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HIGHLIGHTED TOPIC Regulation of Protein Metabolism in Exercise and Recovery
Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed
muscle protein synthesis at rest and following resistance exercise
in young men
Jason E. Tang,
1
Daniel R. Moore,
1
Gregory W. Kujbida,
1
Mark A. Tarnopolsky,
2
and Stuart M. Phillips
1
1
Department of Kinesiology-Exercise Metabolism Research Group, and
2
Pediatrics and Neurology, McMaster University,
Hamilton, Ontario, Canada
Submitted 25 January 2009; accepted in final form 6 July 2009
Tang JE, Moore DR, Kujbida GW, Tarnopolsky MA, Phillips
SM. Ingestion of whey hydrolysate, casein, or soy protein isolate:
effects on mixed muscle protein synthesis at rest and following
resistance exercise in young men. J Appl Physiol 107: 987–992, 2009.
First published July 9, 2009; doi:10.1152/japplphysiol.00076.2009.—
This study was designed to compare the acute response of mixed
muscle protein synthesis (MPS) to rapidly (i.e., whey hydrolysate and
soy) and slowly (i.e., micellar casein) digested proteins both at rest
and after resistance exercise. Three groups of healthy young men (n⫽
6 per group) performed a bout of unilateral leg resistance exercise
followed by the consumption of a drink containing an equivalent
content of essential amino acids (10 g) as either whey hydrolysate,
micellar casein, or soy protein isolate. Mixed MPS was determined by
a primed constant infusion of L-[ring-
13
C
6
]phenylalanine. Ingestion of
whey protein resulted in a larger increase in blood essential amino
acid, branched-chain amino acid, and leucine concentrations than
either casein or soy (P ⬍0.05). Mixed MPS at rest (determined in the
nonexercised leg) was higher with ingestion of faster proteins
(whey ⫽0.091 ⫾0.015, soy ⫽0.078 ⫾0.014, casein ⫽0.047 ⫾
0.008%/h); MPS after consumption of whey was ⬃93% greater than
casein (P ⬍0.01) and ⬃18% greater than soy (P ⫽0.067). A similar
result was observed after exercise (whey ⬎soy ⬎casein); MPS
following whey consumption was ⬃122% greater than casein (P ⬍
0.01) and 31% greater than soy (P ⬍0.05). MPS was also greater with
soy consumption at rest (64%) and following resistance exercise
(69%) compared with casein (both P⬍0.01). We conclude that the
feeding-induced simulation of MPS in young men is greater after
whey hydrolysate or soy protein consumption than casein both at rest
and after resistance exercise; moreover, despite both being fast pro-
teins, whey hydrolysate stimulated MPS to a greater degree than soy
after resistance exercise. These differences may be related to how
quickly the proteins are digested (i.e., fast vs. slow) or possibly to
small differences in leucine content of each protein.
hypertrophy; muscle mass; weightlifting
TWO OF THE MOST POTENT stimulators of skeletal muscle protein
synthesis (MPS) are feeding and resistance exercise (29). The
postprandial increase in circulating essential amino acids stim-
ulates a marked rise in protein synthesis (7, 15, 35); this effect
appears to be due to the amino acids themselves acting as the
stimulus and not an effect due to the modest increases in
insulin that result from amino acid ingestion (16). Resistance
exercise potentiates the anabolic effect of feeding (32, 35).
Several studies examining the consumption of whole proteins
have found that the type of protein, and not simply its amino
acid composition, can differentially modulate the anabolic
response (3, 8, 9, 37). For example, it has been suggested that
milk promotes better whole body nitrogen retention at rest (4,
14), and greater skeletal muscle protein accretion after resis-
tance exercise, compared with soy protein (37). The difference
in the metabolism of milk and soy proteins has been attrib-
uted to their digestion kinetics, wherein milk is digested
slower than soy (4). Milk contains two protein fractions,
whey and casein, which have been characterized based on
their rate of digestion as “fast” and “slow” proteins, respec-
tively (3). Soy, on the other hand, contains a single homo-
geneous protein fraction, which is digested in a manner
more similar to whey than casein (4).
