The effects of protein supplements on muscle mass, strength, and aerobic and anaerobic power in healthy adults: a systematic review.

Sports Med (2015) 45:111–131 DOI 10.1007/s40279-014-0242-2

SYSTEMATIC REVIEW

The Effects of Protein Supplements on Muscle Mass, Strength, and Aerobic and Anaerobic Power in Healthy Adults: A Systematic Review Stefan M. Pasiakos • Tom M. McLellan Harris R. Lieberman

•

Published online: 29 August 2014 Ó Springer International Publishing Switzerland (outside the USA) 2014

Abstract Background Protein supplements are frequently consumed by athletes and recreationally active adults to achieve greater gains in muscle mass and strength and improve physical performance. Objective This review provides a systematic and comprehensive analysis of the literature that tested the hypothesis that protein supplements accelerate gains in muscle mass and strength resulting in improvements in aerobic and anaerobic power. Evidence statements were created based on an accepted strength of recommendation taxonomy. Data Sources English language articles were searched through PubMed and Google Scholar using protein and supplements together with performance, exercise, strength, and muscle, alone or in combination as keywords. Additional articles were retrieved from reference lists found in these papers. Study Selection Studies recruiting healthy adults between 18 and 50 years of age that evaluated the effects of protein supplements alone or in combination with carbohydrate on a performance metric (e.g., one repetition maximum or isometric or isokinetic muscle strength), metrics of body composition, or measures of aerobic or anaerobic power were included in this review. The literature search identified 32 articles which incorporated test metrics that dealt S. M. Pasiakos H. R. Lieberman Military Nutrition Division, US Army Research Institute of Environmental Medicine (USARIEM), Natick, MA 01760-5007, USA T. M. McLellan (&) TM McLellan Research Inc., 25 Dorman Drive, Stouffville, ON L4A 8A7, Canada e-mail: [email protected]

exclusively with changes in muscle mass and strength, 5 articles that implemented combined resistance and aerobic training or followed participants during their normal sport training programs, and 1 article that evaluated changes in muscle oxidative enzymes and maximal aerobic power. Study Appraisal and Synthesis Methods All papers were read in detail, and examined for experimental design confounders such as dietary monitoring, history of physical training (i.e., trained and untrained), and the number of participants studied. Studies were also evaluated based on the intensity, frequency, and duration of training, the type and timing of protein supplementation, and the sensitivity of the test metrics. Results For untrained individuals, consuming supplemental protein likely has no impact on lean mass and muscle strength during the initial weeks of resistance training. However, as the duration, frequency, and volume of resistance training increase, protein supplementation may promote muscle hypertrophy and enhance gains in muscle strength in both untrained and trained individuals. Evidence also suggests that protein supplementation may accelerate gains in both aerobic and anaerobic power. Limitations To demonstrate measureable gains in strength and performance with exercise training and protein supplementation, many of the studies reviewed recruited untrained participants. Since skeletal muscle responses to exercise and protein supplementation differ between trained and untrained individuals, findings are not easily generalized for all consumers who may be considering the use of protein supplements. Conclusions This review suggests that protein supplementation may enhance muscle mass and performance when the training stimulus is adequate (e.g., frequency, volume, duration), and dietary intake is consistent with recommendations for physically active individuals.

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Key Points In untrained individuals, changes in lean mass and muscle strength during the initial weeks of resistance training are not influenced by protein supplementation. Protein supplementation will promote greater gains in lean mass and muscle strength for both trained and untrained individuals as the duration and frequency of resistance training increases. Presently there is some evidence to support the use of protein supplementation to enhance gains in aerobic and anaerobic power during the early stages of training but the mechanisms that account for such improvements are not well described.

