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 doi:10.1152/japplphysiol.00076.2009107:987-992, 2009. First published 9 July 2009; J Appl PhysiolStuart M. PhillipsJason E. Tang, Daniel R. Moore, Gregory W. Kujbida, Mark A. Tarnopolsky andrest and following resistance exercise in young menisolate: effects on mixed muscle protein synthesis atIngestion of whey hydrolysate, casein, or soy protein

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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. Phillips1

1 Department of Kinesiology-Exercise Metabolism Research Group, and   2Pediatrics 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, PhillipsSM.   Ingestion of whey hydrolysate, casein, or soy protein isolate:effects on mixed muscle protein synthesis at rest and followingresistance 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 mixedmuscle protein synthesis (MPS) to rapidly (i.e., whey hydrolysate andsoy) and slowly (i.e., micellar casein) digested proteins both at restand after resistance exercise. Three groups of healthy young men (n 6 per group) performed a bout of unilateral leg resistance exercisefollowed by the consumption of a drink containing an equivalentcontent of essential amino acids (10 g) as either whey hydrolysate,micellar casein, or soy protein isolate. Mixed MPS was determined bya primed constant infusion of  L-[ring-13C6]phenylalanine. Ingestion of whey protein resulted in a larger increase in blood essential aminoacid, branched-chain amino acid, and leucine concentrations thaneither casein or soy  (P 0.05). Mixed MPS at rest (determined in thenonexercised 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 similarresult was observed after exercise (whey     soy     casein); MPSfollowing whey consumption was  122% greater than casein   (P  0.01) and 31% greater than soy  (P 0.05). MPS was also greater withsoy consumption at rest (64%) and following resistance exercise(69%) compared with casein (both  P    0.01). We conclude that thefeeding-induced simulation of MPS in young men is greater afterwhey hydrolysate or soy protein consumption than casein both at restand after resistance exercise; moreover, despite both being fast pro-teins, whey hydrolysate stimulated MPS to a greater degree than soyafter resistance exercise. These differences may be related to howquickly the proteins are digested (i.e., fast vs. slow) or possibly tosmall differences in leucine content of each protein.

hypertrophy; muscle mass; weightlifting

TWO OF THE MOST POTENT  stimulators of skeletal muscle proteinsynthesis (MPS) are feeding and resistance exercise (29). Thepostprandial increase in circulating essential amino acids stim-ulates a marked rise in protein synthesis (7, 15, 35); this effectappears to be due to the amino acids themselves acting as thestimulus and not an effect due to the modest increases ininsulin that result from amino acid ingestion (16). Resistance

exercise potentiates the anabolic effect of feeding (32, 35).Several studies examining the consumption of whole proteinshave found that the type of protein, and not simply its aminoacid composition, can differentially modulate the anabolicresponse (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 differencein the metabolism of milk and soy proteins has been attrib-uted to their digestion kinetics, wherein milk is digestedslower than soy (4). Milk contains two protein fractions,whey and casein, which have been characterized based ontheir 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 mannermore similar to whey than casein (4).

Whey protein is acid soluble and thus is digested quickly andresults in a pronounced aminoacidemia. Data obtained at the

whole body level show that whey induces a transient rise inwhole body protein synthesis and leucine oxidation at rest (3,8, 9). Conversely, casein has a modest effect on whole bodyprotein synthesis but instead inhibits whole body protein break-down (3, 8, 9). Thus, at least at the whole body level, proteindigestion rate appears to be an independent factor regulatingprotein anabolism (8). While the data from whole body proteinkinetics point to differential effects of different proteins onsynthesis and breakdown (3, 8, 9), skeletal muscle only con-tributes 25–30% to whole body protein synthesis (25), and itsturnover 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 ormay 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 limbfollowing whey and casein ingestion (34), but these data do notgive kinetic results and were equivocal depending on thechoice of amino acid tracer studied. Thus to date no study hasdirectly compared changes in skeletal MPS following theconsumption of isolated proteins with differing rates of diges-tion in humans. The purpose of this study, therefore, was tomeasure the response of skeletal MPS following the ingestionof three distinct but high-quality proteins (from a dietarystandpoint), whey, micellar casein, and soy, at rest and afterresistance exercise. We chose to measure these responsesfollowing 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, McMasterUniv., 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.

