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Alternative dietary protein sources to support healthy and active skeletal muscle aging


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To mitigate the age-related decline in skeletal muscle quantity and quality, and the associated negative health outcomes, it has been proposed that dietary protein recommendations for older adults should be increased alongside an active lifestyle and/or structured exercise training. Concomitantly, there are growing environmental concerns associated with the production of animal-based dietary protein sources. The question therefore arises as to where this dietary protein required for meeting the protein demands of the rapidly aging global population should (or could) be obtained. Various non-animal–derived protein sources possess favorable sustainability credentials, though much less is known (compared with animal-derived proteins) about their ability to influence muscle anabolism. It is also likely that the anabolic potential of various alternative protein sources varies markedly, with the majority of options remaining to be investigated. The purpose of this review was to thoroughly assess the current evidence base for the utility of alternative protein sources (plants, fungi, insects, algae, and lab-grown “meat”) to support muscle anabolism in (active) older adults. The solid existing data portfolio requires considerable expansion to encompass the strategic evaluation of the various types of dietary protein sources. Such data will ultimately be necessary to support desirable alterations and refinements in nutritional guidelines to support healthy and active aging, while concomitantly securing a sustainable food future.
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Narrative Review
Alternative dietary protein sources to support healthy and
active skeletal muscle aging
Ino van der Heijden , Alistair J. Monteyne, Francis B. Stephens, and Benjamin T. Wall
To mitigate the age-related decline in skeletal muscle quantity and quality, and the
associated negative health outcomes, it has been proposed that dietary protein rec-
ommendations for older adults should be increased alongside an active lifestyle
and/or structured exercise training. Concomitantly, there are growing environmental
concerns associated with the production of animal-based dietary protein sources.
The question therefore arises as to where this dietary protein required for meeting
the protein demands of the rapidly aging global population should (or could) be
obtained. Various non-animal–derived protein sources possess favorable sustainabil-
ity credentials, though much less is known (compared with animal-derived proteins)
about their ability to influence muscle anabolism. It is also likely that the anabolic
potential of various alternative protein sources varies markedly, with the majority of
options remaining to be investigated. The purpose of this review was to thoroughly
assess the current evidence base for the utility of alternative protein sources (plants,
fungi, insects, algae, and lab-grown meat” ) to support muscle anabolism in (active)
older adults. The solid existing data portfolio requires considerable expansion to en-
compass the strategic evaluation of the various types of dietary protein sources.
Such data will ultimately be necessary to support desirable alterations and refine-
ments in nutritional guidelines to support healthy and active aging, while concomi-
tantly securing a sustainable food future.
Global demographics indicates that the number of indi-
viduals aged 65 years is set to double by the year 2050,
leading to a worldwide older adult population of nearly
1.5 billion.
As a result of the continued progress in hu-
man medicine and health, the fastest-growing subpopu-
lation of the older adults, remarkably, is those aged
80 years or older, which is expected to triple between
2019 and 2050, increasing from 143 to 425 million.
key hallmark of aging is the progressive loss of skeletal
muscle mass and strength/function, referred to as sarco-
penia and dynapenia, respectively: insidious and inevi-
table processes that occur independently of health
The age-related decline of muscle mass and
strength occurs from around the fourth decade onwards
and increases with advancing age.
Increased age-
related muscle loss is associated with increased
Affiliation: I. van der Heijden, A.J. Monteyne, F.B. Stephens and B.T. Wall are with the Department of Sport and Health Sciences, College of
Life Environmental Sciences, University of Exeter, Exeter, United Kingdom.
Correspondence: B.T. Wall, Department of Sport and Health Sciences, College of Life and Environmental Sciences, St Luke’s Campus,
Heavitree Road, University of Exeter, Exeter EX1 2LU, UK. E-mail:
Key words: dietary protein, healthy aging, protein quality, sarcopenia, sustainability.
CThe Author(s) 2022. Published by Oxford University Press on behalf of the International Life Sciences Institute.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (
licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly
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incidence of falls, fractures, and risk of developing
chronic metabolic disease.
Sarcopenia, therefore,
underscores the key role skeletal muscle performs in
supporting healthy aging by profoundly impacting on
both quantity (ie, life-span) and quality (ie, health-span)
of life. Consequently, how sarcopenia is managed at
complex and interconnected societal levels will dictate
the burden it poses to the aging population and global
healthcare systems over the coming decades.
Contemporary management of sarcopenia through
physical activity and dietary protein
A crucial but often overlooked feature of sarcopenia is
that, despite being inevitable, the rate of progression is
highly modifiable by lifestyle. Research over recent deca-
des has reliably demonstrated that muscle tissue of older
adults retains the ability to recondition in response to
environmental stimuli such as physical activity and die-
tary protein intake.
Accordingly, adults who retain
high(er) physical activity levels throughout their lifespan
lose significantly less muscle tissue mass and quality
than their (more) sedentary peers.
physically active (and even highly trained) older adults
still experience some degree of muscle loss that compro-
mises metabolic health and muscle function, and ulti-
mately perpetuates declining health by deterring the
continuation of an active lifestyle.
Such a vicious cycle
can be compounded in those presenting with physical
disabilities, frailty, or disease, who have an impeded ca-
pacity to carry out physical activity, exacerbated at the
greatest extremes by episodic periods of malnourish-
ment and/or muscle disuse during injury or illness.
Typically viewed as an adjunct to physical activity,
dietary protein intake has long been considered an addi-
tional prerequisite for preserving skeletal muscle mass in
older adults.
While the minutiae concerning the op-
timal timing, distribution, amount, etc, of dietary pro-
tein intake required to maximize its effectiveness remain
under intense scrutiny (eg,
), the benefits of combin-
ing regular physical activity/muscle contraction with suf-
ficient dietary protein intake on muscle mass, strength,
and function in older adults have been demonstrated re-
peatedly (eg,
). General agreement has therefore
been adopted by the scientific community that physical
activity (or structured exercise training) and sufficient
dietary protein intake are the cornerstones upon which
recommendations can be built to support healthy muscle
aging (eg,
). Despite this, however, much less re-
search has considered how the dietary protein source
may play into these recommendations. This question
has typically been narrowly focused at identifying the
most effective source(s) for stimulating muscle anabo-
lism (eg,
), but wider and more consequential
questions surrounding the efficacy and viability of di-
verse protein sources, or the potential requirement for
supplemental essential amino acids are emerging.
gap in nutritional understanding appears at a pivotal
time societally, while a number of multidisciplinary (and
often competing) pressures are forcing us to contem-
plate where food should be obtained from to best sup-
port a sustainable and ethical food future. It should be
appreciated that the current knowledge of the impact of
dietary protein intake on skeletal muscle of older adults
is almost exclusively extracted from studies assessing
animal-derived dietary protein sources. This is despite
more than half of the total dietary protein consumed
worldwide being derived from non-animal protein sour-
coupled with growing interest (particularly in the
Western world) in the sustainability and ethical creden-
tials of differing food choices.
