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Protein Applications in
Sports Nutrition—Part I:
Requirements, Quality,
Source, and Optimal Dose
Trisha A. McLain, MS,
1
Kurt A. Escobar, MA,
1
and Chad M. Kerksick, PhD
2
1
Department of Health, Exercise and Sports Sciences, University of New Mexico, Albuquerque, New Mexico; and
2
Department of Exercise Science, School of Sport, Recreation and Exercise Sciences, Lindenwood University, St.
Charles, Missouri
ABSTRACT
PROTEIN, A KEY MACRONUTRIENT,
IS NEEDED BY THE BODY TO
REPAIR AND BUILD NEW CELLULAR
STRUCTURES. EXERCISING INDI-
VIDUALS PARTICIPATING IN BOTH
AEROBIC AND ANAEROBIC ACTIVI-
TIES REQUIRE GREATER AMOUNTS
OF PROTEIN (1.2–1.6 G$KG
21
$D
21
)IN
THEIR DIET. PROTEIN QUALITY IS
EVALUATED PRIMARILY BY ESSEN-
TIAL AMINO ACID CONTENT (8–12 G)
AND DIGESTIBILITY CORRECTED
AMINO ACIDS (PDCAA) SCORES
(1.0–1.2+). FLESH (BEEF, PORK,
POULTRY, AND FISH), DAIRY
(WHEY, CASEIN, MILK, AND
CHEESE), EGG, AND PLANT (VEG-
ETABLE, SOY, ETC.) RANGE IN
QUALITY (PDCAAS: 0.74–1.2+) AND
OTHER PROPERTIES THAT FUR-
THER IMPACT HEALTH. OPTIMAL
DOSING (;20–25 G; 8–12 G
ESSENTIAL AMINO ACIDS) IS
IMPORTANT TO MAXIMALLY STIM-
ULATE MUSCLE PROTEIN SYN-
THESIS AND PROMOTE A POSITIVE
MUSCLE PROTEIN BALANCE.
OVERVIEW
The human body relies on 3 mac-
ronutrients to yield energy that is
used to perform muscular work
and various cellular functions, including
the rebuilding and synthesis of new cells
and tissue. Proteins are structurally dis-
tinguished from carbohydrate and fats
by the presence of an amino or amine
group. Across the human body, proteins
are ubiquitous and considered the “action
molecules” within our biochemistry.
Proteins are comprised of amino acids,
20 of which are used by every cell in our
body to build protein. Unlike carbohy-
drates or fats, no storage of protein oc-
curs throughout the body and increases
and decreases in protein synthesis and
breakdown occur in response to physi-
ological demand. Importantly, studies
have indicated that synthesis of
human skeletal muscle is critically
dependent on the 9 essential amino
acids (histidine, isoleucine, leucine,
lysine, methionine, phenylalanine, thre-
onine, tryptophan, and valine), amino
acids that cannot be endogenously pro-
duced, and must be acquired in ade-
quate amounts (and proportions) in
the diet (65,67). The absolute necessity
of skeletal muscle for the essential amino
acids is an important consideration that
drives ratings of quality, protein source
considerations, and optimal required
protein doses. Nonessential amino acids
(alanine, aspartic acid, glutamic acid, and
serine) (22,49) can be readily produced
inside the human body, whereas other
amino acids may be classified as condi-
tionally essential (arginine, asparagine,
cysteine, glycine, glutamine, proline,
and tyrosine) (22,49), specifically during
periods when the body cannot make
these amino acids in adequate amounts
(disease or high volumes of physical
exercise).
For both athletic and nonathletic pop-
ulations, protein discussions oftentimes
are centered on the nutrient’s ability to
maximize muscle protein synthesis
(MPS), facilitate recovery, and in the
long term, promote greater adaptations
related to strength, power, and accre-
tion of fat-free mass (11,14). However,
protein’s application for widespread
improvements in health is also impor-
tant. A 2-part review was completed to
discuss the available literature sur-
rounding applications of protein in
health, fitness, and sport. The purpose
of the current article is to discuss pro-
tein requirements, protein quality,
sources of protein, and optimal dosing.
To appeal to a wider audience of
coaches, trainers, and practitioners,
no particular focus was made toward
1 population. As such, the reader
should understand that any of the con-
cepts discussed that relate to athletic
performance also hold true for other
applications of protein, such as weight
loss and fat loss. In fact, the literature
Copyright ÓNational Strength and Conditioning Association Strength and Conditioning Journal | www.nsca-scj.com 61
available on the impact of protein on
athletically competitive populations is
lacking. The second part of this review
will focus on topics related to protein
timing, patterns of consumption, and
protein’s impact on fat loss and fat-
free mass accretion.
