effects of different dietary proteins and amino acids on skeletal muscle hypertrophy in young adults...

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Effects of Different

Dietary Proteins andAmino Acids on SkeletalMuscle Hypertrophy inYoung Adults AfterResistance Exercise: ASystematic ReviewHenning Langer, BA and Anja Carlsohn, PhDUniversitat Potsdam, Professur Sportmedizin und Sportorthopadie, Potsdam, Germany

A B S T R A C T

THIS ARTICLE REVIEWS THE

AVAILABLE LITERATURE ON

WHICH PROTEINS, AMINO ACIDS,

OR COMBINATION OF BOTH

SEEM TO BE OPTIMAL TO

ENHANCE HYPERTROPHY AFTER

RESISTANCE EXERCISE IN

YOUNG ADULTS. DEPENDING ON

THE CONTENT OF ESSENTIAL

AMINO ACIDS AND PARTICU-

LARLY LEUCINE, EITHER AN

IMMEDIATE INGESTION OF;

20 GMILK PROTEIN FOLLOWED BY A

SIMILAR AMOUNT ;1 HOUR

LATER, OR A SINGLE BOLUS

OF ;40 G SEEMS TO BE SUIT-

ABLE. GREATER AMOUNTS

MIGHT BE NECESSARY IF A

PROTEIN OF LOWER QUALITY IS

CHOSEN (I.E., PLANT-BASED

PROTEINS) TO MATCH THE

REQUIRED AMINO ACID QUANTI-

TIES AND FACILITATE MUSCLE

GROWTH.

INTRODUCTION

Investigating skeletal muscle hyper-trophy is important for numerousreasons. Recreational and profes-

sional athletes can benefit from hypertro-phy as a muscle with increased cross-sectional area (CSA) can exert moreforce, eventually leading to greaterstrength andpowerpotential (39). Musclehypertrophy has vital implications notonly for performance but also froma health perspective. Muscle tissue playsa major role in regulating our metabolism

and consequentially all diseases that arerelated to it (34,43). Furthermore, buildingup muscle tissue in young subjects couldbe a promising intervention to battleincreasingly prevalent diseases such assarcopenia or cachexia, which are relatedto muscle loss and muscle weakness(5,52). Starting early enough with thiscould be particularly important, ashumans have been reported to becomeresistant to anabolic stimuli during aging(10,33,36,65). How to optimally stimulate

skeletal muscle hypertrophy in young

healthy adults through exercise is a con-troversial topic (14).

Skeletal muscle hypertrophy occurswhen the aggregate of muscle proteinsynthesis (MPS) exceeds the aggregateof muscle protein breakdown (MPB)for an extended period of time, resultingin a positive net protein balance (NPB).It is well-established that in order tostimulate this process, it is essential tooverload the muscle (27,28). However,it has been shown that in a fasted stateand without sufficient protein intake,

the anabolic effects after RE are dimin-ished, as both MPS and MPB rise ina similar fashion resulting in a negativeNPB (1,22,46,60).

Essential amino acids (EAAs) play a keyrole, as they increase MPS and minimizeMPB, causing a higher NPB comparedwith a mixture of nonessential amino

K E Y W O R D S :

milk; whey; casein; soy; leucine; pro-tein; resistance training; hypertrophy

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acids (AAs) (60). The EAA leucine hasbeen suggested as a MPS key modulatorby increasing the activity of importantsignaling proteins (19). Sufficient protein

intake seems to be important to maxi-mize the contractile protein accretionafter RE, and this seems to be partiallydependent on the EAA content of theconsumed protein. A protein high inEAA and with a high digestible indis-pensable amino acid score may betermed as a high-quality protein (23).

Examples of high-quality protein sourcesinclude whey-, soy-, casein-, multicom-ponent proteins or even AA supple-ments such as branched-chain aminoacids (BCAA) or pure leucine. This

review will overview the effects of differ-ent dietary proteins ingested immedi-ately before, during, or after RE. It hasbeen shown that the anabolic effects ofRE differ depending on certain factors,such as level of experience of the sub-

jects, training volume, and training inten-sity. Therefore, the protocols of thereviewed studies will be elucidated indetail to allow for the results to be putin an appropriate perspective.

