The authors are with the Dept. of Kinesiology, McMaster University, Hamilton, ON, Canada L8S 4K1.
International Journal of Sport Nutrition and Exercise Metabolism, 2007, 17, S58-S76
© 2007 Human Kinetics, Inc.
A Critical Examination
of Dietary Protein Requirements,
Benefits, and Excesses in Athletes
Stuart M. Phillips, Daniel R. Moore, and Jason E. Tang
There is likely no other dietary component that inspires as much debate, insofar
as athletes are concerned, as protein. How much dietary protein is required,
optimal, or excessive? Dietary guidelines from a variety of sources have settled
on an adequate dietary protein intake for those over the age of 19 of ~0.8–0.9 g
protein·kg body weight–1·d–1. According to U.S. and Canadian dietary reference
intakes (33), the recommended allowance for protein of 0.8 g protein·kg–1·d–1 is
“the average daily intake level that is sufficient to meet the nutrient requirement of
nearly all [~98%] . . . healthy individuals” (p. 22). The panel also stated, “in view
of the lack of compelling evidence to the contrary, no additional dietary protein is
suggested for healthy adults undertaking resistance or endurance exercise” (33, p.
661). Currently, no group or groups of scientists involved in establishing dietary
guidelines see a need for any statement that athletes or people engaging in regular
physical activity require more protein than their sedentary counterparts. Popular
magazines, numerous Web sites, trainers, and many athletes decry protein intakes
even close to those recommended. Even joint position stands from policy-setting
groups state that “protein recommendations for endurance athletes are 1.2 to
1.4 g/kg body weight per day, whereas those for resistance and strength-trained
athletes may be as high as 1.6 to 1.7 g/kg body weight per day” (1, p. 1544). The
divide between those setting dietary protein requirements and those who might be
making practical recommendations for athletes appears substantial, but ultimately,
most athletes indicate that they consume protein at levels beyond even the highest
recommendations. Thus, one might conclude that any debate on protein “require-
ments” for athletes is inconsequential; however, a critical analysis of existing
and new data reveals novel ideas and concepts that may represent some common
ground between these apparently conflicted groups. The goal of this review was
to provide a critical and thorough analysis of current data on protein requirements
in an attempt to provide some guidance to athletes, trainers, coaches, and sport
dietitians on athletes’ protein intake. In addition, an effort was made to clearly
distinguish between “required” dietary protein, “optimal” intakes, and intakes that
are likely “excessive,” perhaps not from the standpoint of health, but certainly
from the standpoint of potentially compromised performance.
Key Words: amino acid, leucine, oxidation, carbohydrate, performance
Dietary Protein S59
Any person with even a rudimentary familiarity with the area of nutrition and
athletes knows that there is an apparent conflict between the scientific bodies that
have established the dietary guidelines for protein (33) and numerous popular and
even published positions on how much dietary protein athletes require (1). A number
of recent and very informative reviews are available in which recommendations
for dietary protein in athletes are assessed and scrutinized (62, 63, 71, 79). In light
of these recent publications it seems that the reader would be best served to refer
to those articles for all of the classic “arguments” in this area. Thus, instead of
revisiting the same data and points of contention laid out in previous reviews (62,
63, 71, 79), this review has the goal of providing some kind of reconciliatory focus
on the apparent “controversy” over whether or not athletes have elevated needs for
dietary protein, whether the same athletes could derive some benefit from additional
dietary protein over and above the recommended dietary allowances (RDA), and at
what intakes protein might become excessive and a potential risk for compromising
not necessarily health but performance.
Revision of the Canadian recommended nutrient intakes and U.S. RDAs to a
model of nutrient adequacy and an upper limit was the basis for the development of
the dietary reference intakes. The dietary-reference-intake estimations now include
an estimated average (population) requirement, an RDA, and a tolerable upper
limit, which is a threshold above which adverse effects of higher nutrient intakes
appear to increase. Figure 1(A) on the next page shows this model in schematic
form. In addition to the dietary-reference-intake recommendations for nutrient
intake is a series of acceptable macronutrient distribution ranges (AMDRs). The
AMDRs establish a large degree of latitude in what is an acceptable partitioning of
macronutrients that would, with good likelihood, meet the nutritional needs of most
people. These AMDRs are summarized in Table 1 below, which also contains our
attempt to define an athlete-based AMDR derived from knowledge of carbohydrate
“needs” in the case of competitive endurance athletes and retrospective estimates
of protein “needs” for strength- and power-training athletes.