SEARCH STRATEGY AND CRITERIA
The relevant literature was retrieved
from the PubMed and Google Scholar
databases using combinations of search
terms, such as the following: “protein,”
“dose or protein dosing,” “require-
ments,” “quality,” “source,” “exercise,”
“training status,” “whey,” “casein,”
“micellar casein,” “soy,” “egg,” and
“beef.” Only articles written in the
English language were used. Inclusion
of studies was primarily made by author
review and determination of their con-
tent providing necessary basis for the
scope of this review.
PROTEIN REQUIREMENTS
For years, debate has ensued regarding
the efficacy surrounding recommen-
ded amounts of dietary protein. The
recommended daily allowance (RDA)
remains at 0.8 g of protein per kilo-
gram of body mass per day and repre-
sents a protein intake that is sufficient
to meet the needs of nearly all (97.5%)
healthy adult men and women. To
determine protein requirements, the
total intake of nitrogen is often com-
pared against the total excretion of
nitrogen, establishing what is referred
to as nitrogen balance. When excre-
tion exceeds the intake of nitrogen,
the person is said to be in a negative
nitrogen balance, whereas positive
nitrogen balance occurs when intake
exceeds excretion (11). Acute re-
sponses to exercise can create a net
negative balance of protein that can
go on to negatively impact immunity,
recovery, and overall improvements in
strength, endurance, and body compo-
sition. Many studies indicate that ath-
letes (defined as people who are
regularly training and competing in
some manner) ingesting protein
amounts at the RDA value or even
slightly above (1.0 g$kg
21
$d
21
) are
unable to prevent a negative nitrogen
balance, irrespective of the athlete’s
exercise type (e.g., endurance or resis-
tance) or training status (e.g., beginner,
intermediate, advanced, elite)
(11,20,21,32,37,44,61). According to sev-
eral published studies, elevated protein
intake (1.2–1.8 g$kg
21
$d
21
) is well toler-
ated by healthy individuals (2,14,28,29).
However, legitimate shortcomings exist
(i.e., impaired recovery, blunted adapta-
tions, increased catabolism) if inadequate
protein is consumed (31).
ENDURANCE EXERCISE
Accurate assessments of endurance
training volume and intensity, as well
as overall energy intake are important
considerations when evaluating pro-
tein needs. As previously mentioned,
studies involving novice athletes are
available to indicate that regular exer-
cise training elevates protein needs, but
such elevations in protein require-
ments may not be required if the indi-
vidual is consuming a diet that is
providing adequate amounts of energy.
As an illustration, el-Khoury et al. (19)
had 8 healthy men (27 612.5 years;
77.5 66.7 kg; 16.5 65.6% fat) com-
plete a series of highly controlled ex-
periments inside a direct calorimeter,
while providing a diet of 1.0 g/kg of
protein per day and participating in
what they considered to be modest
cycling exercise (two 90-minute
cycling bouts per day at 46% V
̇
O
2
max).
Results of the study indicated that the
1.0 g/kg of protein per day sufficiently
allowed subjects to maintain whole-
body leucine (amino acid) equilibrium.
An investigation by Meredith et al.
(37) asked 6 young (26.8 61.2 years;
71.1 64.5 kg; 64.8 62.8
mL$kg
21
$min
21
) and 6 middle-aged
(52.0 61.9 years; 72.1 63.1 kg;
55.3 65.0 mL$kg
21
$min
21
)men
who had been habitually completing
endurance training (7.5–12.3 h/wk,
11.5–12.8 years of training) to con-
tinue their normal patterns of exercise
and physical activity over separate 10-
day investigative periods. Each study
period required the participants to
ingest 0.61, 0.92, or 1.21 g$kg
21
$d
21
of protein. Researchers found that
a protein intake of 1.21 g$kg
21
$d
21
was needed to promote a positive
nitrogen balance. Tarnopolsky et al.
(62) examined runners and Nordic
skiers with at least 5 years of experi-
ence (22 61 years; 73 61kg;7.16
0.8% fat; $12 h/wk of training) and
foundaproteinintakeof1.6
g$kg
21
$d
21
was needed (using nitro-
gen balance techniques) to meet pro-
tein requirements, a value that was
1.67 times greater than the amount
of protein required by sedentary con-
trols. Similarly, Friedman and Lemon
(21) reported a protein intake of 1.49
g$kg
21
$d
21
wasneededtomaintain
a positive nitrogen balance in elite
endurance runners. This corresponds
with Broun’s lower limit recommen-
dation of protein intakes from 1.5 to
1.8 g$kg
21
$d
21
(9).
STRENGTH AND POWER/
RESISTANCE EXERCISE
Acute responses to a single bout of
resistance training stimulates in-
creases in MPS as well as muscle pro-
tein breakdown; however, in the
absence of protein ingestion, an over-
all negative nitrogen balance results
(5,47). Ingestion of protein (20–25 g)
and/or essential amino acids (8–12 g)
are required to further stimulate MPS
and yield a positive nitrogen balance
(6,8,38,48,64). Therefore, it is the
combination of resistance exercise
and protein feeding over the course
of several weeks that is commonly
associated with increases in strength
and fat-free mass. Data indicate it is
commonly suggested that protein re-
quirements are subsequently elevated
(1.2–1.8 g$kg
21
$d
21
) in an effort to
stimulate and promote fat-free mass
accretion (X
50.69 kg; 0.47–0.91
kg) (14,32,61,62).