SEARCH STRATEGY

A systematic PubMed database searchwas performed using combinations ofthe following terms: protein ingestion,casein, whey, soy, egg, beef,leucine, amino acids, resistance train-ing, resistance exercise, and weight-lifting. Language of the studies waslimited to English (Figure).

CRITERIA

This review included studies thatlooked at the effects of more thanone protein or AA source during theirrespective experiments. The subjects inthose studies had to undergo at least 1bout of intense RE, and indicators ofskeletal muscle hypertrophy had to bemeasured before and after the inter-vention. Examples of such indicatorsare acute changes in muscle proteinturnover or long-term changes in bodycomposition and muscle size. All stud-ies administered the protein in a timeframe of 90 minutes before, during, orafter RE as this seems to be crucial for

the hypertrophic response (1,22,45,53).

Studies with older (+40 years) ordiseased subjects were excluded(Table 1).

A total of 5,868 articles were initially

found. Of this, 12 studies till July 2013fulfilled the defined inclusion criteriaand were included in this review.

DIFFERENT PROTEIN SOURCES

ON MUSCLE PROTEIN TURNOVER

AND HYPERTROPHY

Seven studies analyzed milk proteins(whey and casein) and their differenteffects on postexercise muscle anabo-lism or chronic changes in body com-position. Four of them compared theresults with soy protein.

Tipton et al. (59) included 23 healthy,young, untrained subjects of both gen-ders. Subjects had to perform a singlebout of leg extension, consisting of 10sets of 8 repetitions (reps) at 80% ofthe 1 repetition maximum (1RM) ofthe subjects, with a 2-minute breakbetween each set. One hour after thisbout, 20 g of whey protein (2.3 g leucine),casein protein (1.7 g leucine), or placebo(water) were ingested by the subjects.Vastus lateralis biopsies were taken

55 minutes before, immediately before,and 120 minutes and 300 minutes afterRE. Leg blood flow was measured4535 minutes before RE and at4045 minutes, 8090 minutes, 115120 minutes, 200210 minutes, and290300 minutes after exercise. Bloodsamples were taken at 17 points from35 minutes before exercise to 300 mi-nutes after exercise. Appearance of AAs(phenylalanine and leucine) in the bloodand muscle was measured, and NPBwas calculated using the following for-

mula: NPB 5 (blood flow across theleg) 3 (arterial AA concentration 2venous AA concentration).

Whey protein ingestion resulted ina more rapid increase and larger peakin leucine concentrations in blood andmuscle than casein ingestion, likelybecause of different digestive propertiesof the proteins (2,20,21). However, interms of the potential hypertrophy(NPB), no significant difference betweenthe groups was observed. The placebo

had no effect on the values.

Wilkinson et al. (64) compared a milkprotein containing drink (fat-free milk)with an isonitrogenous and isoenergeticsoy protein drink. Eight young subjects

who were regularly engaged in RE ($

4days per week) participated in 2 bouts ofa unilateral leg workout, separated by$1week. Three standardized exercises (legpress, leg curl, and leg extension) wereexecuted, each for 4 sets at 80% of the1RM with 2-minute rest between eachset. After the exercise, either the soy ormilk drink was ingested. Both included18.2 g of protein, 1.5 g of fat, and 23 g ofcarbohydrates (CHO). Muscle biopsieswere taken immediately before and afterRE, at 60 minutes, 120 minutes, and

180 minutes after workout. Blood sam-ples were drawn 90 minutes before exer-cise, directly before and after exercise,and 30, 60, 90, 120, and 180 minutes afterRE. Blood flow was measured beforeand after workout, as well as every hourafter RE up to 180 minutes. Similarly toTipton et al. (59), fractional synthesis rate(FSR in percentage per hour) was calcu-lated as described by Phillips et al. (46).NPB was calculated by subtracting frac-tional breakdown rate from FSR. Thefat-free milk stimulated FSR to a substan-tially greater extent (34%) than the soydrink, resulting in a significantly greaterNPB area under the curve (AUC). Thisindicates that milk proteins are a betterstimulator of postexercise muscle anab-olism in young resistance athletes thansoy protein.