Table 1 Acceptable Macronutrient Distribution Ranges (AMDR)
as Defined by the Institute of Medicine (IOM) (33) and as Viewed by
Endurance and Strength Athletes as Sufficient
aThe AMDR as defined by the IOM is “a range of intakes for a particular energy source that is associ-
ated with reduced risk of chronic diseases while providing adequate intakes of essential nutrients”
(33, p. 14). bDerived based on recommendations for carbohydrate intake for optimizing performance
(12, 13) and working upward from those estimates, including a required amount of protein based on
retrospective nitrogen-balance estimates (79), as well as allowances for the increased energy needs
of these athletes. Fat percentages are derived by difference. cDerived based on recommendations of
protein “requirements” from retrospective nitrogen-balance analysis (63) and working upward from
those estimates to include sufficient nutrients for health, as well as the elevated energy requirements
for these athletes to maintain and increase skeletal-muscle mass.
Figure 1 — Schematic representation of how increasing dietary intake protein requirements are met
for (A) a sedentary population and (B) a group of athletes, with the “rationale” for why an athlete
may have a higher than normal protein requirement. EAR indicates estimated average (population)
requirement; RDA, recommended dietary allowance; and UL, upper limit.
Dietary Protein S61
Inherent Differences of Opinion
There are a number of interesting and important points about protein to address with
respect to the recommendations made by the Institute of Medicine (IOM) (33):
• The RDA for protein for men and women age 19 years or older is 0.8 g of
good-quality protein·kg–1·d–1. Using “athletic” reference body weights of 80
kg for men and 65 kg for women, the RDA equates to 64 g/d for men and 52
g/d for women.
• There is no tolerable upper limit for dietary protein or for any individual
amino acid, although caution was advised if the intake of specific individual
amino acids exceeded that normally present in the diet from foods (see 
• The AMDR for total protein could not truly be established. Thus, the range of
protein intake recommended in the diet was determined as the amount remain-
ing after fat and carbohydrate needs are met. The IOM report states that “to
complement the AMDRs for fat . . . and carbohydrate . . . for adults, protein
intakes may range from 10 to 35% of energy intake to ensure a nutritionally
adequate diet” (33, p. 844).
• What also needs to be highlighted is the fact that the protein RDA is not estab-
lished as a guideline for how much protein people should be consuming but
instead is a minimal estimate and one that is, even by the admission of those
setting the protein RDA, based on a faulty method.
Part of the apparent disagreement between those deriving dietary guidelines
and athletes and sports practitioners may well be the focus on the RDA, which
establishes a level of protein that will replace losses and thus prevent deficiency.
The methodology used by the IOM in establishing the protein RDA was nitrogen
balance (33, 68). Use of nitrogen balance is likely a completely adequate and
acceptable method for establishing nitrogen or amino acid requirements necessary
to prevent deficiency. It is, however, possible or even likely that the same method is
inadequate to establish intakes optimal for maximizing resistance-training-induced
gains in muscle mass and strength and resistance- or endurance-training-induced
adaptations in metabolic function. In fact, Millward has raised the possibility of such
a concept, calling it the anabolic drive (56, 57). Defining requirements in terms of
preventing deficiency would, from an athlete’s perspective, hardly be considered a
position from which to frame his or her “requirement” for dietary protein; hence,
an inherent tension already exists between the 2 parties.
Nitrogen balance has long been recognized as an inherently flawed method for
determining protein needs because of a number of methodological limitations such
as implausibly high nitrogen balances typically observed with high protein intakes,
increased economy of nitrogen use with low protein intakes, and often estimated
rather than measured dermal and miscellaneous obligatory losses of nitrogen
(33). From an athlete’s perspective it is also important to realize that regardless of
whether nitrogen balance is achieved at a particular protein intake, it is possible
that the level of protein consumed is less than required to optimize all aspects of
muscle mass, muscle function, and muscle metabolic processes. In addition, even
at marginal intakes nitrogen equilibrium can be attained by adaptive and potentially
S62 Phillips, Moore, and Tang
accommodative down-regulation of protein-requiring processes (92). In addition,
one needs to appreciate that as individuals adapt to less than adequate protein
intakes they do so by lowering nitrogen excretion (68, 92, 93) such that there is no
apparent relationship between nitrogen balance and muscle mass, let alone muscle
function, which is a critical measure for athletes but one that has never been mea-
sured in the context of studies of protein adequacy. The last 2 points are difficult
questions to assess, however, and would require long-term studies employing very
intricate and revealing measures. More important from an athlete’s perspective is
the idea of whether protein intakes higher than the RDA translate into improved
competition performance. This is an important consideration if we are to make
arguments directed at optimizing physiological function based on protein intakes
that would likely exceed the RDA; namely, is there benefit to consuming protein
at levels higher than the RDA, and, if so, how much higher?