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matched them on their total essential amino acid (EAA) con-tent since only EAA are needed to stimulate MPS (36). Weemployed a unilateral model of exercise that permitted thecomparison of the effect of protein ingestion on muscle anab-olism both at rest and after resistance exercise within a givenindividual. Our hypothesis was that the consumption of wheyhydrolysate, 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 resistanceexercise.

MATERIALS AND METHODS

Subjects.   Three groups of six healthy young men ( n    18) whoregularly 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). Subjectswere informed of the purpose of the study, experimental procedures tobe used, and potential risks. Written consent was obtained from allsubjects before commencing the study. This study was approved bythe McMaster University and Hamilton Health Sciences ResearchEthics Board. All testing procedures conformed to those outlined inthe Helsinki Declaration of 1963 on the use of human subjects inresearch.

 Experimental protocol.  The protocol was designed to examine theeffect of consuming whey, casein, and soy protein on mixed muscleprotein fractional synthetic rate (FSR) after an acute bout of resistanceexercise. At least 1 wk before their first experimental trial, subjectsparticipated in a familiarization session to become acquainted with thetesting procedures and training equipment to be used. During thefamiliarization 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 performedboth exercises unilaterally such that the contralateral leg served as anonexercised 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 onthose 2 days designed to meet daily caloric (Harris-Benedict equationusing 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 wasdrawn, subjects performed a bout of intense unilateral resistanceexercise consisting of four sets each of leg press and knee extensionexercises at a workload equivalent to previously determined 10- to12-RM with 2 min of passive rest between sets. After the exercisebout, 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 bloodsample was drawn. Subjects then consumed a drink (100 kcal)containing whey (21.4 g), casein (21.9 g), or soy (22.2 g) proteindissolved in 250 ml water with sucralose (1 g, Splenda) for sweeten-

ing and vanilla extract (2 ml) to increase palatability (Table 1). In aneffort to maximize protein synthesis with feeding, the amount of protein in each drink provided 10 g of EAA (7, 24). A small amountof tracer was added to each protein drink (8% of phenylalaninecontent) to minimize changes in blood enrichment after consumingthe drink. Whey protein hydrolysate and micellar casein were ob-tained from American Casein (AMCO, Burlington, NJ) while isolatedsoy protein (Profam 891) was a generous gift from Archer DanielsMidland (ADM, Decatur, IL). A primed-continuous infusion of L-[ring-13C6]phenylalanine (0.05   mol kg1

min1, 2   mol/kgprime; Cambridge Isotope Laboratories, Woburn, MA) was thenadministered through a 0.2-m filter into an antecubital vein catheterafter 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 witha 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 theexercised and nonexercised legs 180 min following the consumptionof the protein drink. As subjects had not previously been infused withL-[ring-13C6]phenylalanine, baseline enrichment of the muscle wasestimated from the enrichment of a mixed plasma protein pelletprecipitated from the preinfusion blood samples (22, 23). The plasmaprotein pellet was processed and analyzed in the same manner as themuscle-bound protein pellet (see below).

 Blood analyses.   Blood samples were collected into evacuatedcontainers containing lithium heparin and deproteinized in perchloricacid (PCA). Whole blood amino acid concentrations were determinedon the PCA extract by high-performance liquid chromatography aspreviously described (37). The remaining whole blood was centri-fuged at 1,200 rpm for 10 min at 4°C to separate the plasma. Plasmawas removed and stored at  20°C until further analysis. Plasmainsulin concentration was determined using standard radioimmunoas-say kits (Diagnostic Products, Los Angeles, CA).