In a follow-up study by Hartman et al.,56 college students who were previ-ously not experienced in RE completeda 12-week trial. Before the start of theprogram, subjects were randomly as-

signed to 1 of the 3 groups (milk, soy,or control) (29). Each subject wastrained 5 days per week on a rotatingsplit program. The split program con-sisted of 3 different types of sessions,for a total of 60 RE sessions over thecourse of the 12 weeks. During the trial,load was progressively increased (start-ing at 80% 1RM), whereas the number ofreps per set decreased. Immediately aftereach workout and again 1 hour later,each subject consumed a drink. The

drink was either skim milk (17.5 g of

Effects of Different Dietary Proteins

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protein would support muscle growthto a greater extent than soy proteincould not be confirmed.

Tang et al. (56) investigated whether

there is a different effect for the isolatedproteins of milk (whey and casein) com-pared with soy. In contrast to Tiptonet al. (59), this study found a significantdifference between casein, soy, andwhey on MPS. Eighteen young andhealthy subjects experienced in weight-lifting (23 sessions per week) com-pleted this trial. A single bout of 2unilateral leg exercises was performed:4 sets of leg press and leg extension at100% of their previously determined1012 RM with 2-minute rest between

each set. Immediately after RE, a drinkof 21.4 g of whey, 21.9 g of casein, or22.2 g of soy protein was ingested. Eachdrink contained approximately 10 g ofEAA. A continuous L-[ring-13C6] phe-nylalanine infusion, blood samples, andmuscle biopsies were used to measureMPS at rest, after the consumption ofthe drink and up to 180 minutes afterthe workout. Despite similar EAA con-tent in all drinks, the ingestion of thewhey drink resulted in a significantly

greater AUC of leucine and higherMPS than the ingestion of soy or casein.Therefore, whey protein seems to bea more potent stimulator of acute mus-cle anabolism after RE than casein orsoy. Several authors suggested that thismight be because of the faster absorp-tion rate of whey when compared withother proteins that are similar in EAAcontent (2,20,21). However, an infusionprotocol of only 3 hours postprandiallyis unlikely to catch the slow actingeffects of casein on MPS and could

therefore show an incomplete picture.Reitelseder et al. (49) conductedanother study comparing whey withcasein. They confirmed what Tiptonet al. (59) found and could not replicateTang et al. (56). They had 17 subjectsnot experienced in RE perform 10 setsof 8 reps at 80% of their 1RM with 3-minute rest between the sets. After RE,a drink of either whey, casein (0.3 g perkilogram lean bodyweight), or control(non caloric) was ingested. MPS was

measured in a similar way as described

in earlier studies (46,56,59,64). Com-pared to the study by Tang et al., theinfusion protocol of this investigationwas twice as long and assessed changes

in MPS over a 6 hour period. Wheyingestion was followed by a greaterpeak in MPS during the early phase(13 hours), whereas casein causeda higher elevation during the later phase(36 hours), resulting in a similar meanMPS over the total period (16 hours).This is supporting the idea that theslower absorption kinetics of casein re-quires a longer infusion period to allowfor a comprehensive comparison to itseffects on MPS versus whey protein.However, it was a special form of casein

that was used in this trial. Unlike micel-lar casein that is found in bovine milkand was used in the other experimentsmentioned above, this form (calciumcaseinate) has different digestive prop-erties that are more similar to whey pro-tein. Micellar casein is not acid solublecausing it to be digested more slowly,whereas caseinates are acid soluble andrelease their AAs much more rapidlyinto the intestine, bloodstream, and ulti-mately peripheral tissues (18). There-fore, the results of this study cannotbe applied to the most common formof casein (micellar casein) and are likelyto be directly linked to this particularform of casein (caseinate).

Recently, another comparison betweenwhey and casein did not show any sig-nificant differences between changes inbody composition (63). Wilborn et al.had 16 female basketball players partic-ipate in this trial of 8 weeks. They par-ticipated in 4 whole-body RE sessionsper week, which were not specified in

terms of intensity. Subjects performed 3conditioning units per week suitedtoward basketball. The subjects wererandomized into a whey or casein pro-tein group. Both groups consumedeither an isocaloric portion of 24 gcasein or whey immediately after eachof the 7 training sessions of the week. Bythe end of the 8 weeks, both groups hadincreased muscle mass and strength ina similar manner. However, the lack ofa nonprotein control group makes it

difficult to distinguish the effects of

the training protocol from the proteinsupplementation.

