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The Journal of Nutrition
Critical Review
The Skeletal Muscle Anabolic
Response to Plant- versus
Animal-Based Protein Consumption
1
Stephan van Vliet,
2,3
Nicholas A Burd,
2,3
and Luc JC van Loon
3
*
2
Department of Kinesiology and Community Health, University of Illinois at Urbana-Champaign, Urbana, IL; and
3
Department of
Human Movement Sciences, Faculty of Health, Medicine, and Life Sciences, School for Nutrition and Translational Research in
Metabolism (NUTRIM), Maastricht University, Maastricht, Netherlands
Abstract
Clinical and consumer market interest is increasingly directed toward the use of plant-based proteins as dietary
components aimed at preserving or increasing skeletal muscle mass. However, recent evidence suggests that the
ingestion of the plant-based proteins soy and wheat results in a lower muscle protein synthetic response when compared
with several animal-based proteins. The possible lower anabolic properties of plant-based protein sources may be
attributed to the lower digestibility of plant-based sources, in addition to greater splanchnic extraction and subsequent
urea synthesis of plant protein–derived amino acids when compared with the ingestion of animal-based proteins. The
latter may be related to the relative lack of specific essential amino acids in plant- as opposed to animal-based proteins.
Furthermore, most plant proteins have a relatively low leucine content, which may further reduce their anabolic properties
when compared with animal proteins. However, few studies have actually assessed the postprandial muscle protein
synthetic response to the ingestion of plant proteins, with soy and wheat protein being the primary sources studied.
Despite the proposed lower anabolic properties of plant vs. animal proteins, various strategies may be applied to augment
the anabolic properties of plant proteins. These may include the following: 1) fortification of plant-based protein sources
with the amino acids methionine, lysine, and/or leucine; 2) selective breeding of plant sources to improve amino acid
profile; 3) consumption of greater amounts of plant-based protein sources; or 4) combining the ingestion of multiple
protein sources to provide for a more balanced amino acid profile. However, the efficacy of such dietary strategies on
postprandial muscle protein synthesis remains to be studied. Future research comparing the anabolic properties of a
variety of plant-based proteins should define the preferred protein sources to be used in nutritional interventions to support
skeletal muscle mass gain or maintenance in both healthy and clinical populations. J Nutr doi: 10.3945/jn.114.204305.
Keywords: plant protein, animal protein, muscle mass, vegetarian, exercise, aging
Introduction
Skeletal muscle mass is regulated via changes in both muscle
protein synthesis (MPS)
4
and muscle protein breakdown. Of the
2, the stimulation of MPS is assumed to be the primary variable
responsible for regulating the maintenance or gain in skeletal
muscle mass (1–4). The 2 main anabolic stimuli that augment
MPS are food intake, in particular dietary protein, and physical
activity (5–7). In addition to providing substrates for newly (de
novo) synthesized proteins, dietary protein–derived essential
amino acids (EAAs) also act as signaling molecules to induce
the MPS response (8). However, the MPS response to food intake
is short lived and appears to last for only 4–5 h after ingestion (9).
Therefore, the consumption of protein in regular intervals
throughout the course of the day is necessary to maximize daily
muscle protein accretion in both postexercise (10) and resting (11)
conditions. Physical activity or exercise performed before food
intake has been shown to sensitize skeletal muscle tissue to the
anabolic properties of protein ingestion (12). Consequently, more
of the dietary protein–derived amino acids (AAs) will be used for
de novo MPS when exercise is performed before protein feeding.
The exercise-induced increase in anabolic sensitivity to dietary
protein–derived AAs has been shown to be sustained for up to
24 h after exercise (13). Therefore, regular physical activity or
exercise combined with adequate dietary protein consumption is
required to preserve or increase skeletal muscle mass and strength (14).
1
Author disclosures: S van Vliet, NA Burd, and LJC van Loon, no conflicts of
interest.
* To whom correspondence should be addressed. E-mail: l.vanloon@
maastrichtuniversity.nl.
4
Abbreviations used: AA, amino acid; EAA, essential amino acid; LBM, lean
body mass; MPS, muscle protein synthesis; PDCAAS, Protein Digestibility
Corrected Amino Acid Score; QMP, quality maize protein; WHO/FAO/UNU,
World Health Organization/Food and Agriculture Organization of the United
Nations/United Nations University.
ã2015 American Society for Nutrition.
Manuscript received September 24, 2014. Initial review completed November 20, 2014. Revision accepted June 30, 2015. 1of11
doi: 10.3945/jn.114.204305.
The Journal of Nutrition. First published ahead of print July 29, 2015 as doi: 10.3945/jn.114.204305.
Copyright (C) 2015 by the American Society for Nutrition
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Many health care professionals, sports nutritionists, and
athletes use the knowledge gained from studies that measure the
dynamics of postprandial MPS to develop nutritional strategies
to facilitate maintenance or gain in skeletal muscle mass, muscle
function, and/or performance in both clinical and athletic
populations. So far, most studies have assessed the MPS response
after the ingestion of free AAs (1–4, 15) or intact animal-based
protein sources, such as milk (and its constituents whey and
casein) (12, 15–25), beef (26–29), and egg (30) protein. In
contrast, few studies have assessed the impact of plant-based
protein ingestion on the postprandial MPS response (19, 21, 31,
32). Moreover, soy protein is the only plant-based protein that
has been studied for its postprandial MPS response in humans.
Studies assessing the postprandial MPS response to soy protein
ingestion have shown that the ingestion of amounts ranging
from 17.5 to 40 g soy protein do not increase MPS to the same
extent as the ingestion of isonitrogenous amounts of whey
protein (19, 21), skimmed milk (32), or beef (31), both in resting
and postexercise conditions.