In a similar respect as endurance exer-
cise, a number of factors combine to
impact the protein requirements of
any given individual. One of the big-
gest factors is the training status of the
individual, as studies indicate that in-
dividuals who are untrained or have
a minimal training background may
have higher protein requirements
compared with athletes who have
VOLUME 37 | NUMBER 2 | APRIL 2015
62
been consistently training for over
a year (45,47). Phillips et al. (45) dem-
onstrated in the same group of initially
untrained participants that 8 weeks of
resistance training (6 d/wk) blunted
the acute MPS response seen at base-
line while consuming .1.2 60.6
g$kg
21
$d
21
of protein. Despite this
attenuated response, the authors re-
ported an elevation in resting muscle
protein turnover without affecting
protein balance, which may suggest
that chronic resistance training results
in reduced protein need or that skel-
etal muscle potentially may become
more efficient at metabolizing protein
as trained-state increases. Other
research by Tarnopolsky et al. pro-
vided estimations of protein require-
ments in experienced American
football and rugby athletes by asking
athletes to consume diets that con-
tained low (0.86 g$kg
21
$d
21
), moder-
ate (1.4 g$kg
21
$d
21
), or high (2.4
g$kg
21
$d
21
) amounts of protein. The
authors concluded that higher
amounts of protein ingestion (1.4
and 2.4 g$kg
21
$d
21
) were needed to
prevent compromised rates of MPS,
seen when the lowest amount of pro-
tein was consumed (61). These con-
clusions were supported by the work
ofLemonwhoindicatedtheprotein
needs of previously untrained novice
bodybuilders participating in a 6 d/wk
split-body program (5–8 exercises)
comprised of 4 sets of #10 repetitions
at 70–85% 1 repetition maximum
(RM) ranged between 1.6 and 1.7
g$kg
21
$d
21
(32). Importantly, Lemon
also reported that no further improve-
mentsinoutcomessuchasstrength
and body composition variables (lean
mass and body fat %) when protein
intake increased from 1.35 to 2.6
g$kg
21
$d
21
.
Trained or untrained engaging in
strength/power exercise does
require a daily protein intake above
the RDA to promote a positive mu-
scle protein balance. Even more sup-
port for these recommendations is
available from the International Soci-
ety of Sports Nutrition (ISSN: http://
www.sportsnutritionsociety.org) who
recommended a protein intake of
1.4–2.0 g$kg
21
$d
21
in their position
stand on protein. Moreover, an excel-
lent review was jointly published by
the former American Dietetic Associ-
ation (now called Academy of Nutri-
tion and Dietetics: http://www.
eatright.org), American College of
Sports Medicine (ACSM: http://
www.acsm.org) and Dietitians of Can-
ada (http://www.dietitians.ca), rec-
ommending a protein intake ranging
from 1.2 to 1.7 g$kg
21
$d
21
(52).
Finally, a review by Phillips that used
a statistical regression approach of
previous studies that used nitrogen
balance techniques concluded that
on average, athletes required
a protein intake of 1.19 g$kg
21
$d
21
.
When a 95% confidence interval was
computed, the upper limit of the rec-
ommended protein intake was deter-
mined to be 1.33 g$kg
21
$d
21
.These
amounts are 49–66% greater than
the RDA (43). Table 1 outlines several
published studies that have reported
protein requirements for both endur-
ance and strength/power athletes.
In summary, optimal protein intake is
an important consideration for any ath-
lete. Whether the athlete is performing
predominantly aerobic or resistance-
based modes of exercise, numerous
studies (found in Table 1) indicate that
protein requirements are increased
approximately 1.5–23above the RDA
(;1.2–1.8 g$kg
21
$d
21
). In light of this
recommendation, 2 points need to be
made. First, a 2004 review by Phillips
as well as numerous other articles report
that typical protein intakes of athletes
fall within this recommended range
even without using dietary strategies
to increase protein intake (43). Second,
the notion that higher dietary intakes of
protein are dangerous and detrimental
to an individual’s health is a dated per-
spective that is permeated by popular
media. The interested reader is encour-
aged to view recent articles and reviews
published on the topic (4,30,35).
PROTEIN QUALITY
When first evaluating the quality of
various protein sources, coaches,
athletes, and practitioners must
understand that skeletal muscle re-
quires adequate amounts (8–12 g) of
the essential amino acids to achieve
maximal rates of MPS (65,67).