The purpose of this article is to provide a compre-
hensive and balanced review of what is currently known
concerning the impact of consuming dietary protein
across a broad range of categorized sources on the skel-
etal muscle of (active) older adults. This is with the aim
of stimulating discussion about how current recom-
mendations, policy, research focus, and clinical practice
may be refined to integrate the multifaceted and nu-
anced issues involved in deciding where dietary protein
should (or could) be obtained from, both now and over
the coming decades, to support healthier aging.
Considering the sustainability credentials of dietary
protein sources
From a nutritional perspective, it has been established
that meat- and dairy-derived dietary proteins are effica-
cious sources of dietary protein for maintaining muscle
mass in older adults. Indeed, acute metabolic stud-
of muscle protein turnover, and longi-
tudinal or interventional studies
of muscle mass
and function, have used meat and dairy protein con-
sumption in older adults as the crux of the current evi-
dence base, and consequently undergird currently
applied (and robust) dietary protein recommendations
). However, rising (aging) populations
; in-
creased urbanization, affluence, and economic develop-
; greater awareness of (increased) protein
requirements in various populations (eg, athletes, older
adults, weight management, clinical situations,
—among other factors—have converged to
explain the rapidly (and presumably exponentially) in-
creasing global consumption of dietary protein.
However, the nutritional evidence base for the efficacy
of non-animal-derived proteins to support healthy mus-
cle aging has not kept pace. If not addressed, this
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mismatch could result in an imminent threat to the sus-
tainable food future.
The production of animal-derived dietary proteins
is typically environmentally costly when quantified in
terms of land and water use, and/or the sum of carbon
emissions (usually referred to as greenhouse gas [GHG]
Emissions), primarily from carbon dioxide (CO
), ni-
trous oxide, and methane. Quantifying precise figures
here would be complex and politically charged.
Estimates are influenced by the diverse methodologies
employed, varying criteria for how to quantify embed-
ded carbon within the chain of production, regional dif-
ferences in food production (due to farming methods
[eg, intensive vs extensive], transport methods, crop
growing conditions [eg, indoors vs outdoors]), and
which variable (ie, land/water use, GHG emission) is
afforded primacy.
As such, defining environmental
sustainability is complex and encompasses various
dimensions. Yet, the most globally applied method for
quantifying the impact of food and protein production
and consumption on environmental sustainability is via
the use of GHG emission data, which are consequently
the most predominantly available data regarding the en-
vironmental impact of dietary protein production.
Therefore, in this review, the sustainability credentials
of dietary proteins are primarily evaluated through the
lens of GHG emissions, while acknowledging that this
does not represent “sustainability” in its entirety.
Current estimates lie at around 5%–15% of total
global GHG emissions being directly attributable to the
production of animal-derived foods.
Also, livestock
takes up approximately 70% and 30% of global agricul-
tural land and water usage, respectively.
In a desire
to interrogate to what extent this proportion of GHG
emissions could be mitigated by altering food choices,
research has attempted to quantify GHG emissions dur-
ing the production of varying protein sources per kilo-
gram edible food. It is generally considered that animal-
derived food production creates considerably more
GHG emissions compared with plant-based foods,
when considering all the steps and processes involved
from farming/cultivation to the availability of the food
product in the store (ie, cradle-to-gate). However, most
animal-derived food sources, in raw format, especially
meat, tend to be more protein dense compared with
non-animal alternatives (with legumes an apparent ex-
ception), as depicted in Figure 1.
Therefore, when
assessing the potential role of the various protein-rich
sources in developing protein recommendations for
muscle mass maintenance, it is appropriate to compare
the environmental impact of divergent food sources per
gram (or portion) of protein, rather than total food
mass or caloric content. To facilitate this comparison,
collapsed data from numerous studies has been used to
express GHG emissions per 30 g portion of dietary pro-
tein (as well as per dose of leucine, branched-chain
amino acids, and essential amino acids, with more rele-
vance to the mechanistic regulation of muscle protein
turnover being discussed later) (see Figures 2 and
Notwithstanding variation depending on
specific source and production methods, these data still
show that meat is the most environmentally expensive
protein source, followed by vegetables and dairy, while
fish-derived dietary protein tends to be substantially
lower in GHG emissions and at a similar level to plant-
based sources.
Interestingly, by correcting to protein
content, plant-based protein sources generally still en-
compass a much lower carbon footprint compared with
animal sources, especially cereals and legumes,
the differences are clearly less marked compared with
the per kilogram edible food comparison. Indeed, the
differences even vanish in some comparisons; eg, due to
their low protein (and amino acid) density (Figure 1),
when corrected per portion of protein, vegetable protein
cultivation generates similar GHG emissions compared
with dairy. Given the majority of protein supply world-
wide is derived from vegetal sources,
it is therefore
of relevance to evaluate whether increasing this propor-
tion still further would in fact benefit environmental
factors but, crucially, also adequately support human
nutrition, especially in older adults.
The relative sustainability of plant-based proteins
is generally attributed to “cutting out the middle man,”
which involves, among others, the inefficiency of
growing crops for animal feed, and the consequent in-
creased land use as well as methane production from
ruminants’ digestive systems, etc., which together ex-
acerbate the costs across all parameters.
For similar
reasons—such as: (more) direct culturing (ie, paths of
lesser resistance from precursor to product) and con-
trolled, small and enclosed production environments
(ie, closed systems), etc.—promising environmental
footprints of other (ie, not plant-based) non-animal-
derived dietary protein sources such as fungal, algal,
and insect proteins are now emerging (Figures 2 and
Indeed, the GHG emission intensities of sev-
eral of these more novel protein sources for human
consumption are considerably lower than both animal-
and plant-derived dietary protein sources on a per ki-
logram edible food and per portion of protein (and
leucine, branched-chain amino acid or essential amino
acid dose) (Figures 2 and 3). However, these sources
are less well advanced from a food production, com-
mercial pipeline, and social acceptability perspective,
and are therefore minimally consumed compared with
animal- or plant-based sources, but they clearly display
attractive sustainability and nutritional profiles to war-
rant further investigation.
