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Dietary protein for athletes: From requirements to metabolic advantage

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The Dietary Reference Intakes (DRI) specify that the requirement for dietary protein for all individuals aged 19 y and older is 0.8 g protein.kg-1.d-1. This Recommended Dietary Allowance (RDA) is cited as adequate for all persons. This amount of protein would be considered by many athletes as the amount to be consumed in a single meal, particularly for strength-training athletes. There does exist, however, published data to suggest that individuals habitually performing resistance and (or) endurance exercise require more protein than their sedentary counterparts. The RDA values for protein are clearly set at "...the level of protein judged to be adequate... to meet the known nutrient needs for practically all healthy people...". The RDA covers protein losses with margins for inter-individual variability and protein quality; the notion of consumption of excess protein above these levels to cover increased needs owing to physical activity is not, however, given any credence. Notwithstanding, diet programs (i.e., energy restriction) espousing the virtue of high protein enjoy continued popularity. A number of well-controlled studies are now published in which "higher" protein diets have been shown to be effective in promoting weight reduction, particularly fat loss. The term "higher" refers to a diet that has people consuming more than the general populations' average intake of approximately 15% of energy from protein, e.g., as much as 30%-35%, which is within an Acceptable Macronutrient Distribution Range (AMDR) as laid out in the DRIs. Of relevance to athletes and those in clinical practice is the fact that higher protein diets have quite consistently been shown to result in greater weight loss, greater fat loss, and preservation of lean mass as compared with "lower" protein diets. A framework for understanding dietary protein intake within the context of weight loss and athletic performance is laid out.
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CURRENT OPINION / OPINION COURANTE
Dietary protein for athletes: from requirements to
metabolic advantage
Stuart M. Phillips
Abstract: The Dietary Reference Intakes (DRI) specify that the requirement for dietary protein for all individuals aged
19 y and older is 0.8 g proteinkg
–1
d
–1
. This Recommended Dietary Allowance (RDA) is cited as adequate for all persons.
This amount of protein would be considered by many athletes as the amount to be consumed in a single meal, particularly
for strength-training athletes. There does exist, however, published data to suggest that individuals habitually performing
resistance and (or) endurance exercise require more protein than their sedentary counterparts. The RDA values for protein
are clearly set at ...the level of protein judged to be adequate... to meet the known nutrient needs for practically all
healthy people...’. The RDA covers protein losses with margins for inter-individual variability and protein quality; the no-
tion of consumption of excess protein above these levels to cover increased needs owing to physical activity is not, how-
ever, given any credence. Notwithstanding, diet programs (i.e., energy restriction) espousing the virtue of high protein
enjoy continued popularity. A number of well-controlled studies are now published in which higher’ protein diets have
been shown to be effective in promoting weight reduction, particularly fat loss. The term higher’ refers to a diet that has
people consuming more than the general populations’ average intake of ~15% of energy from protein, e.g., as much as
30%–35%, which is within an Acceptable Macronutrient Distribution Range (AMDR) as laid out in the DRIs. Of relevance
to athletes and those in clinical practice is the fact that higher protein diets have quite consistently been shown to result in
greater weight loss, greater fat loss, and preservation of lean mass as compared with lower’ protein diets. A framework
for understanding dietary protein intake within the context of weight loss and athletic performance is laid out.
Key words: lean mass, protein turnover, leucine.
Re
´
sume
´
: D’apre
`
s les apports nutritionnels de re
´
fe
´
rence (DRI), les besoins quotidiens de prote
´
ines chez les individus de
19 ans et plus sont de 0,8 g prote
´
inekg
–1
d
–1
. Cette ration alimentaire recommande
´
e (RDA) est dite convenable pour tous.