Taken together, these results indicatethat a combination of milk proteins

as it is naturally occurring in bovinemilk seems to be superior to soy pro-tein in promoting hypertrophic gainsin young healthy adults (29,56,64).When looking at the effects of the iso-lated milk proteins, whey proteinseems to be more facilitating in theearly phase after RE, whereas caseiningestion results in a slower, but moreprolonged effect on MPS (49,56).

It has been suggested that the main rea-son for soy protein being less potent in

augmenting MPS, despite its high-qualityAA profile and fast digestion rate, is theway it is partitioned. Its AAs seem to beprimarily distributed to the splanchnicregion and less toward peripheral tissueslike muscle (3,24,57).

THE ROLE OF LEUCINE AND

OTHER AMINO ACIDS

Results from Wilkinson and Hartmanet al. suggest that milk may be an effi-cient postworkout nutrition and proteinsource (29,64). The protein in bovine

milk as used in those studies usuallycontains 20% whey to 80% casein pro-tein (31). A combination of whey andcasein protein could be a promisingworkout beverage. However, substantialdifferences in absorption kinetics andtime effects of whey and casein wereobserved, leaving the question whetherit is possible to ameliorate the effects offast- and slow-acting proteins on leanmass accretion by combining them.The exact role of AAs within proteinsor added to them remained unclear.

Kerksick et al. (35) compared theeffects of 3 different postworkoutdrinks on body composition after 10weeks of RE. Thirty-six subjects wereassigned to either a whey and caseinprotein blend (48 g: 40 g of whey and8 g of casein), a whey protein that wasstacked with BCAAs and glutamine(48 g: 40 g of whey, 3 g of BCAAs +5 g of L-glutamine), or an isoenergeticCHO placebo that had to be ingestedin a 2-hour window after RE or in the

morning (9 AM) on training-free days.




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The participants were experienced inweightlifting and completed a 10-weekRE program (4 sessions per week), with78 exercises and 2124 sets per session,

respectively. Over the course of thestudy, the training loads were progres-sively increased, and each set throughoutthe entire program was executed untilthe subjects could not perform anothercomplete repetition. The preplannedreps ranged from 6 to 10 per set formost of the exercises and 25 per setfor abdominal exercises. Similarly toCandow et al. (13), the subjects wereinstructed to report their dietary habitsover a 4-day period before the start ofthe trial, during weeks 2, 5, 8, and 10 of

the program. Energy intake from groupto group and week 010 varied between29.2 and 39.8 kcal per kilogram body-weight and protein intake from 1.4 to2.5 g per kilogram bodyweight, respec-tively. Although there was no statisticaldifference between the groups, it isimportant to note that every singlesubject was close to matching oreven exceeding the recommended pro-tein intake per day for RE athletes(37,38,47,50). As a result of this trial,significant gains in LBM measured byDEXA occurred in the whey plus caseingroup only, indicating that this proteinblend would be advantageous comparedwith whey protein that was stacked withEAA and glutamine, which had noeffect on body composition.

In an attempt to further elaborate onthe role of leucine and EAA on MPS,Churchward-Venne et al. (16) looked atthe effects of different whey protein dos-ages, with or without added leucine orEAAs. A regular dose of whey protein

(25 g) was compared with 2 differentgroups of suboptimal dosages of wheyprotein (6.25 g). One group had addedleucine, to match the leucine content ofthe regular dosage in it and the otherhad added EAAs in it, to match everyEAA of the regular dosage besidesleucine. They hypothesized that addingleucine, but not EAA, would result ina MPS response similar to the groupwith the complete whey (19,51,60).They had 24 subjects not experienced

in RE perform a single bout of unilateral

exercise, consisting of 4 sets of 1012reps leg extension and leg press at 95%of their previously determined 1RM.Immediately after this bout, they were

assigned to 1 of the 3 groups in a single-blinded fashion and ingested the bever-age. MPS was measured as describedearlier, using infusion, blood samples,and a biopsy from the worked muscle(vastus lateralis) (46,49,56,59,64). Asa result, they observed equal MPSresponses between the groups in theearly hours (13 hours) after RE, butonly the complete whey group (25 g)was able to sustain this response fora prolonged time (35 hours after exer-cise). This implies that despite the fact

that leucine was suggested to be indis-pensable for MPS and NPB to occur,less of it than previously thought mightbe necessary when other EAAs areavailable in sufficient amounts. In addi-tion, whey in adequate quantities seemsto be a better choice postworkout thanlittle amounts of protein stacked withleucine or other EAAs. Whey in ade-quate quantities seems to be a betterchoice after workout than littleamounts of protein stacked with leucineor other EAAs.