It is surprising that only soy protein has been applied in these
types of studies, because there is considerable interest in the
potential of using plant-based proteins to support muscle mass
maintenance and/or growth. This belief is shown by the number
of recent publications studying the impact of plant-based protein
ingestion on the skeletal muscle anabolic response, provided
either as an isolated protein source (16, 19, 21, 31–33) or as part
of a protein blend (i.e., a combination of both plant- and animal-
based proteins) (33–37).
From the standpoint of global sustainability, plant-based
foods are proposed to be advantageous over animal-based foods
(38–40). It has been suggested that the production of plant-
based foods requires less water, land, and energy. This may
ultimately pose less environmental burden and lower the
financial cost of food production. These notions likely explain
the increasing interest in the potential of using plant-based
protein sources in clinical feeding formulas and sports nutrition
supplements. In addition, there is a growing consumer market
interest in plant-based foods, including plant-based meat sub-
stitutes, entre´
es, convenience foods, and nondairy milks (41,
42). Finally, most of the dietary protein consumed worldwide is
derived from plant- as opposed to animal-based sources (;58%
vs. 42%, respectively; Table 1) (43). Given the above, more
comprehensive insight into the functional capacity of plant-
based proteins to stimulate postprandial MPS is warranted. The
purpose of this review is to assess the potential of plant-based
protein sources to stimulate postprandial MPS and to evaluate
their ability to support muscle mass maintenance/gain in both
healthy and clinical populations.
Protein Quality
Dietary protein quality may refer to the ability of a protein
source to support the increase in MPS after ingestion. Given the
resources needed to compare the in vivo skeletal muscle anabolic
properties of various protein sources, only a few research groups
use contemporary stable isotope AA tracer methodology to
directly assess the capacity of various dietary proteins to
stimulate postprandial MPS. This likely explains the relative
lack of studies comparing the postprandial MPS response to the
ingestion of a wide variety of (plant)-protein sources.
Various alternative measures exist to evaluate dietary protein
quality. The most widely adopted indexes are the Protein
Digestibility Corrected Amino Acid Score (PDCAAS) (Table 2)
(46) and, more recently, the Digestible Indispensable Amino
Acid Score (47). However, both the PDCAAS and Digestible
Indispensable Amino Acid Score do not provide insight into the
true skeletal muscle ‘‘anabolic potential’’ of a specific dietary
protein source. Instead, these ranking methods provide insight
into the minimum amounts of nitrogen and AAs needed to
prevent whole-body protein deficiency. For instance, soy beans
have a high PDCAA score of 0.91 and are on par with beef,
which scores 0.92 (Table 2). On the basis of their respective
PDCAASs, one could expect soy beans to be as effective in
stimulating MPS as beef. However, recent work by Phillips (31)
demonstrated that beef is superior in stimulating postprandial
MPS when compared with an isonitrogenous amount of a soy-
based beef replacement. In agreement is the work comparing
isolated soy protein with dairy proteins, which showed that,
despite a similar PDCAAS (Table 2), these sources differ in their
ability to stimulate MPS in both resting and postexercise
conditions (19, 21, 32).
On the basis of AA tracer studies it has been established that
the potential of a protein source to stimulate MPS depends on
both the dietary protein digestion and AA absorption kinetics of
TABLE 1 Energy and protein consumption per capita per day
1
Energy
intake, kJ/d
Energy
intake, kcal/d
Total protein
intake, g/d
Animal protein intake, %
total protein intake
Plant protein intake, %
total protein intake
Worldwide 10.6 2542 78 42 58
Region
Africa 10.7 2560 67 23 77
Asia 11.3 2706 75 34 66
Americas 13.4 3205 92 56 44
Europe 14.1 3362 102 57 43
Oceania 13.9 3312 98 63 37
Economic class
2
Low income 10.0 2393 63 21 79
Low-middle income 10.9 2597 68 34 66
High-middle income 12.1 2907 83 46 54
High income 13.7 3296 101 58 42
1
Daily energy intake, total daily protein intake, and the percentage of dietary protein intake provided by animal- and plant-based protein
sources. The data are ranked from high to low by means of plant protein intake (%) and clustered by region or economic class. Data are
from reference 43.
2
Data on economic class are from reference 44.
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the ingested food source (17, 22–24, 48) as well as the (essential)
AA composition (2, 49), with the leucine content being of
particular relevance (50). In the following sections, these factors
will be discussed with respect to plant-based proteins.
Protein digestion and absorption kinetics. With the use of
intrinsically labeled protein sources (which have the AA tracer
directly incorporated into the peptide chain of the protein), our
laboratory demonstrated that the protein digestion and absorp-
tion kinetics of the ingested protein source are important for
modulating postprandial MPS (17, 24). In particular, we
demonstrated that enzymatic hydrolysis of intact micellar casein
facilitated dietary protein digestion and AA absorption in vivo in
humans (24). The ingestion of hydrolyzed vs. intact casein
resulted in higher exogenous phenylalanine appearance rates,
increased plasma AA availability, and a tendency to further
increase postprandial MPS rates (24). Our work and that of
others (51, 52) highlight that the protein digestion rate of a food
source (fast vs. slow) is an important and independent predictor
of postprandial MPS. In addition, this work has shown that a
total of 50–70% of the dietary protein–derived AAs from milk
and beef protein become available in systemic circulation when
assessed over a 5- to 6-h postprandial period (17, 22, 29). After
being released into the systemic circulation, these dietary
protein–derived AAs have the potential to facilitate the post-
prandial increase in MPS.