Ethanol was added to plasma to precipitate all plasma proteins. Thesample was then centrifuged at 1,200 rpm for 10 min at 4°C to pelletthe proteins, and the supernatant was decanted. Proteins were hydro-lyzed using 6 N HCl (1,000   l) for 24 h at 110°C. The proteinhydrolysate was passed over a cation-exchange column (Dowex50WX8–200 resin; Sigma-Aldrich), then dried under dried N2   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 supernatantcontaining the muscle intracellular free (MIF) amino acids was col-lected and the procedure repeated. The pooled supernatant was thendried under N2 gas for analysis of the MIF amino acid enrichments, asdescribed below. The remaining muscle pellets were washed twicewith distilled water, once with absolute ethanol, and then lyophilizedto dryness. The dry muscle pellets were subsequently weighed andhydrolyzed with 6 N HCl (400 l/mg) for 24 h at 110°C. The boundprotein hydrolysate was passed over a cation-exchange column(Dowex 50WX8–200 resin; Sigma-Aldrich), then dried under driedN2  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 1.0Arginine, g 0.6 0.8 1.7

Aspartic acid, g 2.2 1.4 2.6Cystine, g 0.4 0.1 0.3Glutamic acid, g 3.6 4.4 4.3Glycine, g 0.4 0.5 0.9Histidine, g 0.4 0.6 0.6Isoleucine, g 1.4 1.2 1.1Leucine, g 2.3 1.8 1.8Lysine, g 1.9 1.6 1.4Methionine, g 0.5 0.5 0.3Phenylalanine, g 0.7 1.0 1.2Proline, g 1.4 2.2 1.2Serine, g 1.1 1.2 1.2Threonine, g 1.0 0.9 0.8Tryptophan, g 0.3 0.2 0.2Tyrosine, g 0.7 1.2 0.8Valine, g 1.0 1.4 1.1Total, g 21.4 21.9 22.2EAA, g 10.0 10.1 10.1

EAA, essential amino acids.

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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 ionsselectively 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 weredetermined by measuring the   N -acetyl-n-propyl ester (NAP) deriva-tive of phenylalanine by gas chromatography combustion-isotoperatio mass spectrometry (GC-C-IRMS; Hewlett-Packard 6890, PaloAlto, CA; Thermo Finnigan Delta Plus XP, Waltham, MA). Derivat-ized amino acids were separated on a 30m DB-1701 column beforecombustion (temperature ramp: 110°C for 2 min; 20°C/min ramp to210°C; 5°C/min ramp to 280°C; hold for 5 min).

Calculations. Mixed muscle protein FSR was calculated from thedetermination of the rate of tracer incorporation into muscle proteinand using the MIF phenylalanine enrichment as a precursor, accordingto the equation:

FSR (%/h) Em1 Em0

E f t 1 t 0 100

where Em0 is the enrichment of the protein-bound isotope tracer fromisolated plasma proteins with the assumption that tracer-naive subjectswould have an  m6 phenylalanine enrichment of virtually zero (i.e.,equivalent in muscle and blood). The enrichment obtained from thepool of all plasma proteins therefore represents a basal measure of isotopic enrichment for  m6 from which the enriched measurementcan be taken. Em1   is the enrichment of the protein-bound isotopetracer from the second biopsy, E f   is the mean MIF tracer enrichmentduring 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 trueFSR; however, we believe this overestimation would be the samebetween 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-factorrepeated-measures ANOVA. When significance was indicated, aTukey honestly significant differences (HSD) post hoc procedure wasused to identify pairwise differences. All statistical analyses wereperformed using SigmaStat 3.10.0 (www.systat.com, Systat Software,Point Richmond, CA), and significance was accepted at  P 0.05. Alldata are presented as means SD.

RESULTS

Plasma insulin concentration.   Plasma insulin at baselinewas similar between all three groups (Fig. 2). There was asmall rise in plasma insulin at 60 min following whey and soy

consumption (both  P 0.05). Plasma insulin was unchangedafter 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 notshown), and leucine in the blood followed the same generalpattern. All proteins stimulated a rise in EAA (whey soy casein; Fig. 3 A) and leucine (whey    soy    casein; Fig. 3 B)

concentration by 30 min postingestion; however, whey proteinresulted in a more pronounced aminoacidemia than eithercasein or soy   (P     0.05). At 60 min postconsumption, theconcentration of EAA and leucine was also higher followingwhey consumption than either casein or soy (whey    soy  casein; all  P 0.05). The AUC for blood leucine after wheyingestion was 73% greater than soy and 200% greater thancasein (Fig. 3 B, inset ).