S. M. Pasiakos et al.

Although Schoenfeld et al. [25] also concluded that protein supplementation augmented gains in muscle mass and strength, their analyses focused on whether the timing of protein ingestion was a critical factor for enhancing muscle adaptation. We recently published two systematic reviews that have addressed the evidence base for the use of protein supplements [26, 27]. Those reviews addressed whether protein supplementation alone or in combination with carbohydrate accelerates glycogen repletion, thereby enhancing acute or repeat endurance exercise performance [26], as well as the effects of protein supplementation on muscle damage and recovery of muscle function and physical performance [27]. This review is focused on the influence of protein supplements on peripheral factors affecting muscle function and subsequent metrics of performance. Specifically, we examined whether consuming supplemental protein facilitates gains in muscle mass, which can increase strength and improve aerobic and anaerobic power.

1 Introduction

2 Methods

Protein supplements are widely consumed by athletes, recreationally active adults, and soldiers [1–4], who generally believe that combining the consumption of protein supplements with exercise will promote gains in lean mass, resulting in improved physical performance [2, 5]. This belief is based on information typically obtained from coaches, teammates, advertising, and family or friends [1, 5], and not based on understanding the peer-reviewed evidence base for the efficacy of protein supplementation. That evidence base may not be as strong as consumers assume. There is no scientific consensus regarding performance benefits associated with protein supplementation [6, 7]. Several reports have reviewed the proposed mechanisms associated with the intended performance benefits of protein supplementation. These reviews address whether protein supplementation alone or combined with carbohydrate attenuates carbohydrate oxidation and hastens muscle glycogen repletion in response to endurance-type exercise, and particularly whether protein supplementation enhances lean mass accretion, muscle strength, and aerobic and anaerobic power [8–17]. However, the evidence that performance changes are associated with these mechanisms has not been systematically evaluated [18–23], and only two recent meta-analyses to date evaluate evidence regarding the effects of protein supplements on actual measures of muscle mass and strength [24, 25]. Cermak et al. [24] provided a strong evidence-based analysis to show that protein supplementation augmented gains in lean mass and muscle strength in both younger and older adults.

PubMed and Google Scholar were searched without restriction to past publication date through to the fall of 2013 using the terms ‘protein’ and ‘supplements’ together with ‘performance,’ ‘exercise,’ ‘competition,’ and ‘muscle,’ alone or in combination as keywords. Searches were limited to English language, peer-reviewed publications reporting findings from healthy adults between 18 and 50 years of age who habitually consumed dietary protein at or above the recommended dietary allowance of 0.8 gkg-1day-1 [28]. Articles were also retrieved from the reference lists of these papers and recent reviews on protein supplements. Studies that manipulated dietary protein and carbohydrate intake to compare the effects of protein supplements on performance were excluded [29]. Studies that compared drink formulations that included protein as well as vitamins and herbal supplements against a placebo trial [30–32] were also excluded since it was not possible to isolate the effects attributed solely to protein supplementation. Studies that combined protein supplementation with creatine were excluded [33–39] except when they compared protein supplementation alone with placebo or carbohydrate. Papers that examined the effects of bovine colostrum [40–47], b-hydroxy-b-methylbutyrate [48], and the ingestion of single amino acids (e.g., arginine or ornithine) were not included in this review [49]. Finally, papers that used resistance training and supplementation for the purpose of long-term weight management by attempting to increase resting metabolic rate were also excluded [50]. Only studies that reported findings with the

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Protein Supplements and Exercise Performance

ingestion of various forms of protein alone or in combination with carbohydrate were reviewed. This review first presents the literature regarding the effects of protein supplementation on muscle mass and strength (Sect. 3.1) for both untrained (Sect. 3.1.1) and resistancetrained (Sect. 3.1.2) participants. Then, it discusses and evaluates the relevant literature on aerobic and anaerobic power (Sect. 3.2). A discussion (Sect. 4), summary evidence statements (Sect. 5), suggestions for future research (Sect. 6) and concluding remarks (Sect. 7) complete the paper.