Whey protein is acid soluble and thus is digested quickly and
results in a pronounced aminoacidemia. Data obtained at the
whole body level show that whey induces a transient rise in
whole body protein synthesis and leucine oxidation at rest (3,
8, 9). Conversely, casein has a modest effect on whole body
protein synthesis but instead inhibits whole body protein break-
down (3, 8, 9). Thus, at least at the whole body level, protein
digestion rate appears to be an independent factor regulating
protein anabolism (8). While the data from whole body protein
kinetics point to differential effects of different proteins on
synthesis and breakdown (3, 8, 9), skeletal muscle only con-
tributes ⬃25–30% to whole body protein synthesis (25), and its
turnover rate is much lower (on the order of 20⫻) than that of
more rapidly-turning-over gut (26, 27) and plasma proteins (6).
Thus protein turnover measured at the whole body level may or
may not be reflective, or even representative, of the anabolism
of muscle proteins. At present, only a single study has mea-
sured the chemical net balance of amino acids across a limb
following whey and casein ingestion (34), but these data do not
give kinetic results and were equivocal depending on the
choice of amino acid tracer studied. Thus to date no study has
directly compared changes in skeletal MPS following the
consumption of isolated proteins with differing rates of diges-
tion in humans. The purpose of this study, therefore, was to
measure the response of skeletal MPS following the ingestion
of three distinct but high-quality proteins (from a dietary
standpoint), whey, micellar casein, and soy, at rest and after
resistance exercise. We chose to measure these responses
following ingestion of similar quantities of these proteins but
Address for reprint requests and other correspondence: S. M. Phillips,
Dept. of Kinesiology-Exercise Metabolism Research Group, McMaster
Univ., 1280 Main St. West, Hamilton, Ontario L8S 4K1, Canada (e-mail:
phillis@mcmaster.ca).
J Appl Physiol 107: 987–992, 2009.
First published July 9, 2009; doi:10.1152/japplphysiol.00076.2009.
8750-7587/09 $8.00 Copyright ©2009 the American Physiological Societyhttp://www. jap.org 987
matched them on their total essential amino acid (EAA) con-
tent since only EAA are needed to stimulate MPS (36). We
employed a unilateral model of exercise that permitted the
comparison of the effect of protein ingestion on muscle anab-
olism both at rest and after resistance exercise within a given
individual. Our hypothesis was that the consumption of whey
hydrolysate, casein, and soy proteins would differentially stim-
ulate MPS, based on the rate at which they are digested
(whey ⬎soy ⬎casein), both at rest and after resistance
exercise.
MATERIALS AND METHODS
Subjects. Three groups of six healthy young men (n⫽18) who
regularly engaged in whole body resistance training (2–3 days/wk)
volunteered to take part in the study. There were no differences in age,
height, or weight between groups (P ⬎0.56; 22.8 ⫾3.9 yr; 179.7 ⫾
5.1 cm; 86.6 ⫾13.9 kg; pooled mean ⫾SD for all subjects). Subjects
were informed of the purpose of the study, experimental procedures to
be used, and potential risks. Written consent was obtained from all
subjects before commencing the study. This study was approved by
the McMaster University and Hamilton Health Sciences Research
Ethics Board. All testing procedures conformed to those outlined in
the Helsinki Declaration of 1963 on the use of human subjects in
research.
Experimental protocol. The protocol was designed to examine the
effect of consuming whey, casein, and soy protein on mixed muscle
protein fractional synthetic rate (FSR) after an acute bout of resistance
exercise. At least 1 wk before their first experimental trial, subjects
participated in a familiarization session to become acquainted with the
testing procedures and training equipment to be used. During the
familiarization session, each subject’s 10-repetition maximum (RM)
was determined for the seated leg press and knee extension exercises
(Universal Gym Equipment, West Point, MS). Subjects performed
both exercises unilaterally such that the contralateral leg served as a
nonexercised control. For the 2 days before each experimental trial,
subjects were asked to refrain from performing any resistance exercise
with their legs. In addition, subjects consumed prepackaged diets on
those 2 days designed to meet daily caloric (Harris-Benedict equation
using an activity factor of 1.6 for all participants) and protein require-
ments for resistance-trained individuals (1.2–1.4 g/kg) (33).