The choice of endpoints in studies of protein requirements also needs to be
evaluated. Although attaining nitrogen balance per se is likely a healthful and
adequate endpoint for sedentary individuals, it is questionable whether the same
can be said for athletes. For those wishing to gain lean mass, for example, positive
nitrogen balance is the desired goal. This is presumably because of the periodic
stimulation of muscle protein synthesis, which, if it is to support the net gain of
new proteins, would require net extra amino acids—for review see (62, 63, 71,
72). For an endurance athlete the goal would likely relate to balancing the loss of
leucine, an amino acid that has been shown to be oxidized to an appreciable extent
during endurance exercise (42–44, 50, 64), and also to support the increased protein
synthesis that occurs after this form of exercise (15, 51, 76). Thus, whatever the end
outcome of any study of dietary protein needs or optimal requirements for athletes,
the model may be quite different than that used by the IOM to define the protein
RDA. A scheme for understanding how an athletes, or their trainers and coaches,
might view their need for dietary protein and what “athlete-specific” outcomes
might be considered is presented in Figure 1(B). Regrettably, at this time it is not
possible to ascertain what levels of protein would promote the necessary adaptations
to support optimal function of all protein-requiring processes or optimal capacity
for athletic performance.
Arguments for Reductions in Protein “Need”
Incongruent to the general belief of many athletes and their coaches, published posi-
tion stands (1), and a number of viewpoints (62, 63, 79), there is another opinion
that exercise per se actually reduces the overall requirement for dietary protein (14,
30, 58, 87). The elegantly controlled studies conducted by Butterfield et al. (14,
87) are often cited in support of this argument but are strongly criticized because
the exercise intensities used in them do not begin to approach those in which most
endurance athletes regularly engage. The implications of such criticisms are of
course that more intense exercise will increase amino acid catabolism or reduce
protein synthesis (i.e., the ability to retain amino acids); however, neither of these
suppositions has ever been investigated.
Two recent longitudinal studies, in which an accrual of lean mass was observed
with resistance training, showed greater economy of nitrogen retention when the
subjects consumed what was determined, through nitrogen balance, to be sufficient
Dietary Protein S63
protein (1.2–1.4 g protein·kg–1·d–1) and energy to cover needs after a strenuous
resistance-training program lasting 12 wk (30, 58). It may be that the anabolic
stimulus of weightlifting is enough to stimulate muscle protein synthesis such
that this tissue becomes a greater site of disposal of amino acids in both fed and
fasted states, possibly at the “expense” of other amino acid–requiring processes. As
such, these data (30, 58) may not necessarily be indicative that resistance training
reduces protein requirements per se, but instead may be evidence of a shift in the
hierarchy of amino acid–requiring processes toward muscle protein synthesis get-
ting a “greater share” of circulating amino acids in both fasted and fed states. The
results obtained with resistance exercise (30, 58) may be markedly different from
those seen with endurance exercise because resistance exercise is fundamentally
anabolic and stimulates protein synthesis, such that loss of amino acids in the fasted
state is reduced for up to 48 h (65). In contrast, the anabolic nature of endurance
exercise is far weaker than that of resistance exercise, and the improved net reten-
tion of amino acids in muscle appears to be much more transient (76).
There is a large body of evidence showing that provision of protein/amino acids
supports increased rates of protein synthesis and positive protein balance after both
endurance exercise (37, 48) and resistance exercise (52, 69, 82, 83, 85). These data,
in and of themselves, provide some credence to an argument for increased protein
for athletes above a requirement level. What is not clear, however, in any of these
studies is exactly how much of the supplemental protein is directed toward muscle
protein synthesis, which goes directly to the question of how much extra protein is
needed to support gains in muscle protein mass. Using urea tracers, a number of
investigations on postexercise amino acid provision have shown no increase in urea
production (52, 69, 82, 83, 85), arguing that the ingested supplement is effectively
and efficiently used by muscle protein synthesis and other amino acid–requiring
processes. On the other hand, the situation of endurance exercise is difficult to
assess because in this case the stimulus is not anabolic and does not ultimately
result in a net accumulation of muscle contractile protein mass (as is the case with
resistance exercise). The argument often given is that extra protein for endurance
athletes is required because endurance exercise increases amino acid oxidation (25,
42–44, 50, 79, 86, 90); however, it has never been shown, at least to our knowledge,
that any amino acid other than leucine is oxidized to a substantial degree during
exercise. Based on an average human body-tissue leucine content of 590 µmol/g
protein (70), if x amount of leucine is oxidized during an exercise bout then x/590
is equivalent to the number of grams of tissue protein broken down. Such a calcula-
tion relies, however, on a number of very tenuous assumptions that are not tested
in most experimental paradigms, so increased leucine oxidation during endurance
exercise may mean an increased need for dietary leucine and not necessarily an
increased need for dietary protein. In a practical sense, however, unless leucine
supplements are ingested, an increased dietary leucine requirement would represent
an increased need to ingest proteins (especially those containing leucine, such as
lean meats and the dairy proteins whey and casein).