For both methodologic and financial reasons, intrinsically
labeled plant proteins are not widely available. Therefore, there
are not many quantitative data describing the digestion and
absorption kinetics of various plant-based proteins. However,
the sparse data available suggest that the reduced ability of soy
protein to support MPS as opposed to whey (21), milk (32), and
beef (31) may in part be attributed to differences in digestion and
absorption kinetics.
In general, it appears that plant-based protein sources may
exhibit lower digestibility than animal-based proteins (53). The
digestibility of the protein source has been defined as the
proportion of dietary protein–derived AAs that is effectively
digested and absorbed, thus becoming available in a form
suitable for body protein synthesis (54). Animal-based protein
sources, including dairy, eggs, and meat, are highly digestible
(>90%) (53). Depending on the processing method (54) and/or
the presence of various ‘‘antinutritional’’ factors (i.e., compounds
in the food source that interfere with digestion and absorption
of the available protein) (55), plant-based sources such as maize,
oat, bean, pea, and potato tend to exhibit lower digestibility
than do animal-based sources, with values ranging from 45%
to 80% (53). However, once freed from these antinutritional
factors, purified plant protein sources such as soy protein isolate,
pea protein concentrate, and wheat gluten display a digestibility
that is similar to that of animal-based protein sources (>90%)
(53).
In addition to digestibility issues, it was reported that the
dietary protein–derived AAs from the plant-based proteins soy
and wheat are more readily converted to urea when com-
pared with the ingestion of milk proteins (56–59). This would
ultimately lower the potential of these plant-based protein
sources to stimulate the skeletal muscle anabolic response. The
exact reason(s) why the ingestion of these plant-based proteins
leads to greater urea synthesis is not fully understood but may
relate to the relative lack of specific EAAs. EAAs cannot be
synthesized de novo in the human body and therefore have to be
supplied through the diet. It is hypothesized that the ingestion of
an ‘‘unbalanced’’ EAA profile results in an unfavorable AA
mixture for gut protein synthesis, which leads to more of the free
AAs to enter the portal vein to hepatic tissue (60). Ultimately,
higher free AA concentrations in the liver serve as a stimulus for
ureagenesis, resulting in less of the dietary-derived AAs becom-
ing available in the systemic circulation to support the post-
prandial increase in MPS. This notion seems to be confirmed by
animal data comparing the ingestion of crystalline AA mixtures,
reflecting the AA composition of casein, egg, and wheat protein
(61). It was found that the ingestion of the representative wheat
mixture resulted in higher free AA concentrations in the liver
and subsequent higher ureagenesis than the ingestion of the
representative casein and egg mixture. Given that all mixtures
were provided as free AAs, differential digestion rates could not
have influenced these findings. Therefore, the observed differ-
ences in urea production are likely attributed to differences in
EAA content of the different protein sources (61). In addition,
recent human data by Yang et al. (21) showed that the ingestion
of soy protein results in higher AA oxidation rates than does
whey protein ingestion. This suggests that more of the AAs
derived from soy protein were directed to urea synthesis when
compared with the AAs derived from the consumption of an
isonitrogenous amount of whey protein. As a consequence, less
of the soy protein–derived AAs were available to stimulate MPS
when compared with the whey protein–derived AAs. Because
the protein digestion rates of both sources are equal (19), these
findings are likely attributed to the differences in EAA content
(Table 3,Figure 1). On the basis of the above, we speculate that
the EAA composition of a protein source is predictive of its
ability to stimulate skeletal muscle anabolism and that all EAAs
should be present in ‘‘sufficient’’ quantities to optimally stimu-
late postprandial MPS.
The lysine and/or methionine contents are lower in plant-
based proteins than in animal-based proteins (69). According to
recommendations by the World Health Organization/Food and
Agriculture Organization of the United Nations/United Nations
University (WHO/FAO/UNU) (46), daily dietary requirements
for lysine are ;30 mg/(kg body weight d) per day or ;4.5% of
the content of the total amount of dietary protein consumed. For
methionine, daily dietary needs are estimated to be ;10 mg/(kg
body weight d) or ;1.6% of the content of the total amount of
dietary protein consumed (46). Note that these percentage
requirements are based on a recommended adult protein intake
of 0.66 g/(kg body weight d). Upon comparison, it becomes
TABLE 2 PDCAAS of common protein foods
1
Source PDCAAS
Milk 1.00
Whey 1.00
Egg 1.00
Soy protein isolate 1.00
Casein 1.00
Beef 0.92
Soy 0.91
Pea 0.67
Oat 0.57
Whole wheat 0.45
1
The PDCAAS is a method sometimes used to assess the ability of a given protein
source to support skeletal muscle anabolism. This method factors in amino acid
contents and digestion kinetics. The PDCAASs of the various protein sources are
ranked from high to low, where a higher score suggests a greater ability to support
skeletal muscle anabolism. PDCAAS, Protein Digestibility Corrected Amino Acid
Score. Data are from reference 45.
Animal- vs. plant-based proteins 3 of 11
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clear that the amounts of lysine and/or methionine present in
plant-based protein sources are lower than in animal-based
protein sources and human muscle protein (Table 3, Figure 2).
The plant-based sources wheat and maize are particularly low in
lysine, with 2.8% as opposed to the ‘‘required’’ 4.5% of lysine.