Subjects arrived at the laboratory on the morning of each experi-
mental trial after an overnight fast. After a baseline blood sample was
drawn, subjects performed a bout of intense unilateral resistance
exercise consisting of four sets each of leg press and knee extension
exercises at a workload equivalent to previously determined 10- to
12-RM with 2 min of passive rest between sets. After the exercise
bout, subjects had a 20-gauge catheter inserted into a dorsal hand vein,
which was kept patent with a 0.9% saline drip, and a second blood
sample was drawn. Subjects then consumed a drink (⬃100 kcal)
containing whey (21.4 g), casein (21.9 g), or soy (22.2 g) protein
dissolved in 250 ml water with sucralose (1 g, Splenda) for sweeten-
ing and vanilla extract (2 ml) to increase palatability (Table 1). In an
effort to maximize protein synthesis with feeding, the amount of
protein in each drink provided ⬃10 g of EAA (7, 24). A small amount
of tracer was added to each protein drink (8% of phenylalanine
content) to minimize changes in blood enrichment after consuming
the drink. Whey protein hydrolysate and micellar casein were ob-
tained from American Casein (AMCO, Burlington, NJ) while isolated
soy protein (Profam 891) was a generous gift from Archer Daniels
Midland (ADM, Decatur, IL). A primed-continuous infusion of
L-[ring-
13
C
6
]phenylalanine (0.05 mol䡠kg
⫺1
䡠min
⫺1
,2mol/kg
prime; Cambridge Isotope Laboratories, Woburn, MA) was then
administered through a 0.2-m filter into an antecubital vein catheter
after consumption of the drink to measure mixed muscle FSR.
Arterialized blood samples were obtained at 30, 60, 90, 120, and 180
min after consumption of the protein drink by warming the hand with
a heating blanket (50°C). The infusion protocol is illustrated in Fig. 1.
Muscle needle biopsy. A percutaneous needle biopsy was taken,
under local anesthetic, from the vastus lateralis muscle of both the
exercised and nonexercised legs 180 min following the consumption
of the protein drink. As subjects had not previously been infused with
L-[ring-
13
C
6
]phenylalanine, baseline enrichment of the muscle was
estimated from the enrichment of a mixed plasma protein pellet
precipitated from the preinfusion blood samples (22, 23). The plasma
protein pellet was processed and analyzed in the same manner as the
muscle-bound protein pellet (see below).
Blood analyses. Blood samples were collected into evacuated
containers containing lithium heparin and deproteinized in perchloric
acid (PCA). Whole blood amino acid concentrations were determined
on the PCA extract by high-performance liquid chromatography as
previously described (37). The remaining whole blood was centri-
fuged at 1,200 rpm for 10 min at 4°C to separate the plasma. Plasma
was removed and stored at ⫺20°C until further analysis. Plasma
insulin concentration was determined using standard radioimmunoas-
say kits (Diagnostic Products, Los Angeles, CA).
Ethanol was added to plasma to precipitate all plasma proteins. The
sample was then centrifuged at 1,200 rpm for 10 min at 4°C to pellet
the proteins, and the supernatant was decanted. Proteins were hydro-
lyzed using 6 N HCl (1,000 l) for 24 h at 110°C. The protein
hydrolysate was passed over a cation-exchange column (Dowex
50WX8 –200 resin; Sigma-Aldrich), then dried under dried N
2
gas
before analysis for isotopic enrichment, as described below.
Muscle analyses. Acetonitrile (10 l/mg) was added to muscle
samples (⬃20 mg) before being manually homogenized, vortexed,
and then centrifuged at 15,000 rpm for 10 min at 4°C. The supernatant
containing the muscle intracellular free (MIF) amino acids was col-
lected and the procedure repeated. The pooled supernatant was then
dried under N
2
gas for analysis of the MIF amino acid enrichments, as
described below. The remaining muscle pellets were washed twice
with distilled water, once with absolute ethanol, and then lyophilized
to dryness. The dry muscle pellets were subsequently weighed and
hydrolyzed with 6 N HCl (400 l/mg) for 24 h at 110°C. The bound
protein hydrolysate was passed over a cation-exchange column
(Dowex 50WX8 –200 resin; Sigma-Aldrich), then dried under dried
N
2
gas before analysis, as described below.