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While these data (Figures 1–3) shine only a brief
light on the complexities of “rank ordering” dietary pro-
teins based on sustainability credentials (a far-reaching
multidisciplinary discussion beyond the scope of the
present review), they at least underscore the wider soci-
etal requirement for us to build a robust evidence base
Figure 1 Mean (6SEM) protein content (% of total mass) of various categories of whole-food dietary protein sources. Protein content
is expressed as raw or dry matter, based on the common type of protein intake (ie, powder or meal) per protein source. White bars represent
animal-based dietary protein sources, light gray bars represent plant-based protein sources, the mid-gray bar represents fungal-derived pro-
tein sources, dark grey bars represent algal-derived protein sources, and the black bar represents insect-derived protein sources. Patterned
bars present protein isolates provided as a reference standard. Values between parentheses indicate the number (n) of included types of die-
tary protein for which the mean (6SEM) protein content is calculated. Data are obtained from Table S1.
Figure 2 Mean (6SEM) greenhouse gas emissions (kg CO
e) required to produce a 30 g portion of protein from various categories
of whole-food dietary protein sources. White bars represent animal-based dietary protein sources, light gray bars represent plant-based
protein sources, the mid-gray bar represents fungal-derived protein sources, and the dark gray bar represents algal-derived protein sources.
Values between parentheses indicate the number (n) of included types of dietary protein for which the mean (6SEM) greenhouse gas emis-
sion content is calculated. Data are obtained from Table S1.Abbreviation: CO
e, carbon dioxide equivalent.
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as to the nutritional efficacy of non-animal-derived pro-
tein sources. Such data are clearly required if divergent
protein sources are to be considered and developed to
support elevating global protein requirements to sup-
port healthier (muscle) aging.
Skeletal muscle protein metabolism and dietary
protein requirements in older adults
Skeletal muscle mass maintenance in both young and
older adults is achieved at the physiological level via a
Figure 3 Mean (6SEM) greenhouse gas emission (kg CO
e) required to produce a portion of 2.5g leucine (A), 5 g branched-chain
amino acids (B), and 15 g essential amino acids (C) from various categories of whole-food dietary protein sources. White bars repre-
sent animal-based dietary protein sources, light gray bars represent plant-based protein sources, the mid-gray bar represents fungal-derived
protein sources, and the dark gray bar represents algal-derived protein sources. Values between parentheses indicate the number (n) of in-
cluded types of dietary protein for which the mean (6SEM) greenhouse gas emission content is calculated. Data are obtained from Table S1.
Abbreviations: BCAAs, branched-chain amino acids; CO
e, carbon dioxide equivalent; EAAs, essential amino acids.
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continuous state of protein turnover, with muscle pro-
teins constantly being synthesized and broken down.
Muscle hypertrophy occurs as the result of a net positive
muscle protein balance, in which muscle protein syn-
thesis rates exceed breakdown rates over time, and pro-
tein is accrued. Skeletal muscle contraction and dietary
protein ingestion are the two key anabolic stimuli regu-
lating muscle protein synthesis.
Regarding the lat-
ter, following dietary protein digestion, amino acids are
absorbed in the gastrointestinal tract, resulting in in-
creased plasma amino acid concentrations and, there-
fore, availability to peripheral tissues, such as skeletal
Once taken up by muscle tissue, dietary
protein-derived amino acids can both directly stimulate
the signaling mechanisms that upregulate muscle pro-
tein synthesis, and can be used as substrate for de novo
muscle protein synthesis.
Concerning physical ac-
tivity, it has been established that a single bout of (en-
durance-, resistance-, or concurrent-type) exercise
stimulates mixed (with emphasis of what protein sub-
fraction is most sensitive, contingent on the nature of
the exercise) muscle protein synthesis rates for at least
24–48 hours, depending on various factors such as exer-
cise intensity and training status.
physical activity also sensitizes muscle tissue to the
aforementioned anabolic properties of amino acids,
which enhances the muscle protein synthetic response
to dietary protein.
This synergy between physical ac-
tivity and dietary protein ingestion appears to last for
each and every meal following exercise for at least
24 h,
and is the metabolic basis for how prolonged di-
etary protein and exercise interventions can be applied
to optimizing muscle tissue maintenance, recondition-
ing, and/or hypertrophy.
The slow, insidious nature of sarcopenia, as well as
its variability in severity and timing of onset, make
gross alterations in muscle protein metabolism difficult
to detect in older adults. Indeed, most,
but not
studies do not detect differences in basal (post-
absorptive) muscle protein synthesis or breakdown
rates between young and older adults. Such findings led
to the emergence of the theory that such an impercepti-
bly small level of daily muscle loss may be better
explained by a reduced responsiveness to daily anabolic
Empirical support followed from multiple
studies to show that a blunted muscle protein synthetic
response to dietary protein ingestion
and physical
occurs in older compared with younger
adults, termed “anabolic resistance.” Such an effect
appears most profound at lower (<20 g) compared with
higher (>20 g) doses of dietary protein,
ing a reduced sensitivity of older muscle to dietary pro-
tein. The etiology of anabolic resistance is complex and
multifactorial, with many physiological effects clearly
present upstream, and therefore impacting on the intra-
cellular molecular regulation of muscle protein synthe-
sis downstream.
For instance, impairments in protein
digestion and/or amino acid absorption, attenuating
postprandial amino acid availability, have been reported
in older adults.
This is likely compounded by in-
creased splanchnic amino acid retention in older
Increasing age is also associated with re-
duced insulin sensitivity,
resulting in blunted post-
prandial microvascular muscle tissue perfusion
likely, muscle amino acid uptake.
While all of these
age-related physiological impairments would ultimately
result in reduced cellular amino acid supply (both as a
signal and substrate) per gram of ingested protein, it is
also true that intramyocellular anabolic signaling path-
ways have been reported as compromised per se in se-
nescent muscle.
Crucially, when viewing the data within the context
of healthy and active aging, these different physiological
levels of postprandial amino acid handling are all also
impaired by physical inactivity.
It has been demon-
strated that short- and long-term muscle disuse (ie, the
removal of muscle contraction) induces rapid muscle
atrophy and worsens (or induces) anabolic resis-
Conversely, while (some degree of) an-
abolic resistance appears inevitable with aging and
disuse, physical activity, especially resistance-type exer-
cise, increases the anabolic sensitivity of both young
and old muscle, and has the potential to partially over-
come anabolic resistance.