Plusieurs athle
`
tes, notamment ceux s’entraı
ˆ
nant a
`
la force, trouveraient cette quantite
´
suffisante pour un seul repas. Selon
quelques e
´
tudes, les individus s’adonnant re
´
gulie
`
rement a
`
des exercices de force et d’endurance auraient besoin de plus de
prote
´
ines que leurs conge
´
ne
`
res inactifs. La ration prote
´
ique recommande
´
e est bien e
´
tablie comme e
´
tant « la quantite
´
de
prote
´
ines suffisante pour combler les besoins de nutriments d’a
`
peu pre
`
s toutes les personnes en bonne sante
´
». Cette ra-
tion tient compte de pertes de prote
´
ines, de la variation interindividuelle et de la nature des prote
´
ines; consommer un sur-
plus de prote
´
ines pour combler les besoins accrus par l’activite
´
physique ne semble pas cre
´
dible. Malgre
´
cela, les re
´
gimes
amaigrissants a
`
forte teneur en prote
´
ines ont encore la cote. De nombreuses e
´
tudes bien structure
´
es indiquent l’efficacite
´
des re
´
gimes hyperprote
´
iques dans la perte de poids, notamment la perte de gras. Par forte teneur en prote
´
ines, on entend
plus de prote
´
ines que n’en consomme la moyenne, soit 30%–35 % comparativement a
`
~15 %, ce qui est compris dans la
fourchette de distribution acceptable des macronutriments (AMDR) selon les DRI. L’observation suivante est pertinente
pour les athle
`
tes et les cliniciens : comparativement aux re
´
gimes hypoprote
´
iques, les re
´
gimes hyperprote
´
iques causent sys-
te
´
matiquement plus de perte de poids, plus de perte de gras et prote
`
gent davantage la masse maigre. Nous pre
´
sentons un
cadre de re
´
fe
´
rence pour bien e
´
tablir les besoins prote
´
iques dans un contexte d’amaigrissement et de performance physique.
Mots cle
´
s:masse maigre, renouvellement des prote
´
ines, leucine.
[Traduit par la Re
´
daction]
Received 16 March 2006. Accepted 26 April 2006. Published on the NRC Research Press Web site at http://apnm.nrc.ca on
27 November 2006.
S.M. Phillips. Exercise Metabolism Research Group, Department of Kinesiology, McMaster University, 1280 Main St. West, Hamilton,
ON L8S 4K1, Canada (e-mail: phillis@mcmaster.ca).
Supported by an unrestricted educational grant from Abbott Laboratories.
Sponsored by Abbott Nutrition.
647
Appl. Physiol. Nutr. Metab. 31: 647–654 (2006) doi:10.1139/H06-035
#
2006 NRC Canada
Introduction
It is likely fair to say that where an optimal dietary pro-
tein intake for athletes is concerned no greater dichotomy
of opinion exists than between those who establish recom-
mendations and the athletes themselves, particularly
strength- and (or) power-training athletes. The published
standards for dietary guidelines the Dietary Reference
Intakes (DRIs), assert that although protein is an essential
nutrient, it is not required above a basal level not much
more than that needed to cover daily body protein losses
(Institute of Medicine 2005). At the same time, a large
group of athletes and an industry devoted to supplemental
protein sources argue that their experience tells them
higher protein diets are beneficial and perhaps even neces-
sary. As with most arguments of this type the truth may
lie somewhere in between; however, it is prudent to re-
view evidence that forms the basis of the arguments on
both sides in an effort to come to an evidence-based con-
clusion on what dietary protein intake is appropriate. In
this review, an appropriate intake of protein is defined as
one that (i) allows maximal functioning of all protein-
requiring processes in the body, particularly protein synthe-
sis; (ii) does not promote significant elevations in urea syn-
thesis and amino acid oxidation, which would create a
situation of excessive amino acid oxidation and nitrogen
loss, or excessive reliance on protein oxidation during pro-
longed exercise; and (iii) might allow beneficial physical
adaptation to occur under certain conditions such as caloric
deprivation. A brief review of how the Recommended Di-
etary Allowance (RDA) for protein is set and also what
evidence exists to support the contention that dietary pro-
tein is required at greater than RDA levels for athletes is
presented.
Establishing the RDA
This section will be relatively brief, since excellent re-
cent reviews on this topic have appeared previously in
this journal (Barr 2006; Zello 2006). The RDA for protein
for persons 19 y and older is set at 0.8 g proteinkg body
mass
–1
d
–1
(Institute of Medicine 2005). This intake is es-
sentially set to cover basal losses of nitrogen and also has
a margin for error of the general population’s Estimated
Average Requirement (EAR), plus two standard deviations.
As a result, the RDA is said to cover the requirements of
97.5% of the population. The DRI data forming much of
the basis of the RDA are from Rand et al. (2003) and en-
compasses 225 individuals and their protein requirement
(Fig. 1). Suffice to say that most athletes, particularly
those engaged in strength training and those concerned
with gaining lean body mass, would believe that they are
most assuredly in the 2.5% of the population not covered
by the RDA. Of note to such athletes, however, is the
fact that the DRI does not set a Tolerable Upper Limit
(TUL) for protein; hence, there appears to be no cap on
protein intake from an adverse health standpoint so far as
the committee establishing the RDA for protein is con-
cerned. As Zello (2006) points out, however, high protein
intakes for those with pre-existing disorders, such as renal
disease, are not recommended.