Table 1. Total and essential amino acid content
of protein drinks
Protein Drink
Whey Casein Soy
Alanine, g 1.1 0.6 1.0
Arginine, g 0.6 0.8 1.7
Aspartic acid, g 2.2 1.4 2.6
Cystine, g 0.4 0.1 0.3
Glutamic acid, g 3.6 4.4 4.3
Glycine, g 0.4 0.5 0.9
Histidine, g 0.4 0.6 0.6
Isoleucine, g 1.4 1.2 1.1
Leucine, g 2.3 1.8 1.8
Lysine, g 1.9 1.6 1.4
Methionine, g 0.5 0.5 0.3
Phenylalanine, g 0.7 1.0 1.2
Proline, g 1.4 2.2 1.2
Serine, g 1.1 1.2 1.2
Threonine, g 1.0 0.9 0.8
Tryptophan, g 0.3 0.2 0.2
Tyrosine, g 0.7 1.2 0.8
Valine, g 1.0 1.4 1.1
Total, g 21.4 21.9 22.2
EAA, g 10.0 10.1 10.1
EAA, essential amino acids.
988 WHEY PROTEIN AND MUSCLE ANABOLISM
J Appl Physiol •VOL 107 •SEPTEMBER 2009 •www.jap.org
Gas chromatography-mass spectrometry. Blood, plasma protein,
and MIF enrichment were determined by making the heptafluorobu-
tyryl isobutyl (HFB) derivative of phenylalanine (28). Isotopic en-
richments were measured by gas chromatography-mass spectrometry
(GC-MS; Hewlett-Packard 5980/5989B, Palo Alto, CA) with ions
selectively monitored at mass-to-charge (m/z) ratios of 316 and 322,
and a skewed abundance distribution correction was applied (38).
Baseline plasma protein and bound muscle protein enrichments were
determined by measuring the N-acetyl-n-propyl ester (NAP) deriva-
tive of phenylalanine by gas chromatography combustion-isotope
ratio mass spectrometry (GC-C-IRMS; Hewlett-Packard 6890, Palo
Alto, CA; Thermo Finnigan Delta Plus XP, Waltham, MA). Derivat-
ized amino acids were separated on a 30m DB-1701 column before
combustion (temperature ramp: 110°C for 2 min; 20°C/min ramp to
210°C; 5°C/min ramp to 280°C; hold for 5 min).
Calculations. Mixed muscle protein FSR was calculated from the
determination of the rate of tracer incorporation into muscle protein
and using the MIF phenylalanine enrichment as a precursor, according
to the equation:
FSR (%/h) ⫽共Em1⫺Em0兲
关Ef䡠共t1⫺t0兲兴 ⫻100
where Em
0
is the enrichment of the protein-bound isotope tracer from
isolated plasma proteins with the assumption that tracer-naive subjects
would have an m⫹6 phenylalanine enrichment of virtually zero (i.e.,
equivalent in muscle and blood). The enrichment obtained from the
pool of all plasma proteins therefore represents a basal measure of
isotopic enrichment for m⫹6 from which the enriched measurement
can be taken. Em
1
is the enrichment of the protein-bound isotope
tracer from the second biopsy, Efis the mean MIF tracer enrichment
during the time period for determination of protein incorporation, and
(t
1
⫺t
0
) is the incorporation time. It is possible that the assumption of
zero for a baseline enrichment would serve to overestimate the true
FSR; however, we believe this overestimation would be the same
between conditions.
Statistical analyses. Subject anthropometric data and leucine area
under the curve (AUC) data were analyzed using t-tests with Bonfer-
roni correction. All other data were analyzed using a two-factor
repeated-measures ANOVA. When significance was indicated, a
Tukey honestly significant differences (HSD) post hoc procedure was
used to identify pairwise differences. All statistical analyses were
performed using SigmaStat 3.10.0 (www.systat.com, Systat Software,
Point Richmond, CA), and significance was accepted at P⬍0.05. All
data are presented as means ⫾SD.