Beyond “Requirements” to “Optimal” Intakes
In the most recent reviews of protein “requirements” for strength-training athletes
it was estimated, based on a meta-analytic regression, that a daily intake of ~1.33
S64 Phillips, Moore, and Tang
g protein·kg–1·d–1 is required to remain in nitrogen balance (66% greater than the
Canadian/U.S. RDA) (63). Protein requirements for endurance athletes were esti-
mated to be ~1.11 g protein·kg–1·d–1 but could be as high as 1.6 g protein·kg–1·d–1
in individuals exercising very intensely (79). Accepting all of the shortcomings of
nitrogen balance, the method used to derive the previous estimates (63, 79) is identi-
cal to the approach that was used to derive the current protein RDA (i.e., an analysis
of pooled nitrogen-balance data from human studies) (33, 68). If these estimates
are reasonable, do these protein intakes represent an optimal level? If we define an
optimal level as being a protein intake that would 1.) support an athlete’s ability
to repair and replace any damaged proteins (resulting potentially from oxidative
stress or mechanical disruption); 2.) adaptively “remodel” proteins in structures
such as muscle, bone, tendon, and ligaments to better withstand the stress and
strain imposed by training and competition; 3.) maintain optimal function of all
metabolic pathways in which amino acids are participatory intermediates (which
includes being oxidative fuels); 4.) support increments in lean mass, if desired; 5.)
support an optimally functioning immune system; and 6.) support the optimal rate
of production of all plasma proteins required for optimally physiological function,
would the previous estimates of protein intake represent an optimal level?
If athletes’ protein requirements were sufficient to support all of the aforemen-
tioned processes, the intake would not be a requirement to prevent deficiency but
rather an intake that is optimal for the athletes’ overall metabolism. In light of this,
such an intake would obviously be greater than that of a sedentary individual because
the nature of exercise is such that there is an up-regulation of protein-utilizing
processes. At the same time, one could argue that optimal levels of dietary protein
should not reach levels that promote excessive production of urea and higher than
necessary oxidative losses of amino acids than those needed for optimal functioning,
as just defined. Why is this? Why not simply consume lots of protein “just to make
sure you’re getting enough”? The simple argument is that ultimately nitrogen is still
toxic to mammalian metabolic systems and cannot be stored or amino acid pool
sizes expanded ad infinitum to accommodate “extra” amino acids. Consequently,
nitrogen consumed in excess of what is immediately required to support the optimal
rates of amino acid–utilizing functions outlined here will ultimately result in urea
production. It is important to recognize that protein ingestion when considered in
this context needs to be evaluated from meal to meal because it is the immediate
handling of ingested nitrogen that will influence the rate of urea production and
amino acid oxidation. It is worthwhile noting that Cuthbertson et al. (18) showed
that an oral dose of 10 g of essential amino acids maximally stimulates muscle
protein synthesis in both the young and the elderly. Because it appears that only
essential amino acids are required to maximally stimulate muscle protein synthesis
(84, 88), these data (18) warrant serious consideration.
If we examine the essential amino acid composition of milk proteins, meat,
and eggs, 10 g of essential amino acids translates to ~25 g of each of these protein
sources (most high-quality proteins are 40% essential amino acids by content),
which represents ~750 mL of skim (nonfat) milk, 4 or 5 eggs, or ~100 g of cooked
lean beef. If we were to use these data and assume that a similar anabolic response
occurs after each meal when consumed, say, 4 times per day, then a daily protein
intake would be, at a minimum, 100 g to achieve the “maximal” anabolic response
in a nonexercising individual. Furthermore, we have data that suggest that the dose
Dietary Protein S65
of protein required to maximally stimulate muscle protein synthesis after an isolated
bout of resistance exercise is similar (or possibly lower at ~8.5 g essential amino
acids or ~20 g protein) to that seen at rest (D.R. Moore and S.M. Phillips, unpub-
lished observations). Thus, from the standpoint of maximally stimulating muscle
protein synthesis, a dose of ~20–25 g of high-quality intact protein (such as dairy,
eggs, or lean meat) appears sufficient. What is missing from these data, however, is
knowledge of how the other amino acid–requiring processes highlighted as being
part of optimal protein intake are stimulated by this dose of protein.