Protein requirements for athletes
Studies in which protein requirements have been exam-
ined in athletes have shown an increased requirement for
protein in strength-trained (Lemon et al. 1992; Tarnopolsky
et al. 1988, 1992) and endurance-trained athletes (Friedman
and Lemon 1989; Meredith et al. 1989; Tarnopolsky et al.
1988). Quite simply, increased protein requirements for indi-
viduals engaging in resistive activities might be expected to
come about owing to the need for extra’ dietary protein
required to synthesize new muscle or repair muscle damage.
On the other hand, endurance exercise is associated with
marked increases in leucine oxidation (Lamont et al. 1999,
2001; McKenzie et al. 2000; Phillips et al. 1993), which
would elevate overall requirements for protein (if other
amino acids are oxidized to an appreciable extent), or at
least for leucine. Conversely, other investigations have
shown that increasing physical activity reduces requirement
for protein (Butterfield and Calloway 1984; Todd et al.
1984). So, why is there a discrepancy? The protein used in
those studies in which protein requirements were reduced
was high-quality egg and milk protein (Butterfield and Cal-
loway 1984; Todd et al. 1984), which may have allowed
subjects to achieve a nitrogen balance at a protein intake
Fig. 1. Data adapted from Rand et al. (2003). (A) Data from 225
individuals showing their protein requirements, based on nitrogen
balance data. Dotted line indicates the 50th percentile for indivi-
dual’s requirement for dietary protein. Note the skewed nature of
the distribution toward protein requirements greater than the RDA.
(B) Data from A shown as a cumulative percentage of individuals
(total of 225) as a function of their protein requirements.
648 Appl. Physiol. Nutr. Metab. Vol. 31, 2006
#
2006 NRC Canada
lower than that seen with lower-quality proteins. In addition,
the intensity of the exercise performed by the subjects in
studies in which protein requirements were elevated
(Friedman and Lemon 1989; Lemon et al. 1992; Meredith
et al. 1989; Tarnopolsky et al. 1988, 1992) was greater than
that in studies in which requirements were reduced (Butter-
field and Calloway 1984; Todd et al. 1984). Thus, the com-
bination of higher-intensity exercise and the fact that leucine
oxidation is proportional to exercise intensity (Lemon et al.
1982) means that a higher exercise intensity may have re-
sulted in a higher requirement for protein.
Although there exist data to support the idea that protein
requirements are higher in persons who are habitually exer-
cising (Friedman and Lemon 1989; Lemon et al. 1992; Mer-
edith et al. 1989; Tarnopolsky et al. 1988, 1992), all of the
aforementioned studies have relied on the nitrogen balance
methodology to establish protein requirements. This method-
ology is still used, in part, to establish the RDA intake for
protein, which gives it some credence. However, there is a
consistent and physiologically non-plausible result with ni-
trogen balance data at high protein intakes, which is an im-
possibly high retention of nitrogen. For example, at a protein
intake of around 2.5–2.8 g proteinkg
–1
d
–1
body builders
have been shown to be in positive nitrogen balance on the
order of 8–20 g Nd
–1
(Lemon et al. 1992; Tarnopolsky et
al. 1988, 1992). Since protein is, on average, 16% nitrogen
by mass, this would mean these athletes would be retaining
50–125 g proteind
–1
, or 200–500 gd
–1
when hydrated, as it
exists in the body; obviously, such a result is physiologically
impossible. This finding is more than likely the result of an
expansion of the circulating urea pool and reflects a physio-
logical limit on the rate at which urea can be excreted. It
may also be that at high protein intakes there is a consistent
overestimation of protein intake and underestimation of ni-
trogen excretion (Hegsted 1976; Young 1986; Young et al.
1987). Many of these shortcomings of nitrogen balance
have long been recognized (Hegsted 1976; Young 1986;
Young et al. 1987). The fact that a non-physiologically rea-
sonable condition forms part of the basis on which conclu-
sions regarding an adequate protein intake are based
highlights the need for another approach to examining pro-
tein requirements; tracer-derived estimations of protein re-
quirements are one alternative method. Using such an
approach it was reported that consumption of a low-protein
diet (0.86 g proteinkg
–1
d
–1
) by a group of strength-training
athletes resulted in an accommodated state in which whole-
body protein synthesis was reduced as compared with medium-
(1.4 g proteinkg
–1
d
–1
) and high- (2.4 g proteinkg
–1
d
–1
) pro-
tein diets (Tarnopolsky et al. 1992). No difference was
seen in whole-body protein synthesis between the medium-
and high-protein diets. On the high-protein diet, amino acid
oxidation was elevated; however, indicating that this pro-
tein intake was excessive compared with requirements. It
should be emphasized that these results do not mean that
1.4 g proteinkg
–1
d
–1
was required to cover dietary protein
needs, but simply that 0.86 g proteinkg
–1
d
–1
was not suffi-
cient to allow maximal rates of protein synthesis. It is not
known what body proteins were being made at a submaxi-
mal rate, but if muscle protein synthesis were adversely af-
fected, then clearly these data would be of relevance to
athletes.