RESULTS
Plasma insulin concentration. Plasma insulin at baseline
was similar between all three groups (Fig. 2). There was a
small rise in plasma insulin at 60 min following whey and soy
consumption (both P⬍0.05). Plasma insulin was unchanged
after the ingestion of casein protein (P⫽0.43).
Blood amino acid concentrations. Changes in the concen-
tration of EAA, branched-chain amino acids (BCAA; data not
shown), and leucine in the blood followed the same general
pattern. All proteins stimulated a rise in EAA (whey ⬇soy ⬎
casein; Fig. 3A) and leucine (whey ⬎soy ⬎casein; Fig. 3B)
concentration by 30 min postingestion; however, whey protein
resulted in a more pronounced aminoacidemia than either
casein or soy (P ⬍0.05). At 60 min postconsumption, the
concentration of EAA and leucine was also higher following
whey consumption than either casein or soy (whey ⬎soy ⬎
casein; all P⬍0.05). The AUC for blood leucine after whey
ingestion was ⬃73% greater than soy and ⬃200% greater than
casein (Fig. 3B, inset).
Plasma and muscle intracellular free phenylalanine enrich-
ment. Plasma and muscle intracellular free phenylalanine en-
richments are shown in Fig. 4, Aand B, respectively. Linear
regression analysis (not shown) indicated that the slopes of the
plasma enrichments over time were not significantly different
from zero (P ⬎0.05), suggesting that plasma enrichments had
reached a plateau and subjects were at isotopic steady state
over the incorporation period.
Mixed MPS. At rest, both whey and soy FSR were signifi-
cantly greater than casein (P ⬍0.01; Fig. 5). Whey FSR tended
to be greater than soy at rest but was not significantly different
(P ⫽0.067). After resistance exercise, FSR was greater com-
pared with rest in all groups (P ⬍0.05; Fig. 5). FSR following
whey consumption was significantly greater than both soy and
casein after resistance exercise (P ⬍0.05).
DISCUSSION
This is the first study to report directly measured rates of
mixed MPS in response to ingesting isolated proteins that are
known to be digested at different rates in humans. We found
that the consumption of whey protein hydrolysate stimulated
MPS to a greater degree than casein both at rest and after
resistance exercise. While FSR in the whey group tended to be
greater than soy in the rested muscle, this did not reach
statistical significance. After resistance exercise whey hydro-
lysate stimulated a significantly larger rise in MPS than soy. In
Fig. 1. Experimental protocol.
Fig. 2. Plasma insulin concentration after ingestion of whey hydrolysate,
casein, or soy protein. *Significantly different from casein for same condition
(P ⬍0.05). All values are means ⫾SD; n⫽6 per group.
989WHEY PROTEIN AND MUSCLE ANABOLISM
J Appl Physiol •VOL 107 •SEPTEMBER 2009 •www.jap.org
congruence with our previous work showing a greater stimu-
lation of MPS with milk vs. soy protein ingestion (37), soy
appears to be less effective at stimulating MPS than whey
protein despite inducing a similar rise in circulating EAA.
Based on previous literature identifying protein digestibility
as an independent factor regulating whole body protein anab-
olism (8), we hypothesized that the pattern of appearance of
amino acids in the systemic circulation following consumption
of whey, casein, or soy would also result in a differential
stimulation of protein synthesis at the muscle level. For exam-
ple, several studies have noted that “fast” proteins stimulate a
large rise in protein synthesis whereas “slow” proteins primar-
ily inhibit protein breakdown, but these results come from data
at the whole body level (3, 8, 9) of which muscle comprises
only 25% (25) and turns over at a much slower rate than, for
example, gut proteins (26, 27). In addition, milk proteins
appear to support greater “peripheral” (i.e., muscle) vs.
splanchnic protein synthesis than do soy proteins (4, 14). Our
data extend these previous studies that measured only whole
body protein turnover by demonstrating that the consumption
of whey hydrolysate and soy isolate (i.e., “fast” proteins) result
in considerably higher rates of muscle protein synthesis than
casein (i.e., “slow” protein), both at rest (whey ⬇soy ⬎
casein) and after resistance exercise (whey ⬎soy ⬎casein). In
our view, these differences are unlikely to be explained by our
use of a whey protein hydrolysate, rather than isolate. This is
because previous data have noted no difference in the pattern
of aminoacidemia following ingestion of 36 g of whole whey
protein or its hydrolysate (5). While the pattern of peripheral
aminoacidemia yields no insight into the actual kinetics of
protein absorption, this fact is of little consequences since the
concentration of amino acids in the peripheral (i.e., non-
Fig. 3. Blood concentration of essential amino acids (A) and leucine (B) after
ingestion of whey hydrolysate, casein, or soy protein. Inset: leucine area under
the curve (AUC). *Significantly different from casein (P ⬍0.05). # Signifi-
cantly different from soy (P ⬍0.05). All values are means ⫾SD; n⫽6 per
group. Some error bars have been omitted for clarity.
Fig. 4. Plasma (A) and muscle (B) intracellular free phenylalanine enrichment
(tracee-to-tracer ratio; t 䡠T
⫺1
). All values are means ⫾SD; n⫽6 per group.
Ex, exercise.
Fig. 5. Mixed muscle protein fractional synthetic rate (FSR) after ingestion of
whey hydrolysate, casein, or soy protein at rest and after resistance exercise.
*Significantly different from casein for same condition (P ⬍0.01). # Signif-
icantly different from soy for same condition (P ⬍0.05). All values are
means ⫾SD; n⫽6 per group.
990 WHEY PROTEIN AND MUSCLE ANABOLISM
J Appl Physiol •VOL 107 •SEPTEMBER 2009 •www.jap.org
splanchnic) circulation would be those that are available for
protein synthesis by peripheral tissues such as muscle (assum-
ing of course equivalent flow). Interestingly, when examining
whole body leucine kinetics, prior studies actually found that
casein consumption promoted a higher whole body leucine
balance than whey (3, 8, 9). While these findings may seem
contradictory to what we observed here, the inhibitory effect of
casein on protein breakdown, almost certainly in the splanch-
nic region (26, 27), was the largest contributor to the greater
whole body leucine balance observed. In addition, the increase
in whole body protein synthesis stimulated by whey was
observed to be quite transient (3, 8, 9). Admittedly, we chose
a 180-min time point in the present study to capture the acute
MPS response, whereas the whole body data represented an
aggregate 7-h response (3, 8, 9). We do not think, however,
that extending our response beyond 3 h would have markedly
affected our MPS results since amino acid concentrations were
back down to baseline levels by 240 min.
Previously, whey and casein proteins were shown to im-
prove net muscle amino acid balance (measured as a-v balance)
to a similar extent, despite a marked difference in the pattern of
aminoacidemia, presumably reflecting the rate of digestion of
each protein (34). In contrast, we observed marked differences,
using direct incorporation measurements, in rates of postexer-
cise skeletal MPS after whey and casein feeding in the present
study. It is known that resistance exercise increases muscle
protein breakdown, albeit to a lesser extent than synthesis (1,
28). Thus, the results of Tipton et al. (34) could have arisen due
to a marked suppression of muscle protein breakdown with
casein and a stimulation of synthesis with whey. Ingestion of
amino acids attenuates the post-resistance exercise-induced
increase in muscle protein breakdown (2, 35); thus in the
present study it is hard to envision that a marked suppression
of muscle proteolysis occurred with casein ingestion that
would not have occurred with whey or soy. We did not
measure proteolysis, however, and thus can only speculate as
to the effect of protein digestibility on muscle protein break-
down due to current methodological limitations that preclude
the direct measurement of muscle protein degradation after
physiological (i.e., bolus) protein ingestion.
The differences in the stimulation of MPS after ingestion of
whey hydrolysate and soy protein and casein at rest and after
resistance exercise are somewhat surprising given their similar
protein digestibility-corrected amino acid scores (PDCAAS)
(13). Indeed, the PDCAAS of these proteins would suggest that
they are high-quality complete sources of amino acids, which
in theory should be able to equally support protein synthesis.