Ultimately, the answer to the question of how much protein is required to, for
example, support optimal immune-system function or to allow optimal flux through
intermediary amino acid–requiring metabolic pathways is relatively difficult to
answer directly. Thus, a default position of many athletes is to consume very large
amounts of protein in the hope that this will be more than enough to satisfy the
myriad of physiological processes that require dietary protein but in effect will do
them little harm from an overall health perspective. However, the potential for a
chronically high-protein diet to influence the metabolic fate of dietary amino acids
requires consideration. For example, habitual consumption of a high-protein (1.8
g·kg–1·d–1) diet increases leucine oxidation at rest and during moderate exercise (10),
demonstrating that the body adapts to relatively high protein loads by increasing
the capacity for amino acid catabolism. Because the pathways for oxidative amino
acid catabolism adapt to the diet and may act as the main regulator of protein stores
(53, 54, 67), it is likely that habitual consumption of a high-protein diet begets the
requirement for greater protein intakes. Therefore, from the standpoint of dietary
sources of protein, consuming large amounts is likely to have little impact on an
athlete’s long-term health (see below); whether it affects performance, however,
is debatable. In the absence of an upper limit for protein (33), should athletes,
dietitians, coaches, or health care providers be concerned about protein intakes
in excess of 2–4 times the RDA? The operative question is really, when do high
protein intakes become “excessive”? What defines excess and what impact could
this have on athletes’ ability to perform or on their long-term health?
From “Optimal” to “Excessive”
Dietary surveys of athletes, particularly strength- and power-training athletes
and bodybuilders, indicate that dietary protein intakes in the range of 2–2.5 g
protein·kg–1·d–1 and up to as high as 3 g protein·kg–1·d–1 (22, 23, 26, 34–36, 80)
are not unusual. Protein intakes are not normally as high in endurance-trained
athletes, usually falling in the range of 1.2–1.6 g protein·kg–1·d–1 (reviewed in )
and tending to be lower in endurance-trained women (4, 19, 77, 78). Hence, as a
general rule it appears that the strength or power athlete and bodybuilders would
be more at risk for excessive protein intakes. A pragmatic question is what are the
true downsides of such high protein intake?
From a health standpoint the response often given is the potential for high
protein intakes to result in reduced peak bone mass and impaired renal function.
Contradicting those arguments is the knowledge that certain populations consume
more protein than the RDA, up to 3.0 g protein·kg–1·d–1, without apparent negative
health effects, at least not those related to dietary protein. For example, the Northern
Canadian and Alaskan Inuit have extraordinarily high protein intakes throughout
S66 Phillips, Moore, and Tang
their lives (39, 40, 61, 74). Based on estimated energy intakes that match an expen-
diture of twice the basal metabolic rate, an intake of 3.0 g protein/kg translates into
an overall protein:energy ratio in the diet of 34%, or very close to the highest end
of the AMDR in terms of protein (10–35%) (55).
Insofar as protein intake and bone are concerned, there are some studies
that have shown increased calciuria with higher protein intakes and a subsequent
increased risk for bone fracture or osteoporosis (24); however, several studies have
supported a contrary position (59, 89). In fact, the relationship between protein and
bone health has recently been highlighted to be a positive one; that is, the more
dietary protein consumed the greater the peak bone mass achieved (reviewed in ).
The mechanism underpinning the greater bone mass with higher intakes of dietary
protein appears to be mediated through levels of IGF-1 (7). Increased protein intake
may also interact with the high forces generated during resistive-type activities,
which are potent stimuli for increasing IGF-I (both systemically and locally) (3, 29,
60), to further increase peak bone mass. Thus, as a health-related reason for why
high dietary protein levels might be deleterious for athletes or for the population in
general, reduced peak bone mass appears to be a dubious argument at best.