Of course, the diet would only be considered low in lysine based
on the assumption that all daily protein consumed is derived
from wheat and/or maize. Oat, hemp, and rice are protein
sources that also provide relatively low amounts of lysine (with
contents of 4.2%, 4.1%, and 3.8%, respectively), especially
when compared with animal-based sources and human muscle
protein that have lysine contents >7%. Interestingly, the plant-
based protein sources that are sufficient in lysine appear to be
relatively low in methionine (Table 3, Figure 2). For instance,
lentil and soy protein have particularly low methionine contents
of 0.9% and 1.3% when compared with animal-based proteins
and human muscle protein (>2.2%). The plant-based protein
sources black bean, wheat, oat, potato, and pea can still be
considered sufficient in methionine according to the WHO/FAO/
UNU (46) recommendations because these protein sources have
a methionine content of ;1.6%. However, the methionine
content of these plant-based sources also remains well below the
methionine content of animal-based protein sources and human
muscle protein (>2.2%). Rice, hemp, and maize protein have
methionine contents that are closer to animal-based protein
sources and human muscle protein, with contents of ;2.2%.
However, as mentioned previously, these specific sources are
relatively low in lysine. Worth mentioning is the recent appear-
ance of many new unconventional plant-based protein sources,
including aquatic algae (e.g., spirulina), oilseeds, and single cell
fungal proteins (e.g., mycoprotein). Although some of these
sources may be higher in lysine and methionine than other plant
protein sources, they still remain well below the amount in most
animal-based protein sources (Table 3, Figure 2). For instance,
spirulina has a lysine content of 5.2% and a methionine content
TABLE 3 Amino acid concentrations of various common dietary protein sources
Source
1
Essential amino acids, %
total protein
Leucine, %
total protein
Lysine, %
total protein
Methionine, %
total protein
Plant sources
Spirulina
2
41 8.5 5.2 2.0
Mycoprotein
3
41 6.2 6.7 1.5
Lentil
2
40 7.9 7.6 0.9
Quinoa
2
39 7.2 6.5 2.6
Black bean
2
39 8.4 7.3 1.6
Maize
2
38 12.2 2.8 2.1
Soy
4
38 8.0 6.2 1.3
Pea
5
37 7.8 6.3 1.6
Rice
2
37 8.2 3.8 2.2
Oat
2
36 7.7 4.2 1.9
Hemp
6
34 6.9 4.1 2.3
Potato
2
33 5.2 5.7 1.7
Wheat
7
30 6.8 2.8 1.9
Animal sources
Whey
8
52 13.6 10.6 2.3
Milk
8
49 10.9 8.6 2.7
Casein
8
48 10.2 8.1 2.7
Beef
8
44 8.8 8.9 2.5
Egg
9
44 8.5 7.1 3.0
Cod
10
40 8.1 8.8 3.0
Human muscle
8
45 9.4 8.7 2.2
1
Human muscle is provided as a reference standard.
2
Data from reference 62.
3
Data from reference 63.
4
Data from reference 19.
5
Data from reference 64.
6
Data from reference 65.
7
Data from reference 66.
8
Data from reference 67.
9
Data from reference 30.
10
Data from reference 68.
FIGURE 1 EAA concentrations of various protein sources. Differ-
entiation is made between plant- and animal-based protein sources.
Human muscle is provided as a reference standard. The dashed line
represents the EAA concentration in whey protein. This line provides a
comparison of the protein source most abundant in EAAs (i.e., whey)
with various other protein sources. EAA, essential amino acid.
4 of 11 van Vliet et al.
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of 2.0%. Although this would be in excess of the amount
‘‘required,’’ as per WHO/FAO/UNU (46) recommendations, the
lysine contents still remain well below the amounts found in
animal-based proteins (which contain amounts ranging from
7% to 10%). Mycoprotein, a fungus protein often marketed as a
high-quality protein source to replace animal-based protein, and
in particular meat, would be considered sufficient in lysine with
a level of 6.7% (63). However, like many of the other plant-
based protein sources, this source is relatively low in methionine
(1.5%), especially when compared with animal-based proteins
(>2.2%). Of particular interest is the plant-based protein source
quinoa, which can be considered a high-quality protein source
on the basis of its relative high lysine (6.5%) and methionine
(2.6%) contents when compared with most other plant-based
protein sources. Furthermore, quinoa protein has a relatively
high total EAA content (39%). It remains to be investigated
whether the ingestion of quinoa or quinoa protein would
translate into superior peripheral AA availability and/or greater
postprandial MPS rates when compared with the ingestion of
isonitrogenous amounts of other (plant) protein sources.
Leucine content. Recent studies have described leucine as the
most potent AA responsible for the postprandial stimulation of
MPS (50). Specifically, a large, transient increase in intracellular
and/or extracellular concentrations of leucine after dietary
protein ingestion is often touted to be driving this response
(70–75). It is now generally believed that the leucine content of a
protein source is an important and independent predictor of its
capacity to stimulate postprandial MPS (50, 76, 77).
Tang et al. (19) examined the pattern of plasma AA
concentrations and postprandial MPS after the ingestion of
similar amounts of whey, casein, and soy protein during
recovery from resistance exercise in healthy young men. It was
shown that the ingestion of whey and soy protein, both acid
soluble (53), resulted in a more rapid appearance of leucine in
the blood than with the ingestion of casein protein. Furthermore,
the highest level of leucinemia and subsequent stimulation of
MPS was observed after the ingestion of whey protein (17). An
interesting observation is that the consumption of soy protein
led to greater MPS rates than did the consumption of the animal-
derived casein protein (17). It was speculated that the slower
digestion rate of casein protein and subsequent slower amino-
acidemia provided a lesser stimulus for muscle anabolism than
did the ingestion of soy protein. These findings further point
toward the rapid, large, and transient increase in systemic
leucine concentrations as an important driver of the postpran-
dial increase in MPS.