Reidy et al. (48) compared the effectsof whey protein with a protein blendconsisting of sodium caseinate (50%),whey, and soy (25% each). Both bev-erages contained approximately thesame amount of leucine (;1.8 g) andEAAs (;8.7 g). Nineteen untrainedsubjects completed a single bout ofleg exercise. They performed 8 sets of10 reps leg extension at increasingintensity from 55 to 70% of their1RM with 3-minute rest between

the sets. FSR was measured usingsimilar methods as described in earlierstudies (16,46,49,56,59,64). The proteindrink was ingested 1 hour after RE.Depending on the subjects groupand their bodyweight (0.30.35 g perkilogram bodyweight), they eitherreceived ;18 g of whey or ;19 g ofthe protein blend. The group hypoth-esized that the individual characteris-tics of each component of the proteinblend would yield unique advantages

over whey protein. No significant

difference in MPS between the groupscould be observed in the early phaseafter RE. However, as expected, theslower kinetics of the caseinate and

the plant protein led to an increasedMPS during the late phase (24 hoursafter RE), whereas no increase in MPScould be observed for whey-only at thattime. This shifted effect of the slowerproteins is also reflected by differencesin the rise and fall of AA concentrationsin the serum. The authors therefore sug-gested that a multicomponent proteincould be a better choice after RE. Sim-ilar to the study by Reitelseder et al. (49),it is important to consider that sodiumcaseinate just like calcium caseinate does

not resemble the digestive characteris-tics of micellar casein and instead isabsorbed much faster (18).

Herda et al. compared bioenhancedwhey (20 g of whey + 5 g of leucine)with normal whey (20 g), a placebo (27g of maltodextrin), and a control group(30). Over 8 weeks of RE, 106 subjects,49 of them experienced in weight train-ing, trained 3 times per week at 80% oftheir 1RM with 2-minute rest betweenthe sets. The subjects were randomly

assigned into 5 different groups: bioen-hanced whey combined with a low-volume RE program, bioenhancedwhey plus moderate volume, normalwhey plus moderate volume, placeboplus moderate volume, and controlplus moderate volume. The differentbeverages were ingested 30 minutesbefore and after workout. On rest days,they were instructed to drink it in themorning in a postabsorptive state. Eachparticipant had to report 3 random daysof dietary intake (2 week days and 1

weekend day) and was asked to main-tain his habits throughout the study.Average kilocalorie intake between thegroups ranged from 2,3206 129 kcal to3,029 6 181 kcal. Protein intake variedfrom between 0.691.96 g per kilogrambodyweight (lowest average groupintake) to 1.262.86 g per kilogrambodyweight (highest average groupintake), respectively. The placebo andcontrol groups consumed significantlyless protein than the other 3 groups.

However, even in those groups some

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individuals excessively exceeded therecommended (37,38,47,50) proteinintake per day. The changes in bodycomposition were evaluated using

peripheral quantitative computed tomog-raphy. No significant changes in muscleCSA or LBM gains could be observedbetween the groups, despite the differ-ence in total protein intake.

Recently, Joy et al. (32) compared theeffects of 48 g of rice protein versus 48g of whey protein after RE for 8 weeks.Subjects were 24 healthy young

males who were experienced in strengthtraining for at least 1 year. The REprotocol consisted of an undulating peri-odization program for 3 days a week.They performed alternating whole-

body sessions at either their 812 RMor 25 RM. The weights were increasedby 25% when the desired number ofreps was achieved. The authors hypoth-

esized that there would be no differencebetween the groups, agreeing withearlier assumptions about a leucinethreshold (4). Indeed, no significant dif-ference between the groups was found.