This likely explains
why older adults who maintain an active lifestyle expe-
rience lesser declines in muscle mass, and fewer detri-
mental consequences, than their more sedentary peers,
despite equivalent dietary protein consumption.
Also, even in the absence of regular physical activity/
training, greater amounts of dietary protein consump-
tion appear to overcome, at least partly, anabolic resis-
which clarifies why older adults consuming
in surplus of the current Recommended Daily
Allowance (RDA) guidelines (0.8 g/kg body weight per
day) tend to lose less muscle mass, strength, and func-
tional capacity than aging populations who consume
protein at, or below, the RDA.
These obser-
vations have led to widespread calls within the scientific
community to raise dietary protein intake recommen-
dations in older adults up to, at least, 1.2 g/kg body
weight per day (eg,
), which is above the
current daily protein intake of many older adults in
Western societies,
and would represent a consid-
erable 50% rise in nationally recognized RDAs. In addi-
tion to daily protein intake, factors such as daily protein
distribution, or protein intake per meal, may also be op-
timized to elicit a greater total postprandial muscle pro-
tein synthetic response over the course of a day.
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Progress regarding the refinement of dietary pro-
tein recommendations for healthy and active muscle ag-
ing will inexorably highlight the pressing need to assess
how the sources of protein can be exploited. As dietary
protein sources have divergent anabolic capacities
), this will mean that protein require-
ments should ultimately be refined to the extent of indi-
cating where certain (animal or non-animal) protein
sources can be used to meet increased protein demands
in aging populations (ie, “source-specific” recommen-
dations in which amounts may differ depending on
source). Consequently, to contemplate the anabolic po-
tential, and therefore viability, of a protein source, it is
necessary to evaluate the current understanding of the
factors determining the anabolic capacity of dietary
protein ingestion and how this pertains to the major
subcategories of dietary protein.
Factors affecting the anabolic potential of a dietary
protein source
The magnitude of the postprandial muscle protein syn-
thetic response to protein ingestion is regulated across
multiple levels. The essential amino acid content and
composition of a protein source is an important modu-
lator of the postprandial muscle protein synthetic re-
Indeed, the postprandial rise in circulating
essential (but not non-essential) amino acids seems to
exclusively drive the acute muscle protein synthetic re-
Moreover, it has been clearly established
that, of the essential amino acids, the branched-chain
amino acids (primarily leucine) are particularly impor-
tant in regulating postprandial muscle protein synthesis
; however, to prevent substrate limitation, it is
generally agreed that all essential amino acids are re-
quired to facilitate an optimal muscle protein synthetic
Leucine not only serves as one of the
amino acid precursors for muscle protein, but also as
the principle signaling molecule that activates the intra-
cellular muscle protein synthetic machinery.
induces protein translation through direct stimulation
of the mammalian target of rapamycin complex 1
(mTORC1), and its downstream effectors (eg, 4E-BP1,
As such, leucine content, and the post-
prandial leucinemia that protein elicits upon ingestion,
are of major relevance to the capacity of a protein
source to stimulate muscle protein synthesis. Although
there are various suggestions that other specific amino
acids may also play important signaling roles,
has been shown that simply enriching a single bolus of
protein with leucine can further enhance the postpran-
dial muscle protein synthetic response.
Collectively, therefore, it is largely assumed that viable
protein sources will contain sufficient leucine, and
essential amino acids (on a per meal basis) to robustly
activate muscle protein synthetic pathways. There is
some debate as to what “sufficient” leucine per dose
may be, with estimates from 1 g–1.5 g
for a measur-
able increase in muscle protein synthesis, to 2.5 g–
3.0 g
for an optimal stimulation offered
within the literature. “Sufficiency” of leucine is likely
modulated by the concordance between the dose
ingested and the subsequent efficiency of absorption,
such that the amount of leucine within a protein bolus
per se is only partially elucidatory. Indeed, this seems to
tease apart the subtle difference between the leucine
“threshold” and “trigger” hypotheses, with the latter re-
ferring to the speed and magnitude of postprandial leu-
cinemia. It should also be noted that there is likely a
minimal level required for all (essential and non-
essential) amino acids on a per meal protein bolus.
Aside from potential (less-well-defined) signaling roles,
theoretically the lack of exogenous availability of any
amino acid would ultimately become substrate limiting
to the continuation of muscle protein synthesis,
depending on factors such as postprandial duration
and/or endogenous amino acid availability, given that
polypeptide chains of complexity require a full comple-
ment of amino acids during their synthesis.
Accordingly, viable dietary protein sources are assumed
to be well balanced with regard to their amino acid
composition, minimizing single amino acid deficiencies
as defined by the World Health Organization/Food and
Agriculture Organization of the United Nations
As previously alluded to, though amino acid com-
position is a key initial component influencing the ana-
bolic potential, differing physiological and metabolic
aspects of postprandial protein handling will arguably
be most consequential. First, dietary protein sources
differ in their digestion and amino acid absorption
characteristics, leading to divergent (total and kinetic)
systemic amino acid availability, which has been sug-
gested to result in different muscle protein synthetic
responses to various isolated protein sources.
For example, for a given protein dose ingested, total
postprandial plasma amino acid availability (ie, the
amino acid bioavailability) can vary between sources,
which will naturally modulate the resultant muscle pro-
tein synthetic response.
It has also been shown that
rapid aminoacidemia (ie, a quicker and larger “signal”),
which occurs when quickly digestible proteins (eg,
whey) are ingested, stimulates muscle protein synthesis
rates to a greater extent (but possibly for a shorter time)
compared with more slowly digested proteins (eg, ca-
Though many of these findings are also
influenced by the high essential amino acid/leucine
contents of whey protein, it has also been shown that
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prehydrolyzing casein
or bolus vs continuous feeding
of whey
to accelerate the digestion and absorption ki-
netics augments the muscle protein synthesis response
to the same protein source (ie, identical amino acid
Dietary protein source and cardiometabolic health
during aging
While it is clear that higher dietary protein intakes are
beneficial for maintenance of muscle mass during ag-
ing, the broader relationship of how this extends to
overall healthy aging requires some nuanced considera-
tions. In general, maintenance of muscle mass with age
correlates with decreased morbidity and mortality, and
is a predictor of longevity and quality of life.