Plasma and muscle intracellular free phenylalanine enrich-ment.  Plasma and muscle intracellular free phenylalanine en-richments are shown in Fig. 4,  A  and  B, respectively. Linearregression analysis (not shown) indicated that the slopes of theplasma enrichments over time were not significantly differentfrom zero (P 0.05), suggesting that plasma enrichments hadreached a plateau and subjects were at isotopic steady stateover 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 tendedto 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 followingwhey consumption was significantly greater than both soy andcasein 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 foundthat the consumption of whey protein hydrolysate stimulatedMPS to a greater degree than casein both at rest and afterresistance exercise. While FSR in the whey group tended to begreater than soy in the rested muscle, this did not reachstatistical 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.

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congruence with our previous work showing a greater stimu-lation of MPS with milk vs. soy protein ingestion (37), soyappears to be less effective at stimulating MPS than wheyprotein despite inducing a similar rise in circulating EAA.

Based on previous literature identifying protein digestibilityas 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 consumptionof whey, casein, or soy would also result in a differentialstimulation of protein synthesis at the muscle level. For exam-ple, several studies have noted that “fast” proteins stimulate alarge rise in protein synthesis whereas “slow” proteins primar-ily inhibit protein breakdown, but these results come from dataat the whole body level (3, 8, 9) of which muscle comprisesonly 25% (25) and turns over at a much slower rate than, forexample, gut proteins (26, 27). In addition, milk proteinsappear to support greater “peripheral” (i.e., muscle) vs.splanchnic protein synthesis than do soy proteins (4, 14). Ourdata extend these previous studies that measured only whole

body protein turnover by demonstrating that the consumptionof whey hydrolysate and soy isolate (i.e., “fast” proteins) result

in considerably higher rates of muscle protein synthesis thancasein (i.e., “slow” protein), both at rest (whey     soy   casein) and after resistance exercise (whey soy casein). Inour view, these differences are unlikely to be explained by ouruse of a whey protein hydrolysate, rather than isolate. This isbecause previous data have noted no difference in the patternof aminoacidemia following ingestion of 36 g of whole wheyprotein or its hydrolysate (5). While the pattern of peripheralaminoacidemia yields no insight into the actual kinetics of protein absorption, this fact is of little consequences since theconcentration of amino acids in the peripheral (i.e., non-

Fig. 3. Blood concentration of essential amino acids ( A) and leucine ( B) afteringestion of whey hydrolysate, casein, or soy protein. Inset : leucine area underthe curve (AUC). *Significantly different from casein  (P    0.05). # Signifi-cantly different from soy  (P 0.05). All values are means SD; n 6 pergroup. Some error bars have been omitted for clarity.

Fig. 4. Plasma ( A) and muscle ( B) intracellular free phenylalanine enrichment(tracee-to-tracer ratio; t T1). 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 aremeans SD;  n 6 per group.

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splanchnic) circulation would be those that are available forprotein synthesis by peripheral tissues such as muscle (assum-ing of course equivalent flow). Interestingly, when examiningwhole body leucine kinetics, prior studies actually found thatcasein consumption promoted a higher whole body leucinebalance than whey (3, 8, 9). While these findings may seemcontradictory 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 greaterwhole body leucine balance observed. In addition, the increasein whole body protein synthesis stimulated by whey wasobserved to be quite transient (3, 8, 9). Admittedly, we chosea 180-min time point in the present study to capture the acuteMPS response, whereas the whole body data represented anaggregate 7-h response (3, 8, 9). We do not think, however,that extending our response beyond 3 h would have markedlyaffected our MPS results since amino acid concentrations wereback 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 presentstudy. It is known that resistance exercise increases muscleprotein breakdown, albeit to a lesser extent than synthesis (1,28). Thus, the results of Tipton et al. (34) could have arisen dueto a marked suppression of muscle protein breakdown withcasein and a stimulation of synthesis with whey. Ingestion of amino acids attenuates the post-resistance exercise-inducedincrease in muscle protein breakdown (2, 35); thus in thepresent study it is hard to envision that a marked suppressionof muscle proteolysis occurred with casein ingestion that