Comparison of the different protein sources reveals that the
leucine content of whey is highest,with 13.6% (Table 3, Figure 3).
Unsurprisingly, whey protein is generally considered the superior
protein source for the stimulation of postprandial MPS when
compared with other rapidly digested protein sources such as soy
protein isolate (19) and hydrolyzed casein (17), which have
leucine contents of 8.0% and 10.2%, respectively. Animal-based
protein sources generally contain more leucine than do plant-
based proteins. Most plant-based sources have a leucine content
of ;6–8%, whereas animal-based protein sources tend to have a
leucine content in the range of 8.5–9% and >10% in the case of
dairy proteins. The higher leucine content may be a key factor
responsible for the proposed greater capacity of animal-based
proteins to stimulate postprandial MPS rates when compared
with the ingestion of various plant-based proteins (19). From this
perspective, maize-derived protein represents an interesting
exception because it has a relative high leucine content of
12.2%. However, comparisons of maize protein isolate to other
animal- and plant-derived proteins for the stimulation of
postprandial MPS remain to be conducted. On the basis of
differences in leucine content between the various plant-based
proteins, it could be expected that substantial differences exist in
the capacity ofindividual plant-based protein sources to stimulate
postprandial MPS rates. Research comparing the anabolic prop-
erties of the various plant- and animal-based protein sources will
be of particular interest to define the preferred protein sources to
be used in nutritional interventions to support skeletal muscle
mass maintenance and/or to facilitate muscle hypertrophy.
FIGURE 2 Lysine (A) and methionine (B) concentrations of various
protein sources. Differentiation is made between plant- and animal-
based protein sources. Human muscle is provided as a reference
standard. The dashed lines represent recommendations for a minimal
intake by WHO/FAO/UNU (46) guidelines. Protein sources with bars
below the dashed line are considered lower than the WHO/FAO/UNU
(46) requirements of the specific amino acid. WHO/FAO/UNU, World
Health Organization/Food and Agriculture Organization of the United
Nations/United Nations University.
FIGURE 3 Leucine concentrations of various protein sources.
Differentiation is made between plant- and animal-based protein
sources. Human muscle is provided as a reference standard. The
dashed line provides a comparison of the protein source most
abundant in leucine (i.e., whey) with the various other protein sources.
Animal- vs. plant-based proteins 5 of 11
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Chronic Studies of Plant-Based Protein
Intake and Muscle Mass
Acute measurements of MPS are often assumed to predict
longer-term phenotypic outcomes (i.e., greater skeletal muscle
maintenance/gain) to nutritional and/or exercise training inter-
ventions (78). However, acute measurements of MPS are not a
quantitative estimation of muscle hypertrophy/maintenance.
Instead, the acute MPS response to various nutritional and/or
exercise interventions should better be viewed as an indicator of
skeletal muscle reconditioning (e.g., muscle repair and remod-
eling), with the potential to provide insight into differences in
skeletal muscle hypertrophy and/or muscle mass maintenance
when performed chronically.
It has been well established that the ingestion of soy protein
results in lower postprandial MPS rates than does the ingestion
of beef (31), whey (19, 21), or milk (32), both at rest and during
recovery from exercise. This begets the question as to whether
chronic intake of plant- vs. animal-based proteins would result
in divergent phenotypic outcomes (i.e., differences in muscle
mass).
Hartman et al. (16) observed that the habitual consumption of
17.5 g milk protein during a 12-wk resistance exercise training
intervention resulted in greater gains in lean body mass (LBM; 3.9
vs. 2.8 kg) than an isonitrogenous amount of soy protein. In
agreement, Volek et al. (79) demonstrated that the ingestion of 24
g whey as opposed to soy protein resulted in greater gains in LBM
(3.3 vs. 1.8 kg) after 36 wk of resistance exercise training in young
men. Similarly, Campbell et al. (80) showed that the consumption
of an omnivorous diet during a 12-wk resistance exercise training
program induced greater gains in LBM and increases in type II
fiber size than did the consumption of a predominantly lactovarian
diet. Later it was demonstrated that increasing daily dietary
protein intake from 0.78 g/(kg body weight d) to 1.15 g/(kg body
weight d) eliminated the differences between the groups who
consumed an omnivorous diet vs. a lactovarian diet (81). Overall,
these findings imply that the ingestion of higher amounts of protein
may reduce the proposed differences in the capacity of different
protein sources (plant vs. animal) to modulate the gains in skeletal
muscle mass during prolonged exercise interventions (82).
Indeed, it could be hypothesized that the ingestion of greater
quantities of plant-based proteins may compensate for the lower
EAA content, thereby improving the potential of plant-based
proteins to support skeletal muscle mass gains. Joy et al. (33)
recently observed that the ingestion of either 48 g rice protein or
an isonitrogenous and isoenergetic amount of whey protein,
immediately after resistance exercise, promoted similar increases
in LBM (2.5 vs. 3.2 kg) during an 8-wk training intervention in
healthy young men. Brown et al. (37) provided their subjects
with either 33 g soy or whey protein and showed similar
increases in muscle mass after prolonged resistance exercise
training. Collectively, the studies that provided greater amounts
of plant-based proteins showed minimized differences in lean
mass gain with resistance exercise when compared with the
ingestion of animal-based proteins. Although more evidence is
required, we argue that plant-based protein supplementation
can be successfully applied to support muscle mass accretion
during prolonged resistance exercise, provided that greater
amounts of plant-based proteins (>30 g/meal) are being con-
sumed. Although the data above are only applicable to the
specific population engaged in resistance exercise, divergent
phenotypic outcomes with regard to plant- vs. animal-based
protein intake have also been observed in other population
groups.