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greater gains in metrics of performance has not been clearly established. 3.1.1 Studies in Untrained Participants

3.1 Muscle Mass and Strength

3.1.1.1 Studies Assessing Protein Versus Non-Energetic Placebo Several studies have examined the effects of protein supplements versus non-energetic placebos on muscle mass and strength using untrained individuals. The results from these studies have been inconsistent. For example, in untrained individuals, Antonio et al. [70] found no effects of protein supplementation on measures of muscle mass and strength following a 6-week combined aerobic and resistance training program. Similarly, Erskine et al. [71] reported no effect of protein supplementation following 12 weeks of resistance training of the elbow flexors. In contrast, an early study by Fern et al. [72] reported greater gains in muscle mass following 4 weeks of resistance training for those participants consuming an additional 2 gkg-1 of protein powder daily. Unfortunately, this study did not include a measure of strength. In addition, Hulmi et al. [73] demonstrated enhanced muscle hypertrophy when whey protein was consumed immediately before and after bi-weekly resistance training sessions for 21 weeks. In addition, protein supplementation attenuated post-exercise myostatin messenger RNA (mRNA) expression and increased cyclin-dependent kinase 2 signaling, suggesting that protein supplementation may potentiate cell growth in response to resistance exercise. Despite concomitant gains in muscle mass and anabolic gene expression, protein supplementation was not associated with consistent gains in strength. Post-training muscle strength was similar between protein and placebo for all measures of isometric and dynamic leg strength, but leg extension force and isometric bench press were greater with protein. As such, the effect of protein supplementation when compared with a non-energy providing placebo on resistance training-induced gains in muscle and strength in untrained adults remains unclear.

The effects of acute and chronic resistance exercise on muscle protein turnover, and the importance of the timing and type of protein ingestion to promote muscle protein synthesis are well established [8–13, 16, 17, 52–56]. Further, substantial evidence shows that amino acids or intact protein supplementation stimulates protein synthesis when consumed immediately before [57, 58], during [59], or within the first few hours after resistance exercise [59–68]. However, recently it was observed that the acute protein kinetic responses to resistance exercise and protein feeding were not correlated to changes in lean mass and muscle hypertrophy following 16 weeks of resistance training [69]. Thus, whether protein supplementation will promote

3.1.1.2 Studies Assessing Protein Versus Carbohydrate It has been hypothesized that supplemental energy from carbohydrate in combination with resistance exercise can produce similar gains in muscle mass and strength in untrained individuals to those produced by consuming supplemental protein. For example, consuming an energymatched protein or carbohydrate supplement before and after resistance training 3 daysweek-1 for 14 weeks produced similar improvements in isometric and isokinetic peak torque (slow velocity) and countermovement jump height [74]. However, squat jump performance, as well as the change in type I and II muscle fiber cross-sectional area, improved more with protein than carbohydrate

3 Results The literature search identified 32 articles incorporating test metrics that dealt exclusively with changes in muscle mass and strength in response to resistance training; five articles that implemented a combined resistance and aerobic training program or followed participants during sports-specific training programs, which included a mix of training strategies and a variety of performance metrics; and one article that evaluated muscle oxidative capacity in response to aerobic training (Fig. 1). A strength of recommendation taxonomy (SORT) [51] was used to document the quality of evidence for conclusions specific for each of these categories. The SORT uses the following criteria: (A) recommendation based on consistent and good-quality experimental evidence; (B) recommendation based on inconsistent or limited-quality experimental evidence; or (C) recommendation based on consensus, opinion, usual practice, case studies or extrapolation from quasi-experimental research. The SORT was created to assist medical practitioners in their assessment of patientoriented evidence [51]. In a similar manner, SORT can be used to assess the evidence-based literature to provide recommendations of protein supplement use by athletes and recreationally active adults.