With respect to cardiometabolic health, weight loss
tends to be the most notable target, with attention on
dietary protein being a key nutritional tool employed
during energy-restricted diets. High(er) protein diets
(25%–35% of total energy intake) have reliably been
shown to be effective for supporting losses of body mass
and fat mass, while maintaining muscle mass, with re-
sultant favorable changes in blood lipid profiles when
compared with standard/low(er) protein diets (10%–
20% of total energy).
In addition, dietary pro-
tein is also linked with increased satiety when compared
with carbohydrate or fat on a calorie-for-calorie basis,
permitting a reduced ad libitum energy intake.
satiating properties of protein have been attributed to,
among other factors, postprandial hyperaminoacide-
elevated gut hormone concentrations (in-
cluding concentrations of hormones such as glucagon-
like peptide 1, cholecystokinin, and peptide YY
and increased diet-induced thermogenesis.
Of rel-
evance, divergent satiating potential has also been at-
tributed to different protein sources. For example, fast-
digesting proteins (eg, whey and soy) resulting in tran-
sient and high peak concentrations of plasma amino
acids and appetite hormones are associated with greater
satiety than more slowly digested protein sources (eg,
Though satiation of different proteins
likely levels out at higher doses,
and the addition of
non-protein nutrients (eg, fiber) typically present in
protein-rich whole foods also modulates various aspects
(eg, chemical, physical, and sensory properties) of food
evaluating how different protein sources can
be applied will be of increasing relevance when consid-
ering (sustainable) dietary options for optimal weight
loss and metabolically healthy aging.
Much of the relevant data of dietary protein beyond
muscle mass per se is out of the scope of the present re-
view, and the reader is referred to the following com-
prehensive reviews (eg,
). However, it would be
remiss not to discuss some aspects of the nuanced rela-
tionship between muscle mass and muscle quality, and
how it relates to dietary protein source. Overweight
adults typically have a higher absolute (although not
proportional to fat mass) muscle mass, but according to
various indices (strength, metabolic function, etc.) the
muscle mass is of lower quality, a fact that is of rele-
vance to dietary protein itself and thus protein source
selected within the diet.
Emerging epidemiological
data have suggested that higher dietary protein intakes
are also capable of adversely affecting markers of cardi-
ometabolic health.
This has been extended by studies
which report that protein intakes of around 30% of en-
ergy requirements can blunt the improvement observed
in insulin sensitivity following 26 weeks of weight
and cause a decline in skeletal muscle insulin
sensitivity in overweight individuals under eucaloric
conditions over 6 weeks.
The beneficial influence of
exercise is a noticeable absence, at present, from this
emerging literature base, and when accounted for may
improve the consistency of the findings.
Of particular interest within the context of the pre-
sent review, however, is the potential role of the protein
source in untangling the relationship between protein
intake and markers of cardiometabolic health. For ex-
ample, it has typically been animal- rather than plant-
derived protein sources implicated in negative effects
upon insulin sensitivity,
possibly attributed to differ-
ences in amino acid composition and content. Indeed,
recent studies indicate that the composition of the pro-
tein per se may also be of relevance, specifically, the
branched-chain amino acid content.
branched-chain amino acids, as discussed above, play a
key mechanistic role in regulating the postprandial
muscle protein synthetic response, studies also show
their presence in muscle are elevated in human models
of physical inactivity,
and in patients with type 2 dia-
Further, increased concentrations of circulat-
ing branched-chain amino acids (and/or their
metabolites) are correlated with insulin resistance and
the onset of type 2 diabetes.
To date, nutritional
intervention data to support this notion are minimal,
though vegan participants supplemented with
branched-chain amino acids for 12 weeks exhibited re-
duced insulin sensitivity,
and branched-chain amino
acid restriction has been reported to improve insulin
sensitivity in rats.
Mechanistic experiments have
shown that branched-chain amino acids are capable of
acutely impairing glucose tolerance and insulin-
stimulated skeletal muscle glucose uptake in various
in accordance with mechanisms laid out
in a “Randle Cycle”–type substrate competition, and/or
direct inhibition of insulin signaling by branched-chain
amino acid metabolites.
These data express the
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need for future work to refine dietary protein recom-
mendations for healthy muscle aging in terms of
amount, but also to what extent it is consumed against
the backdrop of an (in)active lifestyle, as well as the type
(and potentially branched-chain amino acid load) of the
diet and whether protein source can be leveraged to op-
timize this.
Concept of protein “quality”
The reduced anabolic sensitivity of older muscle to die-
tary protein ingestion highlights the importance of
viewing potential alterations in future RDAs for this
age-group through the lens of protein source. To prop-
erly evaluate how the origin of a dietary protein source
may modulate the recommendations for total intake,
the principles of what regulates the muscle anabolic re-
sponse inevitably falls into making some kind of quanti-
tative assessment over a protein’s “quality” to optimize
viable selections for older adults. This is usually
achieved by encompassing a protein source’s amino
acid composition (relative to human requirements) and
its digestibility within composite metrics.
The most
prominent metrics here are known as the Protein
Digestibility Corrected Amino Acid Score (PDCAAS)
and, more particularly, the Digestible Indispensable
Amino Acid Score (DIAAS), which have been used to
rank order protein sources on quality.
PDCAAS determines protein digestibility by
expressing the content of the first limiting essential
amino acid (the minimal requirement as defined by the
FAO/WHO/United Nations University [FAO/WHO/
UNU]143) of a particular protein against the content of
the same amino acid in a reference amino acid compo-
sition, based on amino acid requirements for adoles-
The calculated percentage is subsequently
corrected for the fecal digestibility of a protein source,
as determined in a rat assay.
As proteins with
PDCAAS >1.00 were not considered to have any addi-
tional value in humans, it was decided to truncate
scores at 1.00.
Following global implementation of
PDCAAS towards the end of the last century, several
shortcomings of the PDCAAS metric have been
Specifically, fecal digestibility tends to overes-
timate protein quality, as nitrogen disappearance in the
large intestine is predominantly due to microbial degra-
dation rather than protein digestion, and (virtually all)
amino acids are absorbed in the small intestine rather
than the large intestine.
Moreover, PDCAAS does
not consider the digestibility of individual essential
amino acids, and the use of adolescent amino acid
requirements to infer across the lifespan (including
individuals at an advanced age) has been questioned.
As such, the FAO has recommended the replacement of
the PDCAAS with the DIAAS, which is based on indi-
vidual amino acid digestibility, and estimates protein
quality scores using ileal rather than total tract fecal di-
Instead of applying rat models, DIAAS
uses data involving growing pigs, which is recognized as
a more representative model for estimating amino acid
digestibility by humans.