A greater proportion of daily protein intake derived from
animal- vs. plant-based sources is associated with better muscle
maintenance in older and more clinically compromised individ-
uals (83–86). For instance, long-term vegetarianism in older
women has been reported to compromise muscle mass mainte-
nance when compared with consumers of an omnivorous diet
(18.2 vs. 22.6 kg LBM) (85). Although more long-term studies
are required, it appears that prolonged (lifelong) vegetarianism
can result in lower muscle mass maintenance across the life span.
Aging has been associated with a progressive decline in skeletal
muscle mass and appears to be driven in part by a greater
anabolic resistance of skeletal muscle tissue to dietary protein
ingestion (87). Given the importance of skeletal muscle mass for
metabolic health and physical functioning (88), strategies to
improve the sensitivity of skeletal muscle tissue to the anabolic
properties of plant-based proteins could be of particular interest
for aging populations. In addition, strategies to enhance the
anabolic response to the ingestion of plant-based proteins may
increase consumer demand, thereby supporting global sustain-
ability, and reduce the costs associated with the production of
high-quality-protein–dense foods.
Strategies to Augment the Anabolic Prop-
erties of Plant-Based Proteins
Protein quantity. As mentioned, plant-based proteins contain
fewer EAAs than do animal-based proteins. This is predomi-
nantly due to the lower amounts of lysine, methionine, and/or
leucine (Table 3, Figure 2). In the section above, we argued that
the ingestion of greater amounts of plant-based protein per meal,
consequently ingesting greater amounts of EAAs (and notably
leucine), may compensate for the lower muscle anabolic
properties of plant- vs. animal-based proteins. Further evidence
for such a notion is found in acute work studying the postprandial
MPS response to plant- vs. animal-based protein intake in an
animal model. Specifically, Norton et al. (89) established that the
ingestion of one-third more wheat than whey protein elicited
similar postprandial MPS rates in rodents. However, recent data
in elderly men suggest that the consumption of greater amounts of
plant-based proteins, as a strategy to maximize postprandial MPS
rates, may not per se provide a feasible solution (21). More
specifically, Yang et al. (21) reported that leucine oxidation
rates were elevated after the ingestion of 40 g soy protein when
compared with the ingestion of a similar amount of whey
protein in older individuals. These findings suggest that part of
the soy protein–derived AAs are directed more toward oxidation
than used for de novo MPS when compared with whey protein
ingestion. Moreover, because 20 g whey and soy protein did
not maximize the postprandial MPS response in these older
individuals, the study provided further evidence to support the
idea that aging muscle requires more leucine to maximally
stimulate MPS rates (87).
Previously it was demonstrated that the postprandial MPS
response in healthy young individuals is dose dependent up to
;20 g (;10 g EAAs) of high-quality, animal-based protein (25,
30, 90). In older individuals, maximal stimulation of postpran-
dial MPS tends to be achieved with the ingestion of greater
amounts of high-quality, animal-based protein (>35–40 g) (21,
23, 28). Given the lower leucine contents of plant-based protein
sources, substantial amounts (>40 g) of a plant-based protein
source should theoretically be ingested to maximize postpran-
dial MPS rates in older individuals. It is evident that consuming
such a large amount of protein in a single meal is far from
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practical. Therefore, simply increasing the amount of dietary
protein consumed per meal may not be the most feasible solution
to the problem, especially in a more clinical setting in which food
intake is compromised. However, we must note that research
into the dose dependency of different plant-based protein
sources to maximize MPS is warranted. As a reference, Table
4presents the amounts of various plant-based foods that should
be ingested per meal to allow for the ingestion of the same
amount of leucine (;3 g leucine) as is present in 23 g whey
protein. Such a dose of protein/leucine was previously shown to
maximize postprandial MPS in young individuals both in rested
and postexercise conditions (25).
Leucine fortification. The leucine content of a meal appears to
be of fundamental importance to postprandial stimulation of
MPS, especially in older populations (50) and in more clinically
compromised patient groups (91). Previously, Rieu et al. (72)
found that approximately doubling the leucine content of mixed
meals increased MPS rates in older individuals. Similar obser-
vations were reported by Katsanos et al. (75), who showed that
increasing the leucine content of an AA mixture reversed the
blunted postprandial MPS response in older subjects. We
recently investigated the impact of ingesting intrinsically
L-[1-
13
C]phenylalanine–labeled micellar casein with or without
the addition of free leucine on postprandial MPS in elderly
individuals. The coingestion of free leucine (2.5 g) with 20 g of
casein led to greater postprandial muscle protein accretion than
did the ingestion of casein only (92). Moreover, these stimulating
effects of leucine are likely maintained with chronic adminis-
tration of a higher leucine diet. Casperson et al. (93) observed
increases in both postabsorptive and postprandial MPS rates
after 2 wk of leucine supplementation (4 g/meal for 3 meals
daily). However, the clinical relevance of leucine supplementa-
tion as a dietary strategy to gain muscle mass remains question-
able. We failed to demonstrate measurable increases in skeletal
muscle mass or strength after 3 and 6 mo of leucine supple-
mentation of 7.5 g/d in both healthy (94) and type 2 diabetic (95)
elderly men. The lack of impact of prolonged leucine supple-
mentation on skeletal muscle mass or strength may be attributed
to the fact that the healthy and type 2 diabetic elderly men were
habitually active and were already consuming ample amounts of
dietary protein, namely >1 g/(kg body weight d).