Recent work from our laboratory has shown that novices
undergoing a strength-training program, during which they
gained a significant amount of lean mass (2.5 kg), had a
more positive whole-body nitrogen balance when consuming
1.2 g proteinkg
–1
d
–1
after as opposed to before training
(Hartman et al. 2006). We also observed that tracer-esti-
mated (oral [
15
N]glycine) protein turnover was reduced as a
result of training, consistent with our nitrogen balance data.
Together, these data support the concept that resistance
training in novices, a group in whom one would expect to
see a much greater need for dietary protein (vs experienced
weightlifters who have reduced muscle protein turnover
(Phillips et al. 1999), is actually a protein-conserving stimu-
lus rather than one that results in increased protein needs.
Acute muscle-specific increases in muscle protein balance
last for up to 48 h following resistance exercise and provide
cellular-level support for the idea that resistive exercise is
anabolic per se and results in conservation of body protein
and not increased loss (Phillips et al. 1997, 1999, 2002). Of
note are some recent data showing that even short-lived,
low-intensity, dynamic exercise (Sheffield-Moore et al.
2004), as well as prolonged, intense, dynamic exercise
(Miller et al. 2005), is also stimulatory for protein synthesis,
indicating that they too would provide an anabolic stimulus
to hang on to muscle protein. The weight of evidence con-
tained in these studies (Miller et al. 2005; Phillips et al.
1997, 1999, 2002; Sheffield-Moore et al. 2004) should not
be underestimated in its support for the hypothesis that exer-
cise, be it resistive or dynamic, would actually provide a
stimulus to promote greater intracellular reutilization of
amino acids arising from proteolysis. The end result is that
exercise would actually lower, not raise, protein require-
ments (i.e., the protein needed to replace losses).
Dietary protein in conditions of energy
restriction
With the publication of the new DRIs there came new
recommendations to establish what are termed Acceptable
Macronutrient Distribution Ranges (AMDRs). AMDRs have
been established for carbohydrate, protein, total fat, linoleic
acid, and -linolenic acid (Institute of Medicine 2005). The
AMDRs for carbohydrate, fat, and protein as a percent of to-
tal energy intake are 45%–65%, 20%–35%, and 10%–35%,
respectively, for adults. This is an important and relevant
difference of the new DRIs compared with the older RDA
and Recommended Nutrient Intake (RNI) values into which
no such flexibility of range was built (Health and Welfare
Canada 1990; National Research Council 1989). It is inter-
esting to find that a number of well-controlled studies have
examined the impact of a higher-protein diet, most of which
provided dietary protein within the AMDR, on weight loss
and body-composition changes following energy restriction
in obese individuals (Farnsworth et al. 2003; Foster et al.
2003; Johnston et al. 2004; Layman et al. 2003, 2005;
McAuley et al. 2005; Noakes et al. 2005; Parker et al.
2002). A consistent finding was that higher-protein diets re-
sulted in greater weight loss and a greater amount of that
loss was accounted for by fat mass. By difference, therefore,
higher-protein diets promoted a greater retention of lean
body mass. Blood lipoprotein changes were variable, but in
Phillips 649
#
2006 NRC Canada
almost every case changes in lipoprotein concentrations
were similar with higher-protein diets as they were with
lower-protein (most often higher-carbohydrate) diets. Inter-
estingly, subjects on the higher-protein diets often reported
greater satiety and overall satisfaction with their diets than
their low-protein counterparts (Johnston et al. 2004; Layman
et al. 2003, 2005). Since the goal of energy restriction in a
clinical setting is loss of stored energy as fat, the signifi-
cance of the greater fat loss seen in these studies is particu-
larly noteworthy (Farnsworth et al. 2003; Foster et al. 2003;
Johnston et al. 2004; Layman et al. 2003, 2005; McAuley et
al. 2005; Noakes et al. 2005; Parker et al. 2002). Perhaps of
equal significance to the greater fat loss in these studies is
the preservation of lean mass while undergoing weight loss.
Since skeletal muscle serves as the largest disposal site for
post-prandial glucose (Zierath and Kawano 2003), it also ap-
pears to play a role in post-prandial lipemia (Petitt and Cur-
eton 2003), and is the greatest determinant of our basal
metabolic rate (BMR; (Johnstone et al. 2005)), maintenance
of as much metabolically active skeletal muscle mass as
possible would appear to have substantial implications for
resisting weight gain.
Athletes have long recognized the performance advantage
of maintaining high lean-to-fat mass ratio for almost any
sport; hence, while the clinical implications of weight loss
favouring preservation of lean mass are significant, the
same could be said for those in the athletic realm. Layman
et al. ( 2005) recently showed, using a 16 week randomized
2 2 blocked design trial of both exercise (5 d/week walk-
ing and 2 d/week resistive exercise) and diets of varying lev-
els of protein (higher:1.5 g proteinkg
–1
d
–1
vs lower: 0.8 g
proteinkg
–1
d
–1
), that the effect of higher dietary protein
was as potent in terms of total weight and fat lost as the ad-
dition of exercise in women (body mass index = 33 kgm
–2
)
consuming 7.1MJd
–1
. In fact, weight loss in the group that
consumed higher protein and exercised was 9.8 kg and of
that 96% was lost from the fat compartment, the result
being a reduction in their percentage body fat by almost
6%. By contrast the group that consumed a lower protein
diet and did not exercise, a strategy tried by many when
attempting to lose weight, lost 7.8 kg; however, only 64%
of that loss was fat, meaning that this group also lost 2.7
kg of lean mass. Such findings may not surprise a number
of athletes who, through trial and error, have reached the
same conclusion, which is that to promote the greatest fat
mass loss but avoid lean mass loss during times of caloric
deprivation one must exercise and consume more protein.
Presumably, given the anabolic nature of resistive exercise
(Biolo et al. 1995, 1997; Phillips et al. 1997, 2002), this
mode of exercise would provide the most powerful stimulus
to retain lean body mass. Indeed, in the fasted state, muscle
protein balance is significantly less negative for up to 48 h
following an isolated bout of resistance exercise (Phillips et
al. 1997), showing just how powerful a single bout of resis-
tive exercise can be as a stimulus to conserve muscle protein.
Mechanisms of action
Why then are higher-protein diets more effective than
lower-protein diets in promoting weight loss and particularly
fat loss? In a meta-analytic review, Buchholz and Schoeller
(2004) concluded that Diets high in protein and (or) low in
carbohydrate produced an approximately equal to 2.5-kg [or]
greater weight loss [of body mass] after 12 w[ee]ks of treat-
ment. Neither macronutrient-specific differences in the
availability of dietary energy nor changes in energy expendi-
ture could explain these differences in weight loss.’ In the
final analysis, these authors concluded that there is an
amount of energy expended on a daily basis that is greater
with high-protein diets that cannot be satisfactorily
accounted for by either the increased thermic effect of
protein as a macronutrient (vs either carbohydrate or lipid)
nor by any disproportionate activation of inefficient energy-
consuming metabolic pathways specific to protein consump-
tion. Others have presented contrary arguments stating that
there is evidence that higher protein diets would result in
greater fluxes through metabolically inefficient metabolic
pathways (Fine and Feinman 2004). However, it has been
argued that the magnitude of such an effect is insufficient
to account for the differences seen in total weight loss
(Buchholz and Schoeller 2004).
What is likely true with higher-protein diets is that even
when a person is in an energy deficit the metabolic require-
ment for protein would be reduced somewhat, initially due
to adaptive and ultimately due to accommodative mecha-
nisms (Millward 2004a; Millward and Jackson 2004). At
the same time, it appears that a higher protein intake is as-
sociated with retention of lean mass even while persons are
in an energy deficit (Farnsworth et al. 2003; Foster et al.
2003; Johnston et al. 2004; Layman et al. 2003, 2005;
McAuley et al. 2005; Noakes et al. 2005; Parker et al.
2002). Even in the face of consumption of protein above
requirement levels during energy deficit, excess amino
acids, once deaminated, produce carbon skeletons that
would be oxidized, but that are ultimately very poor lipo-
genic substrates; in fact, only leucine and lysine as purely
ketogenic amino acids (i.e., yielding acetoacetyl CoA)
could likely support significant lipogenesis. Simply put, it
is very metabolically difficult to turn excess protein into
fat. A very interesting question is whether the metabolic
advantage seen with higher than recommended protein in-
takes when people are in energy deficit transfers over to
situations of energy balance? In other words, could some-
one simply shift their macronutrient ingestion ratio from a
relatively high to a lower carbohydrate-to-protein ratio and
achieve fat mass loss? Clearly, from a purely thermody-
namic point of view, such a change would seem unlikely;
however, as discussed by Fine and Feinman (2004), there
is a theoretical basis for how an increase in protein con-
sumption results in metabolically wasteful pathways.
Hence, such an approach, even in persons in true energy
balance, could result in an increased energy expenditure
and hence a relatively slow weight loss.
It is also possible that certain classes of protein may con-
fer an advantage during energy restriction in terms of their
capacity to induce weight loss. Zemel (Zemel 2004) re-
ported that a whey protein-derived angiotensin-converting
enzyme (ACE) inhibitor synergistically enhanced the effect
of dietary calcium on fat loss in energy-restricted aP2-agouti
transgenic mice, which are mice that exhibit a pattern of
obesity gene expression similar to humans (Shi et al. 2001).
Interestingly, calcium and an ACE inhibitor were not as ef-
650 Appl. Physiol. Nutr. Metab. Vol. 31, 2006
#
2006 NRC Canada
fective as either milk or whey in reducing the mass of stored
fat during energy deficit (Zemel 2004). The data of Zemel
(2004) point to an interesting link between whey protein -
derived peptides and fat loss. While Zemel attributes ~40%
of an enhanced weight loss with dairy product consump-
tion to calcium and a modulatory role for 1,25-dihydroxy-
vitamin D (Xue and Zemel 2000; Zemel 2004), recent
work reveals that whey-derived peptides may also be play-
ing an integral role in promotion of fat loss. Certainly, a
unique aspect of whey protein would be in its ability to
support lean mass retention during energy restriction, due
to its its high essential and branched chain amino acid con-
tent and particularly its leucine content, as well as other
bioactive peptides. Clearly, more research is needed re-
garding dairy protein, and whey protein in particular, dur-
ing energy restriction to ascertain how a diet high in these
constituents may enhance fat loss, particularly from central
adipose stores (i.e., trunk fat) (Zemel et al. 2004).
Layman and Walker (2006) have put forth the hypothesis
that it is the leucine content of the higher-protein energy-
restricted diets that is important in maintaining lean mass
and promoting fat loss. They propose that a diet high in
leucine content, mainly through consumption of high-
quality proteins during caloric restriction, would promote
increased muscle protein synthesis and in doing so would
promote retention of muscle protein. Interestingly, leucine
infusion (3.5 g over 4 h) has been shown to reduce urinary
nitrogen loss in fasting obese subjects (Sherwin 1978).
Nevertheless, more work obviously remains to be done in
this area to establish how higher-protein diets are able to
induce greater fat loss and lean mass retention.
Dietary protein in athletes: requirements to
advantage
How then can the assertion that exercise may act as a
conservatory stimulus to retain amino acids and thus muscle
protein be reconciled with data in which a higher-protein
diet appears to be advantageous? Figure 2 is an attempt to
show how this might come about.
There exists good evidence to suggest that both endurance-
and resistance-based exercise can stimulate muscle protein
synthesis (MPS) (Biolo et al. 1995; Miller et al. 2005; Phil-
lips et al. 1997, 1999, 2002; Sheffield-Moore et al. 2004;
Welle et al. 1995, 1999), which would act as an underlying
basis for why exercise would be a protein-conserving stimu-
lus. Although one can argue that muscle protein breakdown
(MPB) is also elevated, in studies in which both MPS and
MPB have been measured, the rise in synthesis has been
shown to be consistently greater in magnitude than any
rise in breakdown (Biolo et al. 1995; Miller et al. 2005;
Phillips et al. 1997, 1999, 2002; Sheffield-Moore et al.
2004; Welle et al. 1995, 1999). In addition, urinary 3-
methylhistidine excretion was shown not to rise even in
the face of substantial increases in MPB (Phillips et al.
1997), indicating that even if proteolysis is elevated it is
not due to substantial degradation of actin or myosin.
It is becoming clearer that higher-protein diets spare
muscle protein and enhance fat loss during periods of energy
restriction (Farnsworth et al. 2003; Foster et al. 2003; John-
ston et al. 2004; Layman et al. 2003, 2005; McAuley et al.