Increased risk of developing renal disease is also an often-stated consequence
of persistently high dietary protein intakes. Protein can form up to 35% of dietary
energy (as reflected in the AMDR), which would almost certainly provide the RDA
and likely much more, unless very low energy was being consumed. In establish-
ing the RDA, the IOM report reviewed the impact of high protein intake on renal
disease and concluded that levels of dietary protein are not related to progressive
decline in kidney function with age (33). Other studies examining protein intake
and renal function support this conclusion (49–51). Martin et al. (49) showed that
protein restriction may be appropriate for the treatment of existing kidney disease
but that evidence for a detrimental effect of high protein intakes on kidney function
was marginal in healthy individuals consuming a high-protein Western diet. The
notion that protein-restricted diets decrease the risk of developing kidney disease
in the general population is not supported by the scientific literature—in fact,
preliminary studies show a positive effect of higher protein diets on risk factors
for kidney disease, including obesity, hypertension, and diabetes (45–47, 66, 94,
95). A review by Bernstein et al. (5) compared the effects of animal and vegetable
protein on kidney function. In short-term clinical trials, egg white, dairy, and soy
consumption did not affect renal function, whereas other animal-protein intake
elicited some response. The researchers noted that “from these studies, it is dif-
ficult to conclude whether or not there is a long-term association between amount
of animal or vegetable protein intake and change in normal renal function” (5, p.
647). Hence, it is difficult to make a convincing argument against a higher than
normal protein intake for those with normal renal function, at least in terms of
adverse health consequences.
The Impact of Energy Intake
A discussion of protein “requirements” and “optimal” protein intakes for athletes
would be incomplete without a discussion of the impact of dietary energy intake.
Assuming that energy balance is a desired goal, increased energy intake is needed
to balance exercising energy expenditure; nevertheless, additional protein intake
Dietary Protein S67
need not be overly high to achieve nitrogen balance. This is particularly true if
the increased energy comes from carbohydrate (73), which, owing to its ability to
stimulate insulin release, can markedly suppress proteolysis, consequently improv-
ing nitrogen balance (9, 16). However, as previously stated, most athletes are not
seeking nitrogen balance (i.e., simply getting enough protein to offset nitrogen
loss) but instead are looking for an optimal protein intake. It is worth noting that,
even in the complete absence of protein intake, after exercise leg-muscle protein
balance can be brought to levels not different from zero (i.e., no net loss or gain of
proteins) simply by ingesting carbohydrates alone (9, 16).
In a previous review (62), we examined studies that had shown a marked fat
loss and a simultaneous “sparing” of muscle mass by inducing an energy deficit with
varying macronutrient ratios. Without going into the same degree of detailed review
here we direct the reader to a recent meta-analysis showing that during hypoen-
ergetic periods it appears that lower carbohydrate (less than 40% of total energy)
and higher protein (> 1.05 g·kg–1·d–1) intake result in increased fat-mass loss and
lean-mass preservation than diets higher in carbohydrate and lower in protein (38).
In addition, Layman et al. (46) showed that a hypoenergetic diet containing lower
carbohydrate and higher protein (carbohydrate-to-protein ratio of 1.6) combined
with the addition of primarily endurance but also some resistive exercise appeared
to be the most effective strategy for promoting fat loss and preserving lean mass.
This finding may not be surprising when one considers that endurance exercise (to
a small degree) (51, 76) and resistance exercise (to a very large degree) (6, 65) are
anabolic in that they stimulate muscle protein synthesis even in the fasted state,
forcing an increased net “conservation” of amino acids arising from proteolysis.
From an athlete’s perspective, however, the important point here is that for most
sports it is recognized that a higher lean-to-fat-mass body-composition ratio typi-
cally translates into a competitive advantage. Thus, we concluded previously (62)
that a lower carbohydrate, higher protein hypoenergetic diet, particularly when
combined with exercise, is likely of substantial benefit for athletes if they wish to
attain the associated performance advantage of modifying their body composition
by losing stored body fat. Of course, such a strategy is not without the obvious
limitation that lower carbohydrate intake for athletes will likely lead to lower muscle
glycogen stores (11–13). In this sense, athletes who adhere to this dietary practice
are depriving themselves of the very fuel that is by far the preferred substrate to
power muscular contraction—reviewed in (11–13).