A possible issue with crystalline leucine supplementation to the
diet may be the plasma- and tissue-depleting effects on the other
BCAAs (valine and isoleucine). For instance, we observed a rapid
decline in fasting plasma valine (223% 62%) and isoleucine
(216% 62%) concentrations within the first 4 wk of interven-
tion (95). Thereafter, plasma valine and isoleucine concentrations
remained stable. Whether this decline in fasting plasma valine
and isoleucine concentration is of physiologic relevance remains
debatable, because the basal concentrations did not decline
further and always remained within a normal physiologic range.
Moreover, many studies showed an improvement in postprandial
MPS with several grams of free leucine supplementation (72, 75,
92, 93). These data suggest that the addition of a few grams of
crystalline AAs to a dietary protein source is unlikely to negatively
affect protein metabolism and may in fact improve the skeletal
muscle anabolic response in settings in which dietary protein
intake is (substantially) lowered.
Work assessing the acute or long-term muscle anabolic
response in humans to the ingestion of plant-based protein
sources fortified with free leucine is currently limited. However,
TABLE 4 Amount of dietary protein to, theoretically, maximize postprandial MPS
1
Source
Leucine, %
total protein
Representative amount of protein to be ingested
per meal for ;3 g leucine, g
Representative amount of the food source
to be ingested per meal, g
Plant sources
Maize 12.3 25 264
Spirulina 8.5 36 63
Black bean 8.4 36 167
Rice 8.2 37 500
Soy 8.0 38 104
Lentil 7.9 39 150
Pea 7.8 39 180
Oat 7.7 35 236
Quinoa 7.2 43 302
Hemp 6.9 45 121
Wheat 6.8 45 299
Mycoprotein 6.2 49 447
Potato 5.2 58 2891
Animal sources
Whey 13.6 23 27
Milk 10.9 28 876
Casein 10.2 30 35
Beef 8.8 35 164
Egg 8.5 36 5
2
Cod 8.1 38 211
1
Amount of protein source to be ingested to maximize postexercise MPS rates in response to feeding in young subjects. Data are ranked
from high to low by leucine content. A higher leucine content suggests that a lower amount of dietary protein from a given source is needed
to maximize postprandial MPS rates. The third column (amount of protein to be ingested per meal) represents a theoretical value using
whey protein as a standard of reference. The amounts of protein calculated represent the amount needed to match the leucine content
found in 23 g whey protein (;3 g). The representative amounts for whey and casein assume isolated protein sources, whereas all other
protein sources are expressed as representative amounts of the intact food source. MPS, muscle protein synthesis.
2
Number of eggs.
Animal- vs. plant-based proteins 7 of 11
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there is evidence to suggest that the addition of free leucine to
plant-based proteins may be a viable option to improve its
anabolic properties. Engelen et al. (96) reported that supple-
menting a soy-based protein meal with free BCAAs (leucine,
isoleucine, and valine) reduced splanchnic extraction and urea
synthesis, consequently shifting dietary protein–derived AAs
toward peripheral (i.e., skeletal muscle) tissue. In addition,
animal work by Norton et al. (66) demonstrated that adding free
leucine to wheat protein, to match the leucine content present in
an isonitrogenous amount of whey protein, resulted in similar
postprandial MPS rates. Overall, the addition of a several grams
of free leucine to plant-based protein formulas and foods may
provide an effective strategy to enhance the anabolic properties
of plant-based protein sources.
Lysine and methionine fortification. As described previously,
plant-based protein sources are generally lower in lysine and/or
methionine than are animal-based protein sources (Table 3, Figure
2). This may impair the postprandial MPS response after the
ingestion of plant-based protein sources when compared with
animal-based proteins (56–59). Thus, similar to leucine fortifica-
tion, it may be hypothesized that adding lysine and/or methionine
to plant-based proteins is a useful strategy to improve their
anabolic properties. However, work studying the postprandial
MPS response to the addition of crystalline lysine and/or
methionine to a plant-based protein source is currently unavail-
able. Nonetheless, some evidence exists that lysine fortification
may be a worthwhile strategy to investigate. Such evidence can be
found in studies documenting differential growth rates among
young children that consume diets predominantly rich in grains
(>50%), either with or without lysine fortification (97–99).
The first large lysine fortification trials were performed in the
late 1960s and early 1970s (100–102). However, these studies
failed to reveal a beneficial effect of lysine fortification of rice,
maize, or wheat on the growth rates of children. A decade later,
thorough review of these studies revealed flaws in terms of design
and conduct (103). Not only did the control and intervention
groups differ vastly in terms of health and socioeconomic status,
in addition to geographical location, the researchers also did not
control for total protein intake or energy status and/or monitor
food consumption. These factors may have confounded the
findings, thereby making it difficult to draw firm conclusions with
regard to the impact of lysine fortification as a nutritional strategy
to augment growth rates (103).
In later, well-controlled experiments, beneficial effects of
lysine fortification have been observed. For instance, Zhao et al.
(97) demonstrated that the fortification of wheat with free
lysine, thereby increasing the lysine content of wheat protein
from 2.5% to 5.5%, resulted in greater gains in height and
weight of infants over a 3-mo period when compared with the
ingestion of an isonitrogenous and isoenergetic nonfortified
control. Moreover, the authors observed a trend for decreased
triceps skinfold thickness, suggesting that the weight gain was
attributed to increases in LBM rather than fat mass (97). Of
note, daily protein intake was ;50 g in both groups, with as
much as ;34 g derived from wheat protein. Similar findings
were made in other work that examined the growth rates of
infants with the consumption of lysine-fortified wheat (98)
and lysine-fortified maize (99). Taken together, these findings
provide additional support for the notion that the fortification
of plant-based foods with crystalline AAs may improve their
anabolic properties. However, future work comparing the
fortification of plant-based protein sources with free AAs vs.
the respective unfortified source with regard to postprandial
MPS response should be conducted to confirm the validity of
such a strategy to improve the skeletal muscle anabolic response.