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114 Fig. 1 Study selection and flow diagram of articles included in the review


Records identified through database searching (n = 53)

Additional records identified through other sources (n = 24)

Records after duplicates removed (n = 77)

Records screened (n = 77)

Full-text articles assessed for eligibility (n = 60)

Studies included in qualitative synthesis (n = 38)

supplementation. Coburn et al. [75] also demonstrated greater gains in skeletal muscle strength and cross-sectional area for volunteers who consumed a leucine/whey protein supplement versus volunteers who consumed an energy-matched carbohydrate supplement during an 8-week (3 daysweek-1) resistance training program. Willoughby et al. [76] evaluated changes in muscle mRNA myosin gene expression, total myofibrillar protein content, body composition and muscle strength in response to 10 weeks of alternating upper and lower body resistance exercises 4 daysweek-1, with a whey, milk, casein and free amino acid protein mixture or an energy-matched carbohydrate supplement consumed 1 hour before and immediately after each session. Total and lean body mass increased in both groups. However, increases in muscle strength were greater for those supplemented with protein than carbohydrate. Changes in myofibrillar protein content and myosin mRNA expression were also greater with protein than carbohydrate supplementation. Weisgarber et al. [77] hypothesized that providing a protein supplement immediately prior to exercise and during training after each set of exercise would provide an optimal stimulus for muscle protein synthesis and gains in

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Records excluded (n =17) • Reviews (n = 15) • Position stands (n = 2)

Full-text articles excluded, with reasons (n = 22) • Dietary manipulation (n = 1) • Included vitamins and herbals (n = 3) • Included creatine (n = 7) • Included bovine colostrum (n = 8) -hydroxy- • methylbutyrate (n = 1) • Included single amino acids arginine or ornithine (n = 1) • Purpose of supplementation was for weight control (n = 1)

muscle hypertrophy and strength over time. However, following 8 weeks of training four times a week, gains in muscle strength were similar for those receiving protein or isocaloric carbohydrate and neither group showed gains in lean mass. Reductions in protein, carbohydrate and total caloric intake over the course of the training program were present in both groups, which may have compromised the potential benefits of the supplementation with training. The effects of protein supplementation on muscle mass and strength may differ based upon resistance training volume, as the skeletal muscle protein synthetic response to resistance exercise varies in magnitude and duration when the volume and length of time the muscle is under tension are manipulated [78–80]. Mielke et al. [81] compared the effects of a leucine-enriched whey protein supplement versus not only an energy-matched carbohydrate supplement but also a non-energetic placebo. The supplements were provided immediately before and after resistance exercise performed three times per week for 8 weeks. The volume of resistance exercise for volunteers consuming protein and carbohydrate was low (one set of 6–8 repetitions of bench press and leg extension). Those consuming the non-energetic placebo completed two sets of each


exercise (6–8 repetitions). Body and lean mass were not affected by training regardless of group assignment and there was no difference among the groups in gains in muscle strength. Thus, despite the smaller volume of training for those groups receiving supplements, either protein or carbohydrate, they improved similarly to those receiving no supplement. However, the low training volume for all groups and the lack of change in lean mass suggest the training stimulus was inadequate. Further, this study did not include a placebo group that performed only one set of each exercise so comparisons with groups receiving supplements are not directly comparable. 3.1.1.2.1 Type of Protein Versus Carbohydrate. Protein source may be particularly important to promote gains in muscle mass and strength, as the digestive properties and essential amino acid (EAA) profile (particularly leucine) of various proteins modulate skeletal muscle protein synthetic responses to exercise [55]. For example, Candow et al. [82] compared the effects of whey or soy protein versus carbohydrate supplementation during a 6-week resistance training program on body composition and muscle strength. Increases in lean mass and 1 repetition maximum (RM) bench press and squat strength increased significantly more for both protein groups compared with those receiving carbohydrate. The authors suggested that protein supplementation (whey and soy) increased muscle protein synthesis independent of source, which contributed to the observed gains in lean mass and muscle strength as compared with carbohydrate. These findings are somewhat surprising given that muscle protein synthesis in recovery from resistance exercise is generally greater after consuming whey versus soy [83], an effect attributed to the higher leucine content and digestive properties of whey protein [84]. However, as acknowledged by Candow et al. [82], since daily dietary protein intake was high, the timing of protein supplementation immediately before and after the training [57, 60, 62] may have been more important than the type of protein ingested. Similarly, Volek et al. [85] compared whey versus soy protein supplementation for gains in lean mass and muscle strength but the resistance training program was extended for a much longer period (9 months) compared with the majority of other studies. Supplementation was isonitrogenous (whey vs soy) and isocaloric (with the carbohydrate only as placebo) and was provided after the exercise sessions on training days or in the morning on non-training days. Gains in lean mass were greater with the whey supplement but gains in 1 RM bench and leg press strength were similar among all groups. The authors suggested the gains in lean mass were likely distributed over the whole body given the number of different exercises performed during the training sessions