Although formalized means exist to
objectively compare various protein
sources, a simple and straight-
forward way to assess the quality of
any given protein source is to evalu-
ate where it is derived. In this respect,
a complete protein is any protein
source that provides adequate
amounts and proportions of the pre-
viously mentioned 9 essential amino
acids to facilitate the rebuilding of
proteins found throughout our body.
Alternatively, incomplete proteins
are any protein source with either
inadequate amounts or ratios of one
or more of the 9 essential amino
acids. Typically, if a protein source
comes from an animal, such as beef,
pork, fish, egg, milk (or dairy prod-
ucts), and poultry, such as chicken or
turkey, it is considered a complete
source of protein. Proteins derived
from plants are typically incomplete
meaning they completely lack one or
more of the essential amino acids.
Soy is an extremely popular vegetable
source of protein that consistently
yields low levels of methionine; how-
ever, recent improvements in
manufacturing have produced high-
qualityisolateversionsofsoythat
can be considered “complete” (51).
In this respect, research by Tang
et al. (59) directly compared similar
doses of (21.4–22.2 g) of whey,
casein, or soy at rest and after a single
bout of lower-body resistance exer-
cise. Results indicated that the ana-
bolic response from soy, both at rest
and in response to acute exercise, was
significantly (P,0.05) less than what
was observed with whey but greater
than casein ingestion. At rest, MPS
changes were reported to be 93%
greater after whey consumption in
comparison with casein and 18%
greater after soy consumption (P5
0.067); MPS changes after soy inges-
tion were 69% greater than changes
seen with casein. After exercise, acute
changes in MPS for whey were 122%
Strength and Conditioning Journal | www.nsca-scj.com 63
greater than casein and 31% greater
than soy while soy was found to be
69% greater than casein. Although
“completeness” of a protein is one
factor on which protein sources can
be evaluated, other factors may also
influence the source of protein
considered. For example, religious
customs, allergies, or various degrees
of vegetarianism will impact what
sources are considered. In addition,
individuals who are primarily
concerned with their health may
prefer protein sources with varying
degrees of saturated fat and cho-
lesterol content, whereas other
people interested in weight loss
might choose to consume various
protein sources that are a better fit
within the confines of their desired
dietary approach.
Table 1
Selected published studies that highlight estimated requirements of protein for various exercising populations
Author Reference Type of athlete Recommendation
Endurance athletes
ACSM, ADA, DC (2009) (48) Endurance 1.2–1.4
a
Brouns (1989) (8) Endurance 1.5–1.8
a
Lemon (1997) (33) Endurance 1.2–1.4
a
Friedman & Lemon (1989) (19) Endurance 1.49
a
Genton, Melzer, & Pichard (2010) (20) Endurance 1.1
a
Meredith (1989) (32) Endurance 1.21
a
Tarnopolsky et al. (1988) (59) Endurance 1.6
a
Tarnopolsky (2004) (57) Low/moderate endurance 1.0
a
Tarnopolsky (2004) (57) Elite endurance 1.6
a
Pendergast et al. (2010) (39) Endurance 15%
c
Strength/power athletes
Lemon (1997) (28) Strength/power/speed 1.7–1.8
a
Pendergast et al. (2010) (39) Anaerobic 15
c
Lemon (1992) (31) Strength 1.6–1.7
a
Phillips (2004) (40) Strength 1.19–1.33
a
ACSM, ADA, DC (2009) (48) Strength 1.2–1.7
a
Genton, Melzer, & Pichard (2010) (20) Strength 1.3
a
Slater & Phillips (2011) (51) Strength 1.6–1.7
a
Pendergast et al. (2010) (39) Strength 1.6
a
Tarnopolsky et al. (1992) (58) Strength 1.76
a
Miscellaneous athletes
Helms, Aragon, & Fitschen (2014) (23) Bodybuilding 2.3–3.1
b
Campbell et al. (2007) (10) Physically active 1.4–2.0
b
Kreider et al. (2010) (27) Physically active 1.4–2.0
a
Lemon, Dolny, & Yarasheski (1997) (29) Moderately active 1.1
a
a
Grams/kilogram of body mass/per day.
b
Grams per kilogram of lean body mass per day.
c
% of daily caloric intake.
VOLUME 37 | NUMBER 2 | APRIL 2015
64
DETERMINING PROTEIN QUALITY
Multiple methods exist to objectively
determine the overall quality of pro-
teins found in various food sources.
One of these methods, net protein uti-
lization (NPU) evaluates how much
protein is used by the body per dose
of protein delivered. Therefore, any
protein source that results in greater
amounts of protein being used per
gram of protein is assigned higher
scores, which suggest them to be of
higher quality. Another method, pro-
tein digestibility corrected amino acid
scores (PDCAAS) are currently the
most commonly discussed and
accepted method of determining pro-
tein quality. The PDCAAS method
uses a formula (provided below) to
calculate a score that represents both
the amino acid requirements of the
human body and its ability to digest
the protein (54).