Timing Is Important
When it comes to stimulating new muscle protein accretion via resistance exercise
it appears that immediate postexercise protein supplementation is beneficial. Results
of a number of studies in which protein was given to subjects postexercise, as a
supplement, appear to agree with a general statement that the timing of protein
consumption postexercise may be a determinant of muscle mass and strength
gains. Although acute studies suggest that muscle is sensitive to the provision of
nutrients (especially amino acids) for up to 3 h after resistance exercise (69), lon-
gitudinal training studies suggest that increases in strength and muscle mass are
greatest when protein is consumed immediately after exercise (2, 20, 31, 32). For
example, Esmarck et al. (20) reported that delaying the postexercise delivery of a
S68 Phillips, Moore, and Tang
protein-containing supplement to elderly men by a mere 2 h completely prevented
exercise-induced hypertrophy and slowed strength gains. In addition, strength and
muscle-mass gains in patients who had just undergone knee surgery were promoted
to a greater degree by protein and carbohydrate consumption than carbohydrate
alone or a placebo (32). Gains in muscle-fiber size were seen with young men
training for 14 wk only if they consumed protein versus isoenergetic carbohydrate
postexercise (2). Cribb and Hayes (17) recently reported that a creatine- and protein-
containing supplement consumed immediately before and after exercise resulted
in more gains in lean mass, strength, and Type II muscle-fiber area than seen in
a group who got the same supplement but at different times of day. We recently
reported that in groups of young men immediately consuming either skim milk, the
equivalent amount of protein as soy, or isoenergetic carbohydrate after resistance
exercise, the greatest lean-mass gains were seen in the milk-supplemented group
(31). Hence, we would propose that our data (31), taken together with previous
data from chronic studies manipulating postexercise protein consumption (2, 17,
20, 32), support the general thesis that immediate consumption of protein, particu-
larly high-quality milk protein (31), after resistance exercise serves to maximize
exercise-induced increases in muscle mass. Furthermore, consumption of energy in
the form of carbohydrate after a resistance-exercise workout, when ingested alone
(i.e., without protein), limits resistance-exercise-induced gains in muscle mass.
Higher Protein and Performance:
A Potential Adverse Effect
Is it possible that there actually is an upper limit for protein intake? The short
answer is yes, and it is 35% of a one’s energy intake, particularly if one is in energy
balance. If taken to extremes higher dietary protein intakes would, unless weight
gain were a desired goal, have to displace another dietary macronutrient. If it is
dietary lipid that is displaced, the outcomes are not likely to be of great concern.
If, however, the increased consumption of dietary protein results in a lower dietary
carbohydrate intake, performance could be compromised. This may be a situation
of greater concern if the athlete has voluntarily assumed an energy deficit to change
his or her body weight and composition. Figure 2 shows how increasingly higher
protein intake, assuming that fat is held constant, will come at the expense of dietary
carbohydrate to the point that carbohydrate could become limiting, according to the
necessary dietary intake levels required to maintain adequate muscle glycogen, for
training and performance (12, 13). This situation would, of course, be exacerbated
by dietary energy restriction.
To restore glycogen during high-intensity and -volume training (i.e., 2 or 3 train-
ing sessions per day), estimated carbohydrate requirements for athletes have ranged
from as little as 5 g up to as high as 8–10 g carbohydrate·kg–1·d–1. It is unlikely, at
least at the very high end of the suggested carbohydrate intakes, that any athletes
other than highly competitive triathletes, runners, or cyclists would require such
intakes to sufficiently maintain the ability to train and perform. Thus, when would
lower carbohydrate intakes begin to compromise performance, and at what specific
level? The answer is likely to be sport and training specific; however, we must stress
that even high-intensity, short-duration muscular efforts (i.e., sprinting and lifting)
Figure 2 — Relationship between dietary energy as a percentage of carbohydrate (energy
in [Ein] from CHO) and protein intake (Ein from protein) plotted against both protein and
carbohydrate intake in g/kg body weight. (A) A 70-kg male athlete consuming 3300 kcal
(13.8 MJ)/d. The dashed lines indicate that consumption at the recommended daily allow-
ance for protein (0.8 g protein·kg–1·d–1—8% of dietary energy as protein) would result in
a carbohydrate intake of 6.3 g·kg–1·d–1. This intake is considered high enough to support
carbohydrate needs for an athlete training at moderate duration and low intensity (13). When
the same athlete holds fat intake constant at ~20% of energy intake but consumes protein
at ~23% of total energy (2.5 g protein·kg–1·d–1), carbohydrate intake at the same energy
intake is 5 g·kg–1·d–1. This intake is considered marginal in terms of what is required for
replenishment of glycogen and performance (13). (B) The same relationships for a 56-kg
female athlete consuming 2500 kcal (10.4 MJ)/d.