However, the concept of PDCAAS and their relevance to
physiological outcomes has drawn some criticism (31); our
results suggest that in the context of skeletal muscle accretion
following resistance exercise this is particularly the case. In the
present study there were marked differences in the patterns of
aminoacidemia, which likely reflect the rate of protein diges-
tion not only between fast and slow proteins, but also within
the fast proteins themselves (i.e., whey and soy). The rise in
EAA (Fig. 3A), BCAA (data not shown), and leucine (Fig. 3B)
was of greater amplitude and considerably more rapid follow-
ing whey consumption compared with soy. These differences
in the rate of EAA appearance in the circulation may be
especially important to the differential stimulation of MPS we
observed after whey or soy ingestion at rest and following
resistance exercise. Recent work has demonstrated that supple-
menting soy protein with BCAA (leucine, isoleucine, and
valine) is required to rescue its anabolic effect in elderly and
clinical populations (11). Leucine has also been shown to
enhance the activation of mTOR-related signaling proteins at
rest and after exercise (17, 19, 21). Thus the greater total
BCAA (⬃7%) and leucine content (⬃28%) in particular may
have contributed to the larger increase in protein synthesis after
whey ingestion compared with soy. We speculate that a critical
“trigger” threshold of EAA, perhaps leucine in particular (10),
has to be reached in the blood before MPS is maximally
stimulated and that this threshold was not reached with soy
ingestion. Such a thesis is supported by our recent work
showing a saturable response in MPS following resistance
exercise (24).
The differences in skeletal MPS that we observed may have
implications for populations with compromised nutrient sensi-
tivity (e.g., the elderly) (7). Indeed, it has been found that the
protein digestibility paradigm observed in young individuals is
actually “reversed” in the old with respect to whole body
protein metabolism (9), and that “fast” protein ingestion is
associated with a greater whole body leucine balance (7). If the
leucine “trigger” concept is correct, then we would speculate
that these results (7) may reflect the inability of casein to
increase blood EAA, BCAA, or leucine concentration high
enough to turn on MPS in older persons who appear to have a
reduced sensitivity to amino acids or an “anabolic resistance”
(7). For example, ingestion of larger doses of leucine has been
shown to enhance feeding-induced increases in MPS in aged
individuals (18, 30). Considering protein ingestion after exer-
cise appears critical to enhance skeletal muscle hypertrophy
with resistance training in the elderly (12), we propose that
elderly individuals would likely obtain the greatest benefit with
respect to stimulating MPS and likely muscle protein accretion
by consuming a “fast” leucine-rich dietary protein such as
whey both at rest and after resistance exercise (18, 20, 30).
Future studies should directly measure skeletal MPS in popu-
lations such as the elderly after consuming different whole
proteins to confirm this thesis.
In summary, we report that the consumption of whey protein
hydrolysate stimulates skeletal MPS to a greater extent than
either casein or soy. Our results suggest that the type of protein
consumed is a modulating factor in determining postprandial
resting and postexercise muscle anabolism in young healthy
men, both at rest and after resistance exercise. Moreover, this
effect may be related to the leucine content of the protein
consumed and how quickly it is digested. Thus, it appears that
when providing an optimal dose of protein (⬃10 g EAA) (7,
24), a rapid increase in EAA (perhaps leucine specifically) is
important for supporting maximal rates of skeletal MPS.
ACKNOWLEDGMENTS
We thank E. T. Prior for outstanding support and technical assistance, and
we thank Archer Daniels Midland for the generous gift of the soy protein used
in this study.
GRANTS
This study was supported by a grant from the US National Dairy Council
to S. M. Phillips. S. M. Phillips was a Canadian Institutes of Health Research
(CIHR) New Investigator Career Award recipient, J. E. Tang was a CIHR
Doctoral Award recipient, and D. R. Moore was a CIHR Canada Graduate
991WHEY PROTEIN AND MUSCLE ANABOLISM
J Appl Physiol •VOL 107 •SEPTEMBER 2009 •www.jap.org
Scholarship Doctoral Award recipient. All gratefully acknowledge these
sources of funding.
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