Protein quality using the PDCAAS
method is determined by comparing
the amino acid profile of a specific
food protein against a reference pro-
tein and amino acid profile. The high-
est potential value is 1.0 and the higher
the PDCAAS, the better the protein. A
score of 1.0 subsequently means that
after the protein is digested, the test
protein provides 100% (or more) of
essential amino acid requirements.
Table 2 provides NPU and PDCAA
scores for different protein sources.
Recently, Canadian scientists dis-
cussed a shortcoming relative to the
PDCAAS method of “no value can
be greater than 1.00.” In fact, they per-
formed their own calculations and indi-
cated that many forms of protein,
including milk solids (1.21), casein
(1.23), whey (1.21), and soy (1.04),
would all have values above 1.00 (46).
Simply based on evaluation of
PDCAAS and consideration of no
other factors, these calculations help
to highlight differences in protein qual-
ity between sources of protein.
PRODUCTION METHODS AND
TECHNIQUES
Often athletes supplement their diet
with protein powders in an attempt
to meet protein requirements, add
convenience, and also to take advan-
tage of any impact offered by protein
or nutrient timing (3,26,27). In general,
a protein concentrate will have any-
wherefrom34to89%proteinby
weight (meaning it will have 34–89 g
of protein per 100 total grams), where
an isolate (i.e., whey or soy protein
isolate) is greater than 90% weight
($90 g of protein per 100 total grams)
with the remaining percentage com-
prising fat and carbohydrate. Hydro-
lyzate formulations are also popular
(66) and are typically produced by
exposing the protein to chemical or
enzymatic hydrolysis, shortening the
protein chain into smaller, more read-
ily digestible peptide chains (39,57).
Studies are available that support use
of a hydrolyzate for time-trial perfor-
mance (53) and recovery (34). How-
ever, more research is needed to
determine the impact of production
methods (concentrates versus isolates
versus hydrolyzates) on physiological
outcomes, such as improvements
in endurance, recovery, maximal
strength, and body composition adap-
tations, seen with both aerobic and
anaerobic training methods. A final
salient point should be made toward
the impact of various types of food
processing and production that may
fundamentally alter the digestibility
as well as the overall bioavailability
of a protein’s constituent amino acids.
In this respect, little data are available
to document any impact these
changes may have on outcomes
related to sports performance, but
evidence does exist of amino acid
alteration and breakdown occurring
as part of food production and pro-
cessing (36). For example, fresh pro-
tein sources exhibit more favorable
amino acid profiles when compared
with identical protein sources that
have undergone some form of pack-
aging and processing. Tuna has been
seen to lose protein content during the
canning process (13). In addition, it
has been observed that packaged
foods, particularly those intended to
have a prolonged shelf-life (ready-to-
eat entre
´es, etc) undergo considerable
decrements in amino acid content as
storage time increases (36).
PROTEIN SOURCE
WHEY PROTEIN
Whey protein is the liquid portion of
milk produced as part of the cheese-
making process and is commonly pro-
duced into concentrate, isolate, or
hydrolyzate versions. Whey protein is
a complete protein and typically ex-
hibits the highest levels of the essential
amino acids (including leucine) and
the greatest amino acid content over-
all. Although the collective dosing of
all of the essential amino acids is
important (65,67), leucine has garnered
particular interest due to its ability to
favorably promote activation and sig-
naling of intracellular events related to
muscle hypertrophy (1,16). On inges-
tion, whey protein is very soluble, re-
sulting in rapid digestion and
a powerful ability to stimulate MPS,
but limited ability to control muscle
protein breakdown (7,18). In addition,
whey protein exhibits high concentra-
tions of the amino acid cysteine
(a powerful antioxidant), as well as
a mixture of immunoglobulins, growth
factors (IGF-1, TGF-1, and others),
and other fractions (lactoferrin and
PDCAAS ð%Þ5mg of limiting amino acid in 1 g of test protein
mg of same amino acid in 1 g of reference protein 3fecal true digestibility ð%Þ3100:
Strength and Conditioning Journal | www.nsca-scj.com 65
lactoperoxidase) that may confer addi-
tional benefits.