would not have occurred with whey or soy. We did notmeasure proteolysis, however, and thus can only speculate asto the effect of protein digestibility on muscle protein break-down due to current methodological limitations that precludethe direct measurement of muscle protein degradation afterphysiological (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 afterresistance exercise are somewhat surprising given their similarprotein digestibility-corrected amino acid scores (PDCAAS)(13). Indeed, the PDCAAS of these proteins would suggest thatthey are high-quality complete sources of amino acids, whichin 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); ourresults suggest that in the context of skeletal muscle accretionfollowing resistance exercise this is particularly the case. In thepresent 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 withinthe fast proteins themselves (i.e., whey and soy). The rise inEAA (Fig. 3 A), BCAA (data not shown), and leucine (Fig. 3 B)was of greater amplitude and considerably more rapid follow-ing whey consumption compared with soy. These differencesin the rate of EAA appearance in the circulation may beespecially important to the differential stimulation of MPS weobserved after whey or soy ingestion at rest and following

resistance exercise. Recent work has demonstrated that supple-menting soy protein with BCAA (leucine, isoleucine, andvaline) is required to rescue its anabolic effect in elderly andclinical populations (11). Leucine has also been shown toenhance the activation of mTOR-related signaling proteins atrest and after exercise (17, 19, 21). Thus the greater totalBCAA (7%) and leucine content (28%) in particular may

have contributed to the larger increase in protein synthesis afterwhey 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 maximallystimulated and that this threshold was not reached with soyingestion. Such a thesis is supported by our recent work showing a saturable response in MPS following resistanceexercise (24).

The differences in skeletal MPS that we observed may haveimplications for populations with compromised nutrient sensi-tivity (e.g., the elderly) (7). Indeed, it has been found that theprotein digestibility paradigm observed in young individuals isactually “reversed” in the old with respect to whole bodyprotein metabolism (9), and that “fast” protein ingestion isassociated with a greater whole body leucine balance (7). If theleucine “trigger” concept is correct, then we would speculatethat these results (7) may reflect the inability of casein toincrease blood EAA, BCAA, or leucine concentration highenough to turn on MPS in older persons who appear to have areduced sensitivity to amino acids or an “anabolic resistance”(7). For example, ingestion of larger doses of leucine has beenshown to enhance feeding-induced increases in MPS in agedindividuals (18, 30). Considering protein ingestion after exer-cise appears critical to enhance skeletal muscle hypertrophywith resistance training in the elderly (12), we propose thatelderly individuals would likely obtain the greatest benefit withrespect to stimulating MPS and likely muscle protein accretion

by consuming a “fast” leucine-rich dietary protein such aswhey 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 wholeproteins to confirm this thesis.

In summary, we report that the consumption of whey proteinhydrolysate stimulates skeletal MPS to a greater extent thaneither casein or soy. Our results suggest that the type of proteinconsumed is a modulating factor in determining postprandialresting and postexercise muscle anabolism in young healthymen, both at rest and after resistance exercise. Moreover, thiseffect may be related to the leucine content of the proteinconsumed and how quickly it is digested. Thus, it appears thatwhen providing an optimal dose of protein (10 g EAA) (7,

24), a rapid increase in EAA (perhaps leucine specifically) isimportant for supporting maximal rates of skeletal MPS.

ACKNOWLEDGMENTS

We thank E. T. Prior for outstanding support and technical assistance, andwe thank Archer Daniels Midland for the generous gift of the soy protein usedin this study.

GRANTS

This study was supported by a grant from the US National Dairy Councilto S. M. Phillips. S. M. Phillips was a Canadian Institutes of Health Research(CIHR) New Investigator Career Award recipient, J. E. Tang was a CIHRDoctoral Award recipient, and D. R. Moore was a CIHR Canada Graduate

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Scholarship Doctoral Award recipient. All gratefully acknowledge thesesources of funding.

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