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and these changes were unlikely to impact strength during a single specific movement. Herda et al. [86] examined whether whey protein, with specifically modified properties of absorption through polyethylene glycosylation, promoted greater resistance training adaptations than standard whey protein following an 8-week program involving three weekly exercise sessions. The study design also included a placebo group receiving isocaloric carbohydrate and a non-supplemented control group. All supplements were provided before and immediately following exercise on training days but only one supplement dose was provided in the morning on nontraining days. The whey supplements were not isonitrogenous as the modified whey product contained an additional 7 g of leucine with each serving in addition to the 20 g of whey protein. Regardless of whether participants received a supplement and regardless of the type of supplement, all groups showed equal gains in lean mass and muscle crosssectional area, and similar improvements in bench and leg press strength and muscle endurance. 3.1.1.3 Studies Assessing Combined Protein and Carbohydrate Results from studies examining the combined effects of carbohydrate and protein on indices of muscle strength and hypertrophy in untrained adults have been inconclusive. For example, increased muscle strength in untrained adults following 10 weeks (5 daysweek-1) of alternating one-legged exercise training were not affected by combined carbohydrate and protein supplementation in a study by Williams et al. [87]. However, because the carbohydrate ? protein supplement was always ingested after training the same leg, but not ingested when the other leg was trained, a carry-over effect of supplementation on the non-supplemented leg during the initial stages of training could not be discounted. Tang et al. [88] reported that rates of muscle protein synthesis were still elevated 70 % versus baseline levels 28 hours after an acute bout of resistance exercise in untrained participants. Using a more traditional whole-body resistance training model, Bird et al. [89] studied the combination of carbohydrate ? EAA on changes in body composition, muscle fiber cross-sectional area and muscle strength, measured throughout 12 weeks of resistance training 2 daysweek-1. Supplementation with carbohydrate, EAA, carbohydrate ? EAA, or placebo was provided throughout the training sessions. Muscle isokinetic leg strength increased similarly for all groups in response to training. Lean mass and muscle fiber type I and II cross-sectional area also increased in all groups, but gains in muscle mass and crosssectional area were greatest in those consuming the combined carbohydrate and EAA supplement. The authors attributed the greater gains in muscle mass for those consuming carbohydrate ? EAA to the well documented