Lastly, DIAAS does not
truncate values at 1.00, allowing further distinction be-
tween various “high”-quality protein sources.
While DIAAS estimates amino acid absorption
more accurately than PDCAAS, animal models are ob-
viously not fully representative of human protein diges-
tion and amino acid absorption kinetics. Also, both
metrics are limited to isolated protein sources, do not
consider the influence of the coingestion of other
nutrients, or the consumption of dietary protein within
a whole food or mixed meal, which modulates post-
prandial protein handling and muscle protein metabo-
Additionally, nuanced metabolic
characteristics of particular amino acids (eg, leucine)
are not accounted for, nor are more holistic systemic
effects that modulate the circulating milieu, (eg, insulin
secretion, incretin responses, etc.).
The limitations
of these metrics in determining protein quality as it per-
tains to muscle anabolism have led to a disconnect
when using them to predict its in vivo anabolic poten-
tial in human experimentation. For example, despite
having similar protein quality scores (ie, PDCAAS
1.00), soy protein stimulated muscle protein synthesis
rates to a lesser extent than an isonitrogenous bolus of
or milk protein,
showing that current protein
quality metrics are not necessarily reflective of the post-
prandial muscle protein synthetic response. Finally, and
crucially, protein demands are constantly influenced
contextually. For instance, physical activity, disuse, or
aging, and the consequential increased/decreased pro-
tein requirements are not captured by protein quality
scores. As a result, though the amino acid composition
of a dietary protein source and its relative PDCAAS and
DIAAS scores may offer a useful foundation when eval-
uating the viability of protein sources for supporting
healthy muscle aging, it is crucial that thorough in vivo
human investigation is carried out. Specifically, it is
necessary to evaluate the multifactorial physiological
and metabolic effects of ingesting differing protein
sources on skeletal muscle tissue to determine their true
protein quality as it relates to the viability of alternative
dietary protein sources. As illustrated in Figure 4, multi-
ple factors are involved in establishing the efficacy of
protein sources to support skeletal muscle maintenance
and reconditioning, and therefore to assess “true” pro-
tein quality. Regarding these factors, an inverse rela-
tionship between the amount of (human) data available
and its conclusiveness in terms of physiological
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endpoints is often observed. For instance, protein com-
position data (ie, essential amino acid composition and
content) is broadly available across all types of protein
sources, but provides the least-refined information re-
garding the in vivo skeletal muscle response.
Ultimately, both context (ie, age, and level of exercising)
and quality of data are required to assess the utility of
multiple types of dietary protein to support healthy
Animal-derived dietary protein sources
Referring to Figure 4; although not complete by any
means, datasets pertaining to animal-derived dietary
proteins are by far the most complete and robust cate-
gory. Animal-derived dietary protein sources generally
encompass a complete amino acid profile, and therefore
are unlikely to elicit any specific amino acid deficiencies
that limit muscle protein synthesis rates. For example,
dairy, meat, and fish protein sources possess, on aver-
age, leucine (8%–9% of total protein), branched-chain
amino acid (18%–20%), and essential amino acid
(43%) contents above recommended requirements
(set at 5.9%, 12.8%, and 27.7% for leucine, branched-
chain amino acid, and essential amino acid contents, re-
spectively) (Figure 5).
Other key amino acids liable
for a deficiency (eg, lysine and methionine [see Table
S2 in the Supporting Information online]) in other
sources are at levels above minimum requirements in
most animal-based protein sources. Further, systemic
amino acid bioavailability is high following the inges-
tion of animal proteins. Recent work demonstrated that
65%, 57%, and 45% of intrinsically stable isotopically la-
belled protein–derived phenylalanine appeared in the
circulation following the ingestion of milk, whey, and
casein protein, respectively.
Similar values have been
reported for minced beef (61%–64%),
beef steak
whole egg (68%),
and egg whites (66%).
While equivalently detailed (intrinsically labelled) data
are not available for other protein sources, direct com-
parisons of postprandial amino acid availability follow-
ing ingestion of animal vs non-animal proteins have
clearly supported the contention of animal protein
exhibiting high (essential) amino acid bioavailability,
suggested as being due to superior protein digestibility
as well as lower rates of essential amino acid extraction
by splanchnic tissues.
As a result of amino
acid composition and bioavailability data, animal-
Figure 4 Graphical illustration of various research approaches involved in evaluating the utility of various protein sources and their
utility within muscle mass maintenance and adaptation. The model represents the inverse relationship between the amount and detail/
impact of data that is generally available. Abbreviations: DIAAS, Digestible Indispensable Amino Acid Score; PDCAAS, Protein Digestibility
Corrected Amino Acid Score.
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derived dietary proteins typically score highly on indi-
ces of protein quality. As can be seen from Table 1,
meat protein typically scores at 1.00 and approxi-
mately 1.10 on PDCAAS and DIAAS, with dairy pro-
teins scoring at 1.00 and 1.16, respectively.
It is therefore not surprising that in vivo evidence
over the last 20 years has repeatedly and extensively
demonstrated that animal-based protein sources ro-
bustly stimulate (different subfractions of) muscle pro-
tein synthesis rates in the acute postprandial (resting or
post-exercise) period in both young and older individu-
als (eg,
). Indeed, the extensive litera-
ture base that exists has enabled researchers and
practitioners to select divergent sources to manipulate
Figure 5 Mean (6SEM) leucine (A), branched-chain amino acid (B), and essential amino acid (C) content (% of total protein) of vari-
ous categories of dietary protein sources. White bars represent animal-based dietary protein sources, light gray bars represent plant-based
protein sources, mid-gray bars represent fungal-derived protein sources, dark gray bars represent algal-derived protein sources, and black
bars represent insect-derived protein sources. Values between parentheses indicate the number (n) of included types of dietary protein for
which the mean (6SEM) leucine, branched-chain amino acid, and essential amino acid content is calculated. Dashed lines represent the
amino acid requirements for adults, derived from the WHO/FAO/UNU guidelines.
Data are obtained from Table S2 in the Supporting
Information online. Abbreviations: BCAA, branched-chain amino acid; EAA, essential amino acid.
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the rate of digestion and absorption kinetics, depending
on the application. For instance, “fast” proteins such as
whey have been successfully applied to maximize post-
exercise muscle protein anabolism,
while slower
proteins (eg, casein) have been used pre-sleep to pro-
vide a more sustained amino acid precursor for
optimizing overnight (post-exercise) muscle protein
synthesis rates and subsequent recovery.
manipulations have been made by considering the form
in which animal proteins are consumed (eg, meat steak
vs mince,
complete vs hydrolysates,
or iso-
lates vs whole foods
). Such data exemplify how
Table 1 PDCAAS and DIAAS of various protein sources
Protein source PDCAAS DIAAS References
Egg 1.00 1.16 Ertl et al (2016)
Cow milk 1.00 1.16 Ertl et al (2016)
Mackerel 1.00 Boye et al (2012)
Tuna 1.00 Su
arez L
opez et al (2006)
Beef 1.00 1.12 Ertl et al (2016)
Chicken 1.00 1.08 Ertl et al (2012)
Lamb/sheep 1.00 1.17 Ertl et al (2012), Su
arez L
opez et al (2006)
Pork 1.00 1.14 Ertl et al (2016)
Barley 0.59 0.47 Ertl et a. (2016)
Maize 0.47 0.42 Ertl et al (2016)
Oat 0.57 0.77 Cervantes-Pahm et al (2014)
Rice 0.64 Cervantes-Pahm et al (2014)
Rye 0.59 0.48 Ertl et al (2016)
Sorghum 0.29 0.29 Cervantes-Pahm et al (2014)
Wheat 0.45 0.40 Ertl et al (2016)
Chick pea 0.84 0.82 Nosworthy et al (2020)
Common beans
Black bean 0.70 0.65 Nosworthy et al (2018)
Faba bean 0.58 0.54 Nosworthy et al (2018)
Navy bean 0.61 0.56 Nosworthy et al (2018)
Pinto bean 0.66 0.61 Nosworthy et al (2018)
Kidney bean 0.65 0.60 Nosworthy et al (2018)
Faba bean 0.58 0.54 Nosworthy et al (2018)
Green 0.51 0.49 Nosworthy et al (2018)
Red 0.55 0.54 Nosworthy et al (2018)
Lupin 0.68 Herreman et al (2020)
Pea 0.78 1.00 Ertl et al (2016), Guillin et al (2021)
Soy 1.00 0.99 Ertl et al (2016)
Hemp seed 0.54 Herreman et al (2020)
Quinoa 0.79 Boye et al (2012)
Carrot 0.90 Su
arez L
opez et al (2006)
Lettuce 0.19 Suarez Lopez et al (2006)
Onion 0.47 Su
arez L
opez et al (2006)
Potato 1.00 Herreman et al (2020)
Spinach 0.90 Su
arez L
opez et al (2006)
Tomato 0.47 Su
arez L
opez et al (2006)
Mycoprotein 0.99 Edwards and Cummings (2010)
Beetles 0.89 Yang et al (2014)
Mealworm 0.82 Jensen et al (2019)
Silkworm 0.86 Longvah et al (2011)
Abbreviations: DIAAS, Digestible Indispensable Amino Acid Score; PDCAAS, Protein Digestibility Corrected Amino Acid Score.
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relatively advanced the literature base on animal-
derived proteins and muscle protein metabolism has be-
come, at least relative to non-animal-derived protein
Acute laboratory studies of hourly muscle protein
turnover rates have ultimately translated to long(er)
term skeletal muscle metabolism and remodeling, in-
cluding within the context of exercise and aging. For ex-
ample, deuterated water techniques have demonstrated
that animal protein intake or supplementation facili-
tates high rates of daily and free-living muscle protein
synthesis, both at rest and in tandem with exercise in
young and older adults.
These findings also cor-
respond with chronic studies demonstrating positive
associations between animal protein intake contributing
to a greater daily total protein intake in surplus of the
current RDA, and the maintenance or gain of muscle
mass, quality, function, and associated clinical parame-
ters in aging populations.
Such work is
now plentiful enough that a series of comprehensive
systematic reviews and meta-analyses have been able to
show the beneficial effects of increasing dietary protein
intake in older adults using animal-derived sources
upon muscle mass maintenance and adaptation, with or
without exercise interventions.
Plant-derived dietary protein sources
The contribution of plant-derived protein sources to
global protein intake is often underappreciated; indeed,
plant proteins represent the predominant source of pro-
tein consumed worldwide.
Moreover, there is rapidly
increasing popularity of various iterations of plant-based
and an expanding variety of commercially avail-
able plant-based protein isolates or plant-protein prod-
ucts (eg, powders, bars, burgers, meat alternative
products, etc.) to support these lifestyle choices. Despite
this, relatively little research attention has been paid to
the muscle anabolic properties of plant-based proteins. A
widely held view seems to exist that plant-derived pro-
teins are less anabolic than animal-derived proteins. This
is primarily due to generally lower protein densities per
gram of product, but may also be the case on a gram-for-
gram of protein basis, due to less favorable amino acid
profiles and/or poorer amino acid bioavailability. Plant
proteins are typically lower in essential amino acids and
less balanced, with most sources displaying at least one
essential amino acid “deficiency” (Figure 5).
The relative lack of in vivo human data concerning
muscle protein synthetic responses to various plant-
derived protein sources makes consideration of how di-
vergent sources differ in their composition crucial. For
example, vegetables are known to be low in protein
(<2% of total mass), whereas cereals have a similar
protein density to that of dairy (8%–10%), and seeds
(dried) and legumes are particularly protein-dense
foods (21%–26%), equivalent to that found in lean
meat (18%) or fish (19%) (Figure 1, see Table S1 in
the Supporting Information online).
However, plant-
protein-containing food typically contributes to a lesser
extent to total energy intake.
Therefore, obtaining
25%–35% of total energy intake (ie, considered as high
protein with respect to total energy intake
) from
(whole-food) plant protein is more challenging than
ingesting an equivalent amount of energy from animal-
based protein sources, and could lead to excess energy
intake, especially in pursuit of optimizing dietary pro-
tein intake beyond the RDA in older adults. Protein
contents of various commercially available plant protein
concentrates or isolates are approximately 51%–81%, a
range that is also reported for animal-derived protein
concentrates and isolates.
Even with comparable pro-
tein isolates, leucine (5%–9% vs 8%–9% of total pro-
tein), branched-chain amino acid (13%–18% vs 18%–
20%), and essential amino acid (32%–38% vs 43%)
contents of plant-based protein sources are generally
lower when compared with animal-based proteins
(Figure 5).