Selective breeding of plants to improve AA composition.
Selective breeding (or genetic manipulation) may also serve as a
useful strategy to improve the EAA composition and digestibility
of plant-based proteins and, as such, improve their anabolic
properties. An example of selective breeding as a method of
improving protein quality of plant-based protein is ‘‘quality
maize protein’’ (QMP). QMP has been produced by selective
breeding of maize with a single gene mutation that results in
increased lysine (and tryptophan) content (104). QMP has a
lysine content of ;4.2% (105), which is near the ‘‘required’’
4.5% as per WHO/FAO/UNU recommendations (46). Although
QMP has nearly double the lysine content than generic maize
(containing ;2.3% lysine), the lysine content still remains well
below the amounts reported for animal-based proteins (>7%).
So far, the postprandial MPS response to the ingestion of QMP
has not been assessed. Nonetheless, QMP consumption has been
shown to augment growth rates of infants in underdeveloped
countries, where maize comprises a large majority of daily
protein intake (106). Thus, the application of selective breeding
or a genetic enhancement strategy to increase the EAA content
may represent an effective strategy to improve the skeletal
muscle anabolic response to plant protein ingestion and could be
of particular relevance in populations who consume diets rich in
plant-based foods (i.e., grains) (Table 1).
Protein blends. Several recent articles have investigated the
anabolic properties of ingesting a combination of plant and
dairy proteins in a single meal (i.e., protein blends) (34–36, 107).
For example, Reidy et al. (107) recently reported that protein
blends may be useful to facilitate the postprandial MPS response
during recovery from a single bout of resistance exercise in
healthy young men. The authors reported no measurable
differences in the MPS response during the 4-h postexercise
recovery period with the ingestion of either 17.7 g whey protein
or 19.3 g of a protein blend (containing a mixture of 25% whey,
25% soy, and 50% casein protein). These results are not
surprising given that nearly equal amounts of EAAs (8.9 60.4
vs. 8.7 60.5 g) and leucine (1.9 60.1 vs. 1.8 60.1 g) were
provided. It is evident that plant-based proteins will contribute
to the postprandial increase in MPS and that they can be applied
effectively in protein blends designed to support muscle mass
gains. This is of particular relevance in postexercise conditions
in which the muscle is even more sensitive to the anabolic
properties of AAs (13, 17). However, animal-based proteins, and
dairy proteins in particular, contain higher amounts of leucine
than do similar amounts of plant-based proteins (Table 3, Figure
3). Consequently, protein blends containing substantial amounts
of plant-based proteins (>50%) may contain less leucine than an
isonitrogenous amount of an animal-based protein source. A
reduction in the amount of leucine consumed may have
relatively little impact on the postprandial increase in MPS
rates during postexercise recovery in healthy young individuals
(108). However, consuming a protein source or protein blend
with a low leucine content will likely be of greater impact on
stimulating postprandial muscle protein accretion in older and
more clinically compromised populations. As mentioned, these
individuals require greater amounts of leucine (>2.5–3g) to
maximize postprandial MPS rates (50, 91).
Although the consumption of a mixture of plant- and animal-
based protein sources is generic to most individuals, vegans
consume a strictly plant-based diet. In the case of plant-based
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protein blends it may be that the consumption of a well-balanced
combination of multiple plant-based protein sources, to allow
for ingestion of a ‘‘complete’’ EAA profile, may improve the
postprandial MPS response when compared with the ingestion
of a single plant-based protein source. In general, plant-based
proteins are only low in 1 or 2 EEAs (Table 3, Figure 2). Thus,
combining plant proteins that are lower in lysine yet higher in
methionine (e.g., wheat, rice, hemp, and maize) with plant
proteins that are higher in lysine yet lower in methionine
(including black bean, oat, soy, lentil, potato, and pea) may
augment the anabolic properties of plant-based protein intake.
Research is warranted to assess whether the ingestion of a plant-
based protein blend, designed to provide for a more balanced
EAA profile, increases postprandial MPS rates when compared
with the ingestion of a single plant-based protein source.
Conclusions
Only a few studies have compared the postprandial MPS response
to the ingestion of plant- vs. animal-based proteins. To date, the
only plant-based protein source that has been extensively studied
in an in vivo human model is soy protein. In this work, the
consumption of soy protein was demonstrated to result in lower
MPS rates than the ingestion of whey, milk, or beef protein. In
addition, the acute skeletal muscle anabolic response was reduced
with wheat protein intake when compared with the consumption
of egg or whey protein in a rodent model. The proposed lower
muscle anabolic properties of plant- as opposed to animal-based
protein sources may be attributed to differences in protein
digestion and AA absorption kinetics, and/or AA composition.
Various strategies that may improve the MPS response after the
ingestion of plant-based proteins include the fortification of plant
proteins with free AAs, the blending of various plant protein
sources to create a more complete AA profile, selective breeding of
plants to improve AA composition, and/or the consumption of
greater amounts of plant proteins. Research is required to
compare the anabolic properties of the many different plant-
based protein sources and to assess the potential of the various
strategies that may augment the postprandial MPS response to the
ingestion of plant-based proteins.
Acknowledgments
SvV, NAB, and LJCvL wrote the manuscript; and SvV and
LJCvL had primary responsibility for the final content. All
authors read and approved the final manuscript.
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