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anabolic effects of protein supplementation on muscle protein turnover. Nevertheless, greater gains in muscle mass did not result in greater gains in muscle strength. Similarly, Vieillevoye et al. [90] compared the effects of a carbohydrate ? EAA supplement versus carbohydrate alone on measures of muscle architecture, strength and body composition following 12 weeks of resistance training twice a week. Supplementation was provided twice daily with one dose occurring immediately following exercise during training days. Although both muscle thickness and angle of pennation for the gastrocnemius medialis determined through ultrasound scans increased more for the carbohydrate ? EAA group, changes in lean mass and improvements in maximal isokinetic force during bench press and leg squat exercise were similar between groups. Interestingly, only one participant was in negative nitrogen balance prior to the start of the program, suggesting that the initial protein intake, which averaged 1.3 gkg-1, was adequate without additional supplementation. Olsen et al. [91] examined the effects of a protein and carbohydrate supplement versus carbohydrate alone on growth and proliferation of muscle satellite cells and myonuclei throughout 16 weeks of resistance training three times a week. Protein supplementation was only provided on training days immediately before and following the workout. Those receiving protein and carbohydrate showed a greater increase in the number of satellite cells and myonuclei per fiber, but improvements in maximal isometric leg extension force were similar between supplement groups. 3.1.1.3.1 Milk and Dairy Products as a Supplement. The study of milk as an inexpensive, high-quality protein ? carbohydrate supplement has increased dramatically in recent years. Walberg-Rankin et al. [92] compared the effects of an energy-matched carbohydrate or low-fat milk supplement consumed following resistance training (3 daysweek-1 for 10 weeks). Improvements in body composition and muscle strength were statistically similar between groups, although mean gains in lean mass tended to be higher with milk (1.6 kg) than the energy-matched carbohydrate control (0.8 kg). Regardless, adaptations to this 10-week resistance training program in previously untrained males were not significantly influenced by supplementation. Hartman et al. [93] compared the effects of an energy and carbohydrate-matched fat-free milk and soy protein ? carbohydrate supplementation in response to 12 weeks of resistance training 5 daysweek-1 in a group of untrained men. A control group consumed an energymatched carbohydrate-only supplement. Lean mass increased in all groups. However, mean gains in muscle

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mass were more pronounced with milk supplementation (6.2 %) compared with the other treatments (3.7 and 4.4 % for control and soy, respectively), and milk tended (p \ 0.1) to improve muscle strength (mean changes of 102, 67 and 62 % for leg press, knee extension and hamstring curls, respectively) more than soy protein ? carbohydrate (98, 60 and 42 %, respectively, for the same exercises above) and carbohydrate alone (87, 46 and 51 %, respectively). In addition, the cross-sectional area of type II fibers in the vastus lateralis increased in all groups but gains in muscle size were greatest for those supplemented with milk. The authors suggested that having subjects train for 12 rather than 10 weeks and perform five rather than three sessions per week could explain the lack of support for milk supplementation observed with the study from Walberg-Rankin et al. [92]. As such, the findings by Hartman et al. [93] support the view that combining protein supplementation in the form of fat-free milk with an adequate resistance-training stimulus in previously untrained adults elicits greater rates of protein accretion compared with soy protein and carbohydrate supplementation alone. In a subsequent study, the combined effects of resistance exercise with milk or energy-matched carbohydrate supplementation for inducing changes in body composition and muscle function in women were examined by Josse et al. [94] during 12 weeks of training. The training program and testing protocol were identical to one previously used by this same laboratory to study training adaptations in males [93]. Lean mass increased and fat mass decreased significantly more with women receiving milk than women receiving carbohydrate. In contrast to Hartman et al. [93], the women consuming milk experienced significantly greater gains in muscle strength as compared with women consuming carbohydrate alone, which was attributed to the lower relative upper body strength (i.e., per kg body mass) for the untrained women compared with men. White et al. [95] also examined whether yogurt was more effective than whey protein or isocaloric carbohydrate in promoting gains in lean mass and strength in previously untrained women. One supplement dose equivalent to one serving of yogurt, which contained 5 g of protein, was provided immediately following the three whole-body resistance training sessions for 8 weeks. In addition, those receiving yogurt consumed a total of three servings per day throughout the study protocol. Consequently, total caloric intake and total protein intake were greater for those receiving yogurt than the other groups. Nonetheless, regardless of supplement provided after training, all groups showed equal gains in lean mass and improvements in muscle strength. 3.1.1.4 Summary: Untrained Participants An overview of the findings from the subsections for untrained


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participants is presented in Table 1 and a subsequent detailed list of the findings for protein supplements is provided in Table 2. Protein supplementation likely has no effect on lean mass and strength when training programs last 8 weeks or less [70, 77, 81, 86, