produces
the highest protein concentration [200]. The disadvantage
is that ion exchange filtration typically denatures some of
the valuable immune-boosting, anti-carcinogenic peptides
found in whey [200]. Cross-flow microfiltration, and ultramicro
filtration are based on the premise that the molecular
weight of whey protein is greater than lactose, and use
1 and 0.25-μm ceramic membranes, respectively, to separate
the two. As a result, whey protein is trapped in the
membranes but the lactose and other components pass
through. The advantage is that these processes do not denature
valuable proteins and peptides found in whey, so
the protein itself is deemed to be of higher quality [200].
The main disadvantage is that this filtration process is
typically costlier than the ion exchange method.
Hydrolyzed proteins
When consumed whole, proteins are digested through a
series of steps beginning with homogenization by chewing,
followed by partial digestion by pepsin in the stomach
[201]. Following this, a combination of peptides,
proteins, and negligible amounts of single amino acids
are released into the small intestine and from there are
either partially hydrolyzed into oligopeptides, 2–8 amino
acids in length or are fully hydrolyzed into individual
amino acids [201]. Absorption of individual amino acids
and various small peptides (di, tri, and tetra) into the
blood occurs inside the small intestine through separate
transport mechanisms [201]. Oftentimes, products contain
proteins that have been pre-exposed to specific
digestive enzymes causing hydrolysis of the proteins into
di, tri, and tetrapeptides. A plethora of studies have
investigated the effects of the degree of protein fractionation
(or degree of hydrolysis) on the absorption of
amino acids and the subsequent hormonal response
[202–207]. Research indicates that amino acids are
absorbed more rapidly when they are consumed as di
and/or tri peptides compared to free form amino acids
or complete proteins [205]. Further, the rate of absorption
may lead to a more favorable anabolic hormonal environment
[202, 203, 206]. Calbet et al. [203] examined
both amino acid appearance and insulin responses
following consumption of protein solutions containing
the same amount of protein, or pure carbohydrates. The
treatments consisted of a pure glucose solution, whey
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 17 of 25
peptide hydrolysates, and cow’s milk containing milk
proteins, lactose and fat. Each of the nitrogen containing
solutions contained 15 g of glucose and 30 g of protein.
Results indicated that peptide hydrolysates produced a
faster increase in venous plasma amino acids compared
to milk proteins. Further, the peptide hydrolysates
produced peak plasma insulin levels that were two- and
four-times greater than that evoked by the milk and
glucose solutions, respectively, with a correlation of 0.8
between plasma amino acids and the insulin response in
the peptide hydrolysates. One of the inherent shortcomings
of this study is that milk proteins are 80% casein
and, therefore, are not ideal candidates to compare with
hydrolyzed whey.
In a more appropriate comparison, Morifuji et al.
[205] investigated the effects of 12.5 g of either hydrolyzed
or non-hydrolyzed soy and whey proteins on
changes in plasma levels of the EAAs, BCAAs, and insulin.
Results indicated that protein hydrolysates produced
greater responses than their non-hydrolyzed counterpart
in plasma for each of the variables (Hydrolyzed whey >
Non-hydrolyzed whey > hydrolyzed soy > Nonhydrolyzed
soy). However, Calbet et al. [202] found that
36 g of hydrolyzed or non-hydrolyzed whey and casein
led to no differences in the plasma amino acid/BCAA
responses in the whey groups. The hydrolyzed casein,
however, did result in a greater amino acid response
than the nonhydrolyzed casein. Finally, both hydrolyzed
groups resulted in greater gastric secretions, as well as
greater plasma increases, in glucose-dependent insulinotropic
polypeptides [208].
Buckley and colleagues [207] found that a ~ 30 g dose
of a hydrolyzed whey protein isolate resulted in a more
rapid recovery of muscle force-generating capacity
following eccentric exercise, compared with a flavored
water placebo or a non-hydrolyzed form of the same
whey protein isolate. Indeed, the effect of this hydrolysate
was such that complete recovery of muscle forcegenerating
capacity had been achieved by six hours post
supplementation, while the normal whey and placebo
groups’ strength remained depressed 24 h later. In agreement
with these findings, Cooke et al. [209] had 17
untrained men complete an eccentric-based resistance
training bout to invoke muscle damage and supplemented
with either carbohydrate or a hydrolyzed whey
protein isolate. Three and seven days after completing
the damaging exercise bout, maximal strength levels
were higher in the hydrolyzed whey protein group compared
to carbohydrate supplementation. Additionally,
blood concentrations of muscle damage markers tended
to be lower when four ~30-g doses of a hydrolyzed whey
protein isolate were ingested for two weeks following
the damaging bout. Beyond influencing strength recovery
after damaging exercise, other benefits of hydrolyzed
proteins have been suggested. For example, Morifuji et
al. [210] using an animal model reported that the ability
of whey hydrolysates to increase skeletal muscle glycogen
replenishment after exercise was greater when compared
to BCAA ingestion. Furthermore, Lockwood et al.
[204] investigated the effects of ingesting either 30 g of
hydrolyzed whey or two varying forms of whey protein
concentrates during a linear resistance-training protocol
over 8 weeks. Results indicated that strength and lean
body mass (LBM) increased equally in all groups. However,
fat mass decreased only in the hydrolyzed whey
protein group. While more work needs to be completed
to fully determine the potential impact of hydrolyzed
proteins on strength and body composition changes, this
initial study suggests that hydrolyzed whey may be efficacious
for decreasing body fat. Finally, Saunders et al.
[7] had thirteen trained male cyclists complete a simulated
60-km time trial where they ingested either carbohydrate
or carbohydrate and protein hydrolysate at equal
intervals throughout the race as well as at the conclusion
of the race. The authors reported that co-ingestion
of a carbohydrate and protein hydrolysate improved
time-trial performance late in the exercise protocol and
significantly reduced soreness and markers of muscle
damage. Two excellent reviews on the topic of hydrolyzed
proteins and their impact on performance and recovery
have been published by Van Loon et al. [211] and
Saunders [212].
Digestive enzymes in proteins
Digestion is the physiological process of rendering the
food we eat into smaller components that allow key
nutrients to be assimilated into our body’s tissues. The
prevalence of digestive enzymes in sports nutrition
products has increased during recent years with many
products now containing a combination of proteases
and lipases, with the addition of carbohydrates in plant
proteins. Proteases can hydrolyze proteins into various
peptide configurations and potentially single amino
acids. It appears that digestive enzyme capabilities and
production decrease with age [213], thus increasing the
difficulty with which the body can break down and
digest large meals. Digestive enzymes could potentially
work to promote optimal digestion by allowing upregulation
of various metabolic enzymes that may be
needed to allow for efficient bodily operation. Further,
digestive enzymes have been shown to minimize quality
differences between varying protein sources [214].
Individuals looking to increase plasma peak amino acid
concentrations may benefit from hydrolyzed protein
sources or protein supplemented with digestive enzymes.
However, more work is needed before definitive
conclusions can be drawn regarding the efficacy of
digestive enzymes.
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 18 of 25
Protein safety
Despite a plethora of studies demonstrating safety, much
concern still exists surrounding the clinical implications
of consuming increased amounts of protein, particularly
on renal and hepatic health. The majority of these concerns
stem from renal failure patients and educational
dogma that has not been rewritten as evidence mounts
to the contrary. Certainly, it is clear that people in renal
failure benefit from protein-restricted diets [215], but
extending this pathophysiology to otherwise healthy
exercise-trained individuals who are not clinically
compromised is inappropriate. Published reviews on this
topic consistently report that an increased intake of
protein by competitive athletes and active individuals
provides no indication of hepato-renal harm or damage
[216, 217]. This is supported by a recent commentary
[134] which referenced recent reports from the World
Health Organization [218] where they indicated a lack of
evidence linking a high protein diet to renal disease.
Likewise, the panel charged with establishing reference
nutrient values for Australia and New Zealand also
stated there was no published evidence that elevated intakes
of protein exerted any negative impact on kidney
function in athletes or in general [219].
Recently, Antonio and colleagues published a series of
original investigations that prescribed extremely high
amounts of protein (~3.4–4.4 g/kg/day) and have
consistently reported no harmful effects [220–223]. The
first study in 2014 had resistance-trained individuals
consume an extremely high protein diet (4.4 g/kg/day)
for eight weeks and reported no change in adverse outcomes
[223]. A follow-up investigation [220] required
participants to ingest up to 3.4 g/kg/day of protein for
eight weeks while following a prescribed resistance training
program and reported no changes in any of the
blood parameters commonly used to assess clinical
health (e.g., there was no effect on kidney or liver function).
Their next study employed a crossover study
design in twelve healthy resistance-trained men in which
each participant was tested before and after for body
composition as well as blood-markers of health and performance
[221]. In one eight-week block, participants
followed their normal (habitual) diet (2.6 g/kg/day) and
in the other eight-week block, participants were prescribed
to ingest greater than 3.0 g/kg/day resulting in
an average protein intake of 2.9 g/kg/day over the entire
16-week study. No changes in body composition were
reported, and importantly, no clinical side effects were
observed throughout the study. Finally, the same group
of authors published a one-year crossover study [222] in
fourteen healthy resistance-trained men. When prescribed
to a high protein diet, the participants were
instructed to ingest 3 g/kg/day and achieved an average
intake of 3.3 g/kg/day and when following their normal
diet they consumed 2.5 g/kg/day. This investigation
showed that the chronic consumption of a high protein
diet (i.e., for 1 year) had no harmful effects on kidney or
liver function. Furthermore, there were no alterations in
clinical markers of metabolism and blood lipids.
Key points
 Multiple review articles indicate that no controlled
scientific evidence exists indicating that increased
intakes of protein pose any health risks in healthy,
exercising individuals.
 Statements by large regulatory bodies have also
indicated that concerns about one’s health
secondary to ingesting high amounts of protein
are unfounded.
 A series of controlled investigations spanning up to
one year in duration utilizing protein intakes of up
to 2.5–3.3 g/kg/day in healthy resistance-trained
individuals consistently indicate that increased
intakes of protein exert no harmful effect on blood
lipids or markers of kidney and liver function.
Conclusion
In alignment with our previous position stand, it is the
position of the International Society of Sports Nutrition
that the majority of exercising individuals should
consume at minimum approximately 1.4 to 2.0 g of protein
per kg of bodyweight per day to optimize exercise
training induced adaptations. Importantly, this recommendation
also falls within the Institute of Medicine’s
Acceptable Macronutrient Distribution Range (AMDR)
of 10–35% protein [224]. The amount is dependent
upon the mode and intensity of the exercise, the quality
of the protein ingested, as well as the energy and carbohydrate
status of the individual. However, it should be
noted that there is preliminary evidence that consuming
much higher quantities of protein (> 3 g/kg/d) may confer
a benefit as it relates to body composition. Concerns
that protein intake within this range is unhealthy are unfounded
in healthy, exercising individuals. An attempt
should be made to consume whole foods that contain
high-quality (e.g., complete) sources of protein; however,
supplemental protein is a safe and convenient method of
ingesting high-quality dietary protein. The timing of
protein intake in the period encompassing the exercise
session may offer several benefits including improved recovery
and greater gains in lean body mass. However,
perhaps the most important issue regarding protein
intake during the peri-workout period is that it serves as
an opportunity to eat thus elevating one’s total daily
protein intake. In addition, consuming protein pre-sleep
has been shown to increase overnight MPS and nextmorning
metabolism acutely along with improvements
Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 19 of 25
in muscle size and strength over 12 weeks of resistance
training. Intact protein supplements, EAAs and leucine
have been shown to be beneficial for the exercising individual
by increasing the rates of MPS, decreasing muscle
protein degradation, and possibly aiding in recovery
from exercise. In summary, increasing protein intake
using whole foods as well as high-quality supplemental
protein sources can improve the adaptive response to
training.
Acknowledgements
The authors are particularly grateful for the thorough and excellent review
by Jorn Trommelen (Maastricht University, The Netherlands) and Raza Bashir
(Iovate Health Sciences International Inc., Canada). We would like to thank all
the participants and researchers who contributed to the research studies
and reviews described in this position stand.
Funding
No funding was provided.
Authors’ contributions
RJ, BIC, PJC, SDW & CMK prepared and compiled the draft for review and
editing by coauthors. All other coauthors reviewed, edited, and approved
the draft, and the final manuscript.
Competing interests
RJ has received grants to evaluate the efficacy and safety of proteins, serves
on scientific advisory boards, and has served as an expert witness, legal and
scientific consultant. CMK consults with and receives external funding from
companies who sell supplemental protein, has received remuneration from
companies for delivering scientific presentations at conferences and writes
online, print and other media on topics related to exercise, nutrition and
protein for related companies. Has served as an expert witness and provided
testimonies related to exercise, supplementation and nutrition. BIC writes
and is compensated for various media outlets on topics related to sports
nutrition and fitness; has received funding for research related to dietary
supplements; serves on an advisory board for a sports nutrition company
and is compensated in product donations, and is a consistent expert witness
and legal consultant in matters related to dietary supplement. PJC designs
and sells exercise training and nutrition certifications to professionals in the
fitness industry. SDW and TMS are employees of BioTRUST Nutrition. MP has
received grants to evaluate the efficacy of dietary supplements, serves on a
scientific advisory board for a sports nutrition company, and as a scientific
consultant. TNZ has received external funding from companies who sell
protein supplements, has received remuneration from companies for
delivering scientific presentations at conferences, and authors online articles
related to exercise, nutrition and protein. Has served as an expert witness
and provided testimonies related to exercise, supplementation and nutrition.