Several plant-based protein sources are
also low/deficient in specific essential amino acids; eg,
lysine (eg, cereals, seeds), methionine (legumes, vegeta-
bles), or leucine (vegetables), with contents below the
recommended WHO/FAO/UNU requirements of 4.5%,
1.6%, and 5.9% of total protein, respectively (Figure 5,
see Table S2 in the Supporting Information on-
However, a few exceptions worth mentioning
are spinach, pea, maize, quinoa, and lentils because of
their balanced, nondeficient essential amino acid profile
and relatively high leucine content, especially in com-
parison with other plant-based protein sources
(Figure 5).
Still, due to low(er) protein/essential
amino acid contents in raw forms (particularly per calo-
rie ingested), coingestion of / fortification with specific
amino acids (eg, leucine, methionine, lysine)
or essential amino acid supplementation,
or further
food processing are viable strategies for optimizing pro-
tein intake (without excess energy consumption) from
primarily plant-based diets.
Human experiments have reliably demonstrated
that plant-derived protein sources display inferior pro-
tein digestibility/amino acid bioavailability in compari-
son with animal-based comparators,
with pea
protein as a notable exception.
In combination with
amino acid content, this results in lower scores on the
protein quality indices for plant-derived proteins (ie,
PDCAAs and DIAAS of 0.29–1.00; see Table 1). Inferior
postprandial plant protein amino acid availability has
been attributed to greater splanchnic extraction of
plant- than animal-derived amino acids, and higher
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amino acid oxidation (ie, ureagenesis) and deamination
Greater ureagenesis may be caused by the lack
of specific essential amino acids, resulting in an imbal-
ance in the essential amino acid mixture required for
(gut) protein synthesis. As a consequence, a greater pro-
portion of free amino acids will be directed to the liver,
which will serve as a stimulus for ureagenesis.
Also, the naturally occurring presence of antinutritional
compounds found in plant proteins (eg, trypsin inhibi-
tors, phytates, and tannins) may further impede protein
digestibility, absorbability, and subsequently systemic
amino acid availability.
Removal of these compo-
nents by various processing methods (such as roasting,
extrusion, soaking, and blanching) has been shown to
enhance protein digestibility and absorbability
(reviewed in
). Contrarily, while removal of such fac-
tors appears to positively affect protein digestion and
absorption kinetics, it is unclear whether such factors or
compounds play a role in the postprandial anabolic re-
sponse. Moreover, potentiating anabolic effects of vari-
ous nonprotein factors (including vitamins, minerals,
fiber, and other bioactive compounds) have been postu-
For example, fermentation of dietary fibers by
gut microbiota results in the production of short-chain
fatty acids (primarily acetate, propionate, and butyrate),
which have been linked with the postprandial anabolic
As protein is mostly consumed within a
whole food, the presence of such factors should there-
fore not be neglected, and it raises the importance of
thinking of protein concentrates/isolates as distinct
from protein within a food, meal or diet.
The translation of findings about the protein qual-
ity of plant- vs animal-derived protein sources to in
vivo muscle protein synthesis date is broadly in line
with what one would expect. What is less often recog-
nized, however, is that this area of research is still in its
infancy. To date, comparisons have shown that inges-
tion of isolated milk proteins (whey, casein, and/or
complete milk protein) typically,
but not al-
elicits a greater acute (ie, 0–4 h) muscle pro-
tein synthetic response compared with the ingestion of
an isonitrogenous bolus of plant-based protein in both
young and older muscle. In the few studies available,
consuming a greater amount of plant- vs animal-
derived protein seems to “rescue” this response,
not always.
These protein-matched comparisons are
limited in their diversity, only comparing milk proteins
with soy, wheat, or potato proteins—plant-based sour-
ces that are considerably lower in essential amino acid
content and exhibit lower postprandial amino acid bio-
availability compared with milk proteins.
As a result, the data thus far are perhaps unsurprising,
and leave open important questions, and the urgent
need to expand the dataset. For example, it has also
been suggested that an inferior anabolic capacity of
plant proteins may be due to the presence of rate-
limiting amino acids, which could be compensated for
by consuming more of the protein. Such a proposition
has implications for total protein intake recommenda-
tions and, therefore, the sustainability credentials of a
given diet. An attractive alternative to simply consum-
ing more plant-based protein is the idea of blending
sources to optimize amino acid profiles, an idea sup-
ported by data demonstrating that soy–dairy,
and milk–wheat
blends stimulate
muscle protein synthesis to a similar extent to protein-
matched dairy-derived protein comparators.
Several key factors remain to be elucidated regard-
ing the postprandial regulation of muscle protein syn-
thesis upon plant-based protein ingestion. For example,
it is not clear whether the proposed amino acid defi-
ciencies in many sources exist in vivo, and if so,
whether they may explain the lower anabolic potential
of plant-derived proteins. If they do, it is not clear
whether this is a direct result of differing initial amino
acid composition, or an indirect result (due to lower
bioavailability of the “same” protein as a result of infe-
rior protein digestibility/absorbability/greater splanch-
nic extraction). Further, whether such an in vivo amino
acid deficiency would represent limitation(s) to the ana-
bolic signaling capacity (eg, due to differing leucine or
branched-chain amino acid contents within isonitroge-
nous doses) or rather substrate limitation for (continua-
tion of) muscle protein synthesis (eg, due to differing
essential amino acid contents, such as lysine or methio-
nine, across isonitrogenous sources) has also not been
clarified. These are consequential questions that require
definitive data, in models of aging and exercise, when
determining how to recommend plant-derived proteins
to support healthy aging in the face of age-related ana-
bolic resistance.
Studies attempting to translate acute muscle pro-
tein synthetic data following plant-based protein inges-
tion to long(er) term muscle protein turnover
measurements are currently restricted to a single study
looking at potato protein supplementation over a 14-
day free-living period.
In this study, the authors
showed that potato protein isolate can be used as an ef-
fective supplemental protein source to increase daily
protein consumption and enhance daily muscle protein
synthesis rates at rest and during resistance training
(compared with no protein supplementation).
However, it remains unclear how this would fare com-
pared with other (animal or plant) proteins. Indeed, the
few long-term studies that have investigated skeletal
muscle adaptive responses to plant-based proteins have
reported mixed findings regarding their anabolic capac-
ities. For example, some,
but not all,
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chronic resistance exercise training studies in both
young and older adults show that plant-protein supple-
mentation (with soy,
or pea,
adopting a plant-based diet
can facilitate similar