S70 Phillips, Moore, and Tang
rely heavily on carbohydrate (21, 41, 49, 75, 81). Given that it is resistance-train-
ing and power-lifting athletes who tend to consume more protein, such individuals
may be at greater risk for lower than optimal carbohydrate intakes to support the
most intense training effort possible. Data from Martin et al. (49) showed that with
3 sets of biceps curls (8–10 reps/set) performed at a weight providing 80% of the
subjects’ single-repetition maximum load (1-RM), muscle glycogen concentration
was reduced by almost 35% from starting levels. Similar results have been obtained
by others (21, 75, 81). In addition, carbohydrate provision improves weightlifting
performance during a bout of resistance exercise lasting 60 min or more, which is
a duration that is similar to traditional training bouts performed by many athletes
(27, 28). These results provide strong support for the idea that carbohydrate is an
important and potentially limiting substrate even during resistance-exercise workouts
(27, 41, 81). Another line of evidence to support the concept that dietary carbohydrate
would be important for strength-training athletes is that postexercise ingestion of
carbohydrate appears to augment the rise in muscle protein synthesis brought about
by protein/amino acids (8, 52, 69, 83, 84, 91). The quantity of carbohydrate neces-
sary to achieve this effect (35 g) is minimal in comparison with the carbohydrate
intakes suggested to completely replenish muscle glycogen, which are in the range
of 1–1.2 g carbohydrate·kg–1·h–1 (11–13). Nevertheless, from a practical standpoint
athletes need to consider their postexercise carbohydrate intake, in addition to their
protein intake, in order to optimize performance.
Conclusions and Practical Recommendations
To attain peak levels of performance athletes clearly need to be aware of their dietary
intake of protein, as well as carbohydrate and a number of other micronutrients and
minerals. Highly detailed and refined guidelines for intakes, however, are likely to
be confusing for most athletes. Notwithstanding, it appears that emerging dietary
guidelines for protein are in the range of 1.2–1.6 g protein·kg–1·d–1. This level is
greater than the RDA, with the general recommendation that the RDA is a protein
intake designed simply to alleviate deficiency. More important, it is an intake that
appears, based on experimental evidence (mostly nitrogen balance), to be adequate
and more than sufficient. Should athletes aim to meet or exceed this intake? Quite
simply, in the absence of evidence suggesting that higher intakes are beneficial, it
is not yet possible to say that they will be beneficial. What appears to be critical,
as with the recommendations for carbohydrate, is that timing of ingestion is very
important. Put simply, protein should be consumed early during the postexercise
recovery phase (i.e., immediately to 1 h after exercise). Protein quality also appears
to be important in maximizing the accretion of muscle proteins, so athletes would
do well to focus on high-quality protein sources such as dairy protein, eggs, and
lean meat. When athletes find it inconvenient to consume such protein sources,
more portable protein sources, particularly protein supplements, offer a practical
alternative. The content of these protein supplements should be closely scrutinized
by athletes for quality, however, because protein bars and drinks are highly hetero-
geneous in terms of their composition. The high-quality protein dose that appears
to maximally stimulate muscle protein synthesis is close to 20–25 g; above this
point protein synthesis is not additionally stimulated, but increases in amino acid
oxidation and urea synthesis may result.
Dietary Protein S71
As a closing remark, it is tempting to dismiss the notion of protein intake
for athletes as relatively unimportant in the grand scheme because all athletes
appear to consume enough. Adequate protein consumption is not always the
case, however, particularly when female athletes are concerned. More important,
athletes, dieticians, and coaches alike would be remiss in their attention to details
and advice to simply assume that the athletes get enough protein and that there
is nothing more that they have to be concerned about. As noted by Burke et al.
(12), dietary guidelines for athletes are almost unanimous in their recommenda-
tion of high carbohydrate intakes for enhancing performance, and yet many top
athletes do not appear to achieve the levels recommended. Quoting Burke et
al., “The real or apparent failure of these athletes to achieve [exceed] the daily
CHO [protein] intakes recommended by sports nutritionists does not necessar-
ily invalidate the benefits of meeting [following] such guidelines” (12, p. 267).
Thus, hidden in the details of the recommended guidelines for protein intake for
athletes are a number of points regarding timing and composition (quality), as
well as consumption in combination with macronutrients such as carbohydrate;
attention to these details, we contend, will enable athletes to perform to the best
of their potential.
D.R.M. and J.E.T. were both supported by a Canadian Institutes of Health Research (CIHR)
doctoral scholarship award. S.M.P. is supported by research funding from CIHR, the
National Science and Engineering Research Council of Canada, U.S. National Dairy
Council, and Canadian Foundation for Innovation. S.M.P. received reimbursement for
travel from DSM Food Specialties to present this paper.
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