Several studies have clearly shown that
delivering a dose of whey protein iso-
late ranging from 20 to 40 g is an effec-
tive means to maximally stimulate
MPS in healthy young (20–25 years)
and older (65–75 years) participants
(24,59,63). Burke et al. reported a 2-fold
greater increase in lean body mass and
greater strength increases when whey
protein was ingested (in comparison
with carbohydrate ingestion) by 42
young men (18–31 years; 80–87.6 kg)
who reported 4.2–5.6 years (4–5 d/wk
and 7.1–8.3 h/wk) of resistance training
experience (10). Similar outcomes
(greater increases in lean mass and
strength) were reported when identical
doses (1.5 g$kg
21
$d
21
;;122 g/d) of
whey protein isolate or casein protein
were ingested by resistance training
young men (26.5 66 years, 81.9 68
kg) in a placebo-controlled, double-
blind manner (15). The strongest sup-
port for whey comes from a 2009 review
article prepared by Phillips et al.,
Table 2
Estimated net protein utilization and protein digestibility corrected amino acid scores for multiple sources of protein
(sources used: (10,43))
Protein source EAA content (g/100 g) NPU PDCAA Overall comments
Whey 63–66 92 1.15 Liquid portion of milk and commonly produced in concentrate,
isolate, and hydrolyzate versions. A high-quality complete protein
source. Exhibits a high speed of digestion that translates into sharp
increases in amino acid levels and robust increases in MPS. Minimal
impact on protein breakdown and exhibits multiple bioactive
fractions that may aid in immune function, antioxidant status, and
other health-related attributes
Casein 45–49.3 78 1.23 Thick or curd portion of milk and insoluble in acidic conditions, which
leads to clumping or gel forming in stomach and digesting slower.
High-quality complete source of protein that exhibits powerful
ability to prevent protein breakdown and marginal impact on
protein synthesis
Milk protein 48.9 86 1.21 Combination of whey and casein and mandated by FDA to resemble
the protein profile of bovine milk. Limited research is available, but
overall is comprised of high-quality protein sources. Research on
protein blends show promise
Egg 50 72 1.00 High-quality protein source and used as reference protein for NPU
determination. A 20-g dose of egg protein maximally stimulates
MPS after resistance exercise. Contains high amounts of albumin,
a key protein for transport throughout body
Soy 49–62 72 1.04 Extracted from soybean plant and considered a good source of
protein. Soy has excellent digestibility, intermediate digestion
speed, and excellent antioxidant profile. Excellent protein source
for vegetarians and contains isoflavone glucosides, which are
linked to positive health outcomes. Isolate versions should be
considered complete and excellent sources of protein
Flesh proteins — — 0.85–0.92 Beef, poultry (chicken, turkey, and game fowl) and fish are all
complete proteins and can contain other healthful nutrients,
including iron, B vitamins, and essential fatty acids. Need for proper
storage and lack of convenience may preclude use
Vegetable
proteins
— — 0.74 Lower quality (incomplete) protein sources, complementary
combinations are needed. Have high contents of many other
vitamins, minerals, and fibers
Gelatin — NA 0.08 Gelatin is produced from the collagen found oftentimes inside the
hide and bones of both swine and bovine. Contains protein,
collagen, and various amino acids. The overall protein quality of
gelatin is extremely poor
EAA 5Essential Amino Acids; MPS 5muscle protein synthesis.
VOLUME 37 | NUMBER 2 | APRIL 2015
66
who combined the results from several
studies and indicated that whey protein
ingestion was responsible for the
greatest (;3 kg) increase in lean
mass (DXA was used in 7 studies,
hydrostatic weighing was used in 2
studies), when compared with out-
comes when soy and casein were in-
gested (46).
MICELLAR CASEIN
Micellar casein protein is the thick or
“curd” portion of milk produced as
part of the cheese-making process.
Casein is classified as a high-quality
complete source of protein with high
levels of the essential amino acids.
Casein digests slower than whey
due to its insoluble characteristics in
gastric solutions, leading to a release
of amino acids into the bloodstream
(7,18). Consequently, the slower
release of amino acids promotes
a prolonged positive net balance of
proteinthatresultinmodeststimula-
tion of protein synthesis, but
a powerful attenuation of protein
breakdown (7,18).
A study by Tang et al. (59) compared
acute MPS response at rest and after
resistance exercise and reported that
in comparison with similar amounts
of whey protein and soy protein,
casein ingestion exhibited the small-
est responses. However, Soop et al.
(56) concluded that when blended
with other proteins, casein’s contri-
butionresultedinsignificantly
greater amino acid accretion rates
when measurements were extended
out for several hours. In addition,
Reidy et al. (50) reported favorable
outcomes for a blend of whey, casein,
and soy on their impact of increas-
ing MPS.
MILK PROTEIN
The Food and Drug Administration
(FDA) states that products contain-
ing milk protein concentrate (or iso-
late) should contain all of the
proteins naturally found in milk,
and these proteins should exist with
the same ratios as what are naturally
found in milk. Briefly, whole milk is
approximately 87% water and 13%
solids. The solid portion is 27% pro-
tein that further breaks down into
a natural ratio of 80% casein and
20% whey protein. Currently, limited
research is available on milk proteins
in conjunction with exercise. Two
studies compared blends of whey
and casein (1 study used fat-free milk
[80% casein, 20% whey] (23),
whereas the other study provided
40gofwheyand8gofcasein(29)
against carbohydrate and protein
control groups in conjunction with
heavy resistance training programs
(10–12 weeks of upper- and lower-
body workouts targeting all major
muscle groups, 4–5 days per week
using 3–4 sets of 8–12RM loads with
2–3 minutes of rest between sets).