2005; Noakes et al. 2005; Parker et al. 2002). With these re-
sults in mind it would appear to be wise to counsel both ath-
Fig. 2. Theoretical relationship between habitual protein intake and maximal functioning of protein synthetic processes for all body proteins
in sedentary individuals (solid line), resistance-trained athletes (short dashed line), and the perceived relationship between dietary protein
intake and maximal functioning of synthesis of body proteins in resistance-trained athletes (long dashed line), as well as the relationship
between dietary protein intake and amino acid catabolism, representing deamination, oxidation, and ultimately urea synthesis (dashed line
with double dots). (A) Dietary protein RDA. (B) Estimated safe protein intake for resistance-training athletes (Phillips 2004). (C) Range of
protein intakes at which adaptive metabolic advantage, including enhanced fat loss and retention of lean mass during energy restriction, of
higher dietary protein intakes might be seen.
Phillips 651
#
2006 NRC Canada
letes and patients for whom weight loss is a desired goal
that up to 35% of their energy can come from protein and
that this may result in a metabolic advantage in terms of
the change in body composition achieved during energy re-
striction.
What must be made clear, however, is that 35% of an en-
ergy-restricted diet for an obese female (100 kg) consuming
only 7 MJd
–1
(~1700 kcald
–1
) would amount to ~150 g of
protein or 1.5 g proteinkg
–1
d
–1
. An athlete (e.g., a 20-y-old
male hockey player) who may weigh closer to 90 kg and is
consuming, while still being energy restricted, something
closer to 10.4 MJd
–1
(~2500 kcald
–1
) would be getting
219 g or 2.4 proteinkg
–1
d
–1
(close to the 1 gram protein per
pound (lb) of body mass often recommended in magazines),
assuming 35% of his total energy were from protein. If the
same young hockey player were to consume 20% of his en-
ergy as fat he would have a displacement of dietary carbo-
hydrate by protein and would only be getting 280 g of
carbohydrate (3.1 g carbohydratekg
–1
d
–1
), which is well be-
low what is recommended for optimal athletic performance
(Burke et al. 2004). This simple example highlights a pitfall
for athletes who, although they may be seeking a lean phy-
sique, adopt a diet that is simply not going to allow them to
perform over the long-run owing to incomplete muscle gly-
cogen restoration (Burke et al. 2004). It is also quite apparent
that at some point the benefit associated with a high-protein
diet would have to diminish and result only in elevated blood
urea and amino acid oxidation (i.e., a stimulation of amino
acid breakdown and use of amino acids as fuel), as well as in-
complete glycogen restoration. At this point, the performance-
oriented utility of consuming higher protein intakes would
be lost, at least for some athletes. However, the anecdote
is very powerful in athletic circles and is proportional in
its power based on the success of the athlete delivering it.
Thus, it would take only one successful athlete to promote
the idea that they won while consuming a particular diet
for the practice to take hold. One question that athletes
often fail to consider when adopting a new dietary regime
is whether the diet works because of, or in spite of, what
they are consuming. In fact, it has been shown that a num-
ber of athletes consume suboptimal carbohydrate intakes
despite their elite status (Burke et al. 2001b), primarily
due to their preoccupation with body mass and body fat.
However, it is this author’s opinion that athletes would
likely tolerate all manner of diets if they believe it will en-
hance their performance.
The pitfalls of consuming a higher-protein diet and the
potential for increased amino acid oxidation and loss of ni-
trogen between meals is an argument consistently put forth
by Millward (e.g., Millward 2004b). However, many ath-
letes, particular strength-training athletes and those seeking
gains in lean body mass, will consume protein far above re-
quirement levels (which may actually be lower owing to the
anabolic nature of exercise) in the hopes that the extra pro-
tein will enhance protein synthesis and ultimately lead to a
greater lean mass accretion. Some studies have shown that
protein supplementation during intensive training can bring
about greater gains in lean mass (Burke et al. 2001a; Deibert
et al. 2004; Demling and DeSanti 2000), but a lack of strict
controls and a fully randomized design make it difficult to
draw a solid conclusion on this point. Of relevance, an opti-
mal dose of protein to achieve maximal muscle protein syn-
thesis and hence muscle mass gains with resistance training
is still unknown. Cuthbertson et al. (2005) recently reported
that an oral dose of 10 g of essential amino acids maximally
stimulated muscle protein synthesis in young individuals at
rest. Considering that exercise per se is anabolic and acts as
an independent stimulus for protein synthesis it is likely that
an optimal dose of protein to maximize the anabolic re-
sponse would be even less than 10 g. More to the point,
once an increase in muscle mass is attained a high-protein
diet would appear to be even more unnecessary.