AAF has no conflicts to report. SMA serves on the advisory board for
Dymatize Nutrition. ASR has received grants to evaluate the efficacy of
dietary supplements and serves on the scientific advisory board for sports
nutrition companies. JRS has received grants to evaluate the efficacy of
dietary supplements and has previously served on scientific advisory boards
for a sports nutrition companies. PJA serves on the American Heart
Association Advisory Board (Capital Region); serves on the Scientific Advisory
Boards for Dymatize Nutrition and Isagenix International LLC; serves as a paid
consultant to Isagenix International LLC; Founder and CEO of PRISE LLC a
health and wellness consultant company that owns the GenioFit App. MJO
serves on the advisory board for Dymatize Nutrition and has received
external funding from companies who sell supplemental protein. LWT has
received external funding to evaluate the efficacy of dietary supplements
and has previously served in a consulting role for a sports nutrition
company. CDW has received external funding from supplement companies
to do research, served on multiple advisory boards for supplement
companies, and has served as a consultant, advisor, and spokesperson for
various nutrition companies. DSK works for a contract research organization
that does conduct clinical trials for dietary supplement and pharmaceutical
companies. RBK has received externally funded grants from industry to
conduct research on protein and protein-containing supplements, serves as
a scientific and legal consultant, and is a university approved scientific
advisor for Nutrabolt. DSW has received grants to evaluate the efficacy of
dietary supplements, serves on a scientific advisory board for a sports
nutrition company, and as scientific consultant, and has received remuneration
from a company for delivering scientific presentations at conferences.
JRH has received grants from various dietary supplement and pharmaceutical
companies to investigate the efficacy of various supplements including
protein. He has also been hired to serve as an expert witness on behalf of
supplement companies in various legal proceedings. JK is an independent
consultant for Isagenix. JA is the CEO and co-founder of the ISSN. The ISSN is
supported in part by grants from raw good suppliers and branded
companies that sell dietary protein supplements.
Ethics approval and consent to participate
This paper was reviewed by the International Society of Sports Nutrition
Research Committee and represents the official position of the Society.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
Author details
1
Increnovo LLC, Milwaukee, WI, USA. 2
Exercise and Performance Nutrition
Laboratory, School of Health Sciences, Lindenwood University, St. Charles,
MO, USA. 3
Performance & Physique Enhancement Laboratory, University of
South Florida, Tampa, FL, USA. 4
Metabolic Precision Certifications,
Queensland, Australia. 5
BioTRUST Nutrition, Irving, TX, USA. 6
The Center for
Applied Health Sciences, Stow, OH, USA. 7
Department of Geriatrics, University
of Arkansas for Medical Sciences, Little Rock, AR, USA. 8
IFNH Center for
Health & Human Performance, Department of Kinesiology & Health, Rutgers,
the State University of New Jersey, New Brunswick, New Jersey, USA.
9
Applied Physiology Laboratory, Department of Exercise and Sport Science,
University of North Carolina, Chapel Hill, NC, USA. 10
Institute of Exercise
Physiology and Wellness, University of Central Florida, Orlando, FL, USA.
11
Human Nutrition and Metabolism Laboratory, Health and Exercise Sciences
Department, Skidmore College, Saratoga Springs, NY 12866, USA.
12
Department of Nutrition, Food and Exercise Sciences, Institute of Sport
Sciences and Medicine, Florida State University, Tallahassee, USA.
13
Biokinetics, Exercise and Leisure Studies, University of KwaZulu-Natal,
Durban 4000, South Africa. 14
Human Performance Laboratory, University of
Mary Hardin-Baylor UMHB, Belton, TX 76513, USA. 15
Department of Nutrition
& Endocrinology, QPS, Miami, FL, USA. 16
Exercise & Sport Nutrition Lab,
Human Clinical Research Facility, Department of Health & Kinesiology, Texas
A&M University, College Station, TX, USA. 17
Exercise and Biochemical
Nutrition Laboratory, Department of Health, Human Performance, and
Recreation, Baylor University, Waco, TX, USA. 18
Department of Human
Movement Sciences, Carroll University, Waukesha, WI, USA. 19
Department of
Health and Human Performance, Nova Southeastern University, Davie, FL,
USA.
Received: 3 June 2017 Accepted: 5 June 2017
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Jäger et al. Journal of the International Society of Sports Nutrition (2017) 14:20 Page 25 of 25