Both studies reported greater
improvements in fat-free mass
(determined through DXA) and
strength (23,29).
SOY
Soy protein is the most popular vege-
table protein (extracted from the soy-
bean plant) and is considered a good
source of protein, whereas isolated ver-
sions (.90% protein by weight) are
excellent and should be considered
a complete protein. Soy exhibits an
intermediate digestion speed (faster
than casein, slower than whey) and
an excellent antioxidant profile (high
levels of isoflavones, saponins, and cop-
per) (50). Vegetarians, and particularly
vegan athletes, should strongly con-
sider adding soy to their dietary regi-
men to offset increased protein needs
and their relative lack of essential
amino acid content. Soy contains iso-
flavone glucosides (55), which are
linked to favorable outcomes related
to bone health and cholesterol metab-
olism (68).
Recent research involving young men
(22.8 63.9 years; 179.7 65.1 cm;
86.6 613.9 kg) compared the protein
synthesis responses to identical doses
of whey protein hydrolyzate, micellar
casein, and soy protein isolate (each
contained 10 g of essential amino
acids) at rest and after resistance exer-
cise (59). The acute exercise bout
consisted of 4 sets of unilateral leg
press and knee extension exercises
at a 10–12RM load with 2 minutes
of passive rest between sets. Ingestion
of assigned supplements occurred
immediately after completion of post-
exercise assessments. Rates of MPS
increased for all 3 protein sources
when drink ingestion occurred at rest
and when combined with resistance
exercise (59). Under both resting
and postresistance exercise condi-
tions, whey protein exhibited the
most robust increases in MPS. How-
ever, in comparison with casein, soy
ingestion stimulated 64% greater MPS
levels at rest and 69% greater levels
when ingested after resistance exer-
cise (59). Of particular interest, 27
untrained healthy participants (18
women and 9 men) aged between
18–35 years of age completed a 6-
week study that involved resistance
training (4–5 sets of 6–12 repetitions
at 60–90% 1RM across a 4-day per
week split-body routine) and either
soy or whey protein supplementation
(both protein sources were dosed at
1.2 g/kg body mass plus 0.3 g of
sucrose/kg body mass) in a double-
blind, placebo-controlled manner.
Daily supplementation was split into
3 equal doses (0.5 g/kg) provided
before each workout, after each work-
out and before going to bed. It was
determined that either protein source
ledtosignificantimprovementsin
lean mass (determined using DXA)
and strength, with no significant dif-
ferences found between the 2 protein
sources (12).
FLESH PROTEINS
When used as a supplement to the
diet, whey, casein and soy dominate,
but ingestion of flesh proteins (beef,
poultry, and fish) is quite common
within a Western diet. As indicated
in Table 1, many flesh proteins
exhibit excellent essential amino acid
profiles and all are considered com-
plete protein sources. Within
research, flesh proteins are less com-
monly used, and this is primarily due
to the lack of convenience, as well as
need for preparation and proper
Strength and Conditioning Journal | www.nsca-scj.com 67
storage, whereas whey, casein, and
soy proteins are readily powdered
and mixed into solution before inges-
tion. Studies involving flesh proteins
in relation to outcomes that directly
link to exercise, such as fat-free mass
accretion, strength changes, or
changes in MPS are relatively scarce.
Symons et al. (58) published a study in
young (41 68 years, n 510) and older
(70 65 years, n 510) healthy, phys-
ically active (not athletically trained)
individuals and reported that beef
ingestion (4 ounces, 113 g, ;10 g of
essential amino acids) was able to
increase MPS rates to levels similar
to what is seen with other high-
quality sources of protein.
OPTIMAL PROTEIN DOSE
An important question for every coach
or athlete to ask is, “How much protein
should I consume in one sitting or one
dose?” In 2009, Moore et al. (40) had 6
young (22 62 years; 86.1 67.6 kg;
1.82 60.1 m) active men ($4months
of training experience) ingest 0, 5, 10, 20,
or 40 g of whole egg protein after com-
pleting a single bout of lower-body
resistance exercise and having MPS
rates determined. Four sets of 8–10 rep-
etitions completed to muscular failure
were performed on bilateral machine-
based exercises (leg press, knee exten-
sion, leg curl) with approximately 2
minutes of rest between sets. Each set
was completed within 25 seconds.