Table 2Protein sources, duration, and outcome

Reference Protein and amino acid contentof the beverages

Indicators of hypertrophy Duration Results

Tipton et al. (59) 20 g whey or 20 g casein NPB Single bout NPB[; no difference between

the groups

Wilkinson et al. (64) 18.2 g milk protein or 18.2 g soyprotein

NPB Single bout NPB[; milk. soy

Hartman et al. (29) 17.5 milk protein or 17.5 soyprotein

CSA, LBM 12 wk CSA[, LBM[, milk. soy

Candow et al. (13) ;30 g whey, ;30 g soy proteinor ;30 g maltodextrin

LBM 40 d LBM[; no difference betweenthe groups

Tang et al. (56) 21.4 g whey, 21.9 g casein, or22.2 g soy protein

MPS Single bout MPS whey . soy . casein

Reitelseder et al.(49)

;20 g whey or ;20 g casein MPS Single bout MPS no difference betweenthe groups

Wilborn et al. (63) 24 g whey or 24 g casein LBM 8 wk LBM[; no difference betweenthe groups

Kerksick et al. (35) 40 g whey + 8 g casein or 40 gwhey + 3 g BCAAs + 5 gglutamine

LBM 10 wk LBM[; whey plus casein .whey plus BCAA andglutamine

Churchward-Venneet al. (16)

25 g whey, 6.25 g whey + leucine(matched with 25 g whey),6.25 g whey + EAA (matchedwith 25g whey, besidesleucine)

MPS Single bout MPS[; no difference betweenthe groups in the earlyresponse; MPS remainedelevated during the latephase only in the 25 g wheygroup

Reidy et al. (48) ;18 g whey or ;19 g proteinblend (25% whey, 50%sodium caseinate, 25% soy)

MPS Single bout MPS[; no differences betweenthe groups in the earlyresponse; MPS remainedelevated during the latephase only inmulticomponent proteingroup

Herda et al. (30) 20 g whey, 20g + 5 g leucine,27 g maltodextrin or placebo

CSA, LBM 8 wk LBM[; no difference betweenthe groups

Joy et al. (32) 48 g whey or 48 g rice protein LBM 8 wk LBM[; no difference betweenthe groups




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The rice group increased their lean massby 2.5 kg, which was measured usingDEXA. The whey group gained 3.2 kgof lean mass during the intervention.

Both increased their 1RM of the per-formed exercises in a similar manner.The authors concluded that the studyprovides evidence for the theory ofa leucine threshold. They assume thatthis threshold is within the rangeof 1.73.5 g of leucine. Similar toWilborn et al., this study was limitedby the lack of a control group.

DISCUSSION

Although in contrast to the majority

of the findings, the results of Herdaet al. are in line with a number ofother studies that could not find anypositive effects for protein or AA sup-plementation combined with RE(7,12,25,26,61,62). However, it is theonly study fulfilling the criteria of thisreview that failed to show a positiveeffect of protein supplementation onindicators of skeletal muscle hyper-trophy. There are two main reasonsthat could explain these results. Oneis that most subjects in the study of

Herda et al. were not experienced inRE, and it is a common observationthat novice lifters tend to respondmarkedly well to the onset of RE,unlike advanced athletes. This mightbe because of a decreased MPS sig-naling in advanced athletes, causedby chronic loading of the muscles(17,41,42,54). It is possible that thebeginners in the study by Herdaet al. could have benefited from theirinitial sensitivity to RE, resulting insignificant hypertrophy despite sub-optimal nutritional circumstances.The authors agree on the chance thatthe naturally high basal MPS rates inyoung men, in conjunction with anefficient RE program, could havealready been such a great stimulusthat all additional effects of proteinsupplementation got whitewashed(30). Additionally, some of the sub-

jects in each of the groups exceededthe current recommendations fordaily protein intake by a great margin.

Potentially, this could have slightly

distorted the results despite a signifi-cant mean difference in overall pro-tein intake between the groups.

Yet it is possible that overall daily protein

and calorie intake are paramount to pro-tein ingestion with training. However,there is no doubt that a high-quality pro-tein source enhances acute RE-inducedMPS and NPB (16,48,49,56,59,64).Moreover, acute NPB after exerciseand EAA ingestion reflects 24-hourNPB (58), and there is evidence for pro-tein to augment long-term LBM gains toa greater degree than placebo or CHO:A recent meta-analysis has shown thatimprovements in LBM favor proteinsupplementation over placebo (15). Thiswas regardless of the age or training sta-tus of the subjects (15). This indicatesthat studies unable to find an effect forprotein supplementation to furtherenhance the effects of RE may have beenunderpowered.

Eventually, based on the current liter-ature, isolated or mixed milk proteinsand regular bovine milk seem to havethe most potential to facilitate musclegrowth compared with other proteins(29,49,56,64).