During periods of energy restriction it does appear benefi-
cial to consume protein above current requirement levels,
although while in energy deficit the surfeit protein would be
directed toward lean mass maintenance rather than gains. To
date, one study has reported that muscle mass gains can oc-
cur while in a substantial exercised-induced energy deficit
(Demling and DeSanti 2000), whereas another reported that
higher protein could only maintain muscle mass (Deibert et
al. 2004). Thermodynamically, tissue protein accretion is an
energy requiring process; hence, the results of Demling and
DeSanti (2000) are at odds with the concept that substantial
muscle growth can be achieved when an overall energy def-
icit is in place. For skeletal muscle accretion to occur, in the
face of energy deficit, it would require that energy would
have to be preferentially directed toward muscle protein
anabolism during a time when hormonally, and energeti-
cally, catabolism is favoured. Clearly, further work needs to
be done on this topic, particularly in high-risk populations
such as obese patients and those with type 2 diabetes or
pre-diabetes for whom lean mass retention, and potentially
gain, during weight (i.e., fat) loss would be far more benefi-
cial than simply losing body mass, which would also include
lean mass.
Summary
Available evidence suggests that protein requirements are
not likely elevated, if they are elevated at all, by substantial
amounts in persons completing exercise of either a dynamic
or resistive nature. Ultimately, a debate on protein require-
ments appears to be moot for most athletes anyway, since
their habitual intakes, particularly those of males, far exceed
the RDA and even the most liberal estimates of requirement,
which when estimated from existing nitrogen balance data in
strength-trained athletes is ~1.3 g proteinkg
–1
d
–1
(Phillips
2004) or ~1.1 g proteinkg
–1
d
–1
in endurance-trained ath-
letes (Tarnopolsky 2004). Accumulating evidence suggests
that energy-restricted higher-protein diets may offer some
benefit in terms of promoting fat loss while maintaining
lean mass. It may be that higher-quality proteins consumed
during such an energy-restricted period offer a further ad-
vantage owing to their high leucine content; however, addi-
tional work is required to confirm this thesis.
Acknowledgements
S.M.P. is a Canadian Institutes of Health Research
(CIHR) New Investigator award recipient. Funding during
the time of this work was from the Natural Science and En-
gineering Research Council (NSERC) of Canada and CIHR
652 Appl. Physiol. Nutr. Metab. Vol. 31, 2006
#
2006 NRC Canada
and is gratefully acknowledged by S.M.P. Thanks to Jason
Tang for edits to the final version of this manuscript.
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... Contributing factors to metabolic rate are tissue growth and excessive repair of tissue damage. Either will increase metabolic rate (Clark 2015, Henry 2007, Phillips 2006, Tarnopolsky 2000, Thomas et al. 2008. Additionally, metabolic rates change in response to exertional stress (physical activity, exercise) as measured by a person's level metabolic expenditure (VO2) during that physical activity (ACSM 2006) . ...
... This combination of use of nutrients within the various metabolic pathways allows us to derive nutrient balance. For example liver, skeletal muscle and adipose cells will synthesize larger macromolecules (structural and contractile proteins, glycogen, fatty-acids and lipid droplets) for maintaining or regaining homeostasis within the tissues when intake is greater than utilization for energy balance, or to act as stores of nutrients for times when nutrient use outpaces intake (Carbone and Pasiakos 2019, Fern et al. 2015, Jensen 2002, Phillips 2006. It is important to note that the misconception is that storage will take place strictly as lipids in adipose tissue based on the Caloric imbalance. ...
... A concept that is important to discuss with students entering health careers is the changing need for lipids and amino acids across the age range of patients that they will encounter (Kerksick et al. 2017, Uauy et al. 1999. Additionally, there is a varying degree of understanding as to the necessity for macronutrients, especially protein, by those who exercise or are attempting to shift body composition for health benefits (Antonio et al.2015, Carbone and Pasiakos 2019, Clark 2015, 2019Lambert et al. 2004, Phillips 2006, Phillips et al. 2015, Tarnopolsky 2004. There is also a higher carbohydrate load, due to tissue growth and increased neuron function, that is seen in children, juveniles, and adolescents that may explain the increased need and desire for carbohydrates within their diet (Giovannini et al. 2000, Stephen et al. 2012). ...
... Endogenous oxidative loss must be replaced with a dietary source of amino acids in order to provide a substrate (like a building block) to repair and remodel the tissues during the recovery period [4]. As the need for amino acids increases after exercise, a higher protein consumption (1.2-2.0 g of protein/kg/day) is recommended for athletes compared with the current recommended daily allowance of protein (0.8 g/kg/day) [5]. However, as the need for amino acids during the recovery phase are modulated by the type or duration of exercise, as well as the training status of athletes [1,[6][7][8][9], a pattern of protein consumption should be optimized according to the nature of exercise and the subjects' characteristics. ...
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