Progressive increases in MPS were
found up to the 20-g dose, but no fur-
ther increase in MPS was seen from 20
to 40 g. Furthermore, rates of protein
oxidation significantly increased after
the 40-g dose, which is used as an
indicator of excessive protein intake
(42,70). In 2012, Yang et al. (69) exami-
ned changes in MPS after providing 0,
10, 20, or 40 g of whey protein isolate
to 37 elderly men (71 64 years; 26 6
2.7 kg/m
2
) both at rest and after
completing a single bout of lower-
body resistance exercise. The exercise
bout consisted of 3 sets of unilateral
knee extension at a load that approxi-
mated a 10RM. Each set was completed
within 25 seconds and two-minutes
rest was given between sets. In resting
conditions, a 20-g dose, again, was the
lowest dose that stimulated maximal
rates of MPS, whereas a 40-g dose
stimulated MPS to the greatest extent
when ingested after a single bout of
lower-body resistance exercise.
Smaller doses (5–10 g) robustly increase
MPS, but achieved rates might not reach
maximal levels. Thus, if the athlete does
not have the opportunity to maximally
dose with protein or a coach or school
cannot afford such provisions, smaller
protein doses (5–10 g) can be viewed
as a “better than nothing” approach
(41). Importantly, repeated studies indi-
cate that elderly muscle (65–70 years) is
more resistant to the stimulating effect of
certain amino acids (25), in particular leu-
cine, and this needs to be taken into
account when considering optimal dose
for an aged client or athlete. Briefly, leu-
cine content has been demonstrated in
the literature to favorably promote
greater activation and signaling of intra-
cellular events that promote muscle
hypertrophy (1,16). In summary, recent
studies in both young (20–25 years) and
elder (65–75 years) participants indicate
that the optimal dose of protein lies
somewhere close to 20 g with slightly
higher amounts needed in elder popula-
tions when combined with single bouts of
unilateral and bilateral resistance exercise
using 3–4 sets of 8–10 RM loads (41,69).
Moreover, it is important to keep in mind
that the essential amino acid content (and
composition) is likely the driving force
behind these observed increases in
MPS leading one to conclude that a dose
of 8–10 g of essential amino acids (;20 g
of whey protein isolate) should be con-
sidered optimal (11,17,46,60).
CONCLUSIONS
One of the 3 macronutrients, protein,
operates primarily to repair, regenerate,
and synthesize new proteins across the
human body. Multiple published reports
(11,43,52,61) indicate that individuals
who regularly perform exercise training
have an increased protein requirement
from the RDA of 0.8 g$kg
21
$d
21
to
1.2–1.8 g$kg
21
$d
21
.Wheyandcasein
are the most commonly used supple-
mental proteins, whereas flesh proteins
and dairy sources are routinely ingested
in a Western diet, all of which are con-
sidered excellent sources of protein. Rat-
ings of protein quality predominantly
consist of NPU and PDCAA scores
(.1.0) that, respectively, provide indica-
tions of the extent to which ingested
nitrogen is used and incorporated into
tissue protein or the relative amino acid
content of a given protein source. An
optimal dose of protein is an amount
that maximally stimulates rates of MPS
without significant increases in protein
oxidation; studies indicate an optimal
dose lies somewhere around 20 g per
dose for younger individuals and
between 20 and 40 g for older/elder in-
dividuals. In conclusion, the following
take-home points are provided:
Individuals who regularly perform
exercise training have an increased pro-
tein requirement from the RDA of 0.8
to 1.2–1.8 g$kg
21
$d
21
(11,43,52,61).
High-quality sources of protein are
recommended. Complete protein
sources are those which provide all
of the essential amino acids in ade-
quate amounts and ratios approxi-
mately to human metabolic needs.
Animal sources (beef, chicken, tur-
key, fish, milk, cheese, dairy, egg,
etc.) of protein are considered com-
plete proteins and are recommen-
ded. Plant sources of protein are
missing one or more essential amino
acids; isolates of soy are the only
exception.
Supplemental sources of high-
quality proteins, such as whey and
casein are popular, but not necessar-
ily required. However, regular provi-
sions of amino acids promote
a positive muscle protein balance
and the added convenience of sup-
plemental proteins may be of benefit.
An optimal protein dose is one that
stimulates MPS and promotes a pos-
itive balance of muscle protein. Opti-
mal doses of protein are considered
to be approximately 20–25 g in
younger individuals and 20–40 g
for older individuals.
Conflicts of Interest and Source of Funding:
The authors report no conflicts of interest
and no source of funding.
VOLUME 37 | NUMBER 2 | APRIL 2015
68
Trisha A.
McLain is a PhD
student in the
Health, Exer-
cise, and Sports
Science Depart-
ment at the
University of
New Mexico. Tri-
sha currently
serves as a student
representative for
the NSCA’s Nutrition, Metabolism and
Body Composition Special Interest Group
and the International Society of Sports
Nutrition.
Kurt A. Escobar
is a PhD student
and teaching
assistant in the
Department of
Health, Exercise,
and Sports Sci-
ences at the Uni-
versity of New Mexico.
Chad M.
Kerksick is cur-
rently an Assis-
tant Professor of
Exercise Science
in the Exercise
Science department in the School of Sport,
Recreation and Exercise Sciences at
Lindenwood University.
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