Bovine milk is, despite its high caseincontent, rapidly absorbed, resulting inan acute MPS response that is surpris-ingly close to isolated whey (64). Thegrowth-facilitating effects of milk pro-teins seem to be greater than for otherproteins not only on the short-term butalso on the long-term in young healthyadults (29,56,64). Furthermore, bothcomponents of milk protein (wheyand casein) are high in leucine andother EAAs, but only whey seems to

effectively augment acute postworkoutMPS because of its fast absorbablecharacteristics (56).

In general, it appears that combiningboth milk proteins, as it naturally hap-pens in bovine milk, could be a promis-ing strategy in further optimizing theMPS response after RE. The fast-acting characteristics of whey are com-plemented by the slow-acting charac-teristics of casein, resulting in a rapid,but also prolonged increase in MPS

(49,64). This seems to be the case

regardless of the type of casein(29,35,64). Adding other proteins suchas soy to those milk proteinbasedblends might be advantageous as well,

but further research is needed to con-firm the benefits of such multicompo-nent proteins (48).

Moreover, adding leucine or otherAAs to a sufficient amount of protein,which is already high in EAA, doesnot seem to further augment skeletalmuscle hypertrophy (30,35). However,if the amount of high-quality proteinand leucine is below what seems to bethe sufficient amount (20 g and 11.7 g,respectively) (4,32,40), adding EAA orleucine could help to still get the opti-

mal, early response after RE (16). Alter-natively, exceeding those amounts andconsuming enough of a protein that ispoor in leucine until one transgressesthe proposed threshold of 11.7 g couldbe feasible as well, if a protein of higherquality is not available (32). However, ifthe amount of protein is insufficient andthe shortcoming is compensated withthe addition of leucine or EAA, it wouldbe advisable to endorse such a compro-mise with a full dose of a high-quality

proteinshortly afterward, as only a com-plete protein seems to allow for a pro-longed MPS (16).

It is difficult to compare acute studiesthat measured muscle protein turnoverwith long-term interventions whichmeasured changes in muscle size. Littleis known on the long-term adaptationsof skeletal muscle to different levels ofprotein intake and timing. For example,effect sizes of muscle growth found inlong-term interventions are commonlysubstantially smaller than expected

based on acute studies that measuredmuscle protein turnover (30,40). Futureexperiments should target the physio-logical changes that might occurbetween acute studies and long-terminterventions. For accurate recommen-dations, further research is needed todetermine the optimal ratio of fast- toslow-acting proteins and the role ofother AAs besides leucine. Moreover,other high-quality protein sources suchas beef protein or egg protein have been

shown to ameliorate muscle protein



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turnover to a similar degree as milk pro-teins (40,44,55). However, to date, thiswas performed exclusively in elderly,dissociated from RE or without com-

paring it to another protein during thesame trial. Those findings are yet to beapplied to resistance trained young sub-

jects and longterm interventions.

CONCLUSION

Most of the results suggest that in-gesting milk protein directly beforeor after RE is advantageous forindividuals interested in optimizingskeletal muscle hypertrophy. Theavailable literature indicates that

a combination of fast- and slow-acting milk proteins (i.e., whey andcasein) could provide the most per-petual anabolic effects. The exactamount that should be ingested isdependent on the quality of the pro-tein and its content of EAA, specifi-cally leucine, with 11.7 g of leucineappearing to be optimal to facilitatemuscle protein accretion in youngsubjects. Approximately 20 g of a milkprotein followed by a similar amount;1 hour later, or alternatively ;40 g

in a single bolus feeding seem to besufficient to maximize the acute MPSresponse after RE, as well as long-term hypertrophy. Exceeding thoseamounts might be necessary only ifa protein of lower quality (i.e. ,plant-based protein) is chosen. Par-ticularly, individuals with low proteinintake and protein sources of poorquality could benefit from proteinsupplementation around training.

Conflicts of Interest and Source of Funding:

The authors report no conflicts of interestand no source of funding.

Henning Langeris a graduatestudent (MSc

program, Biologyof Human Perfor-mance andHealth) at Maas-tricht University

(Netherlands).

Anja Carlsohn isan AssistantProfessor at theUniversity of

EducationSchwaebischGmuend(Germany).

ACKNOWLEDGMENT

The authors thank Fabian Ottawa forhis assistance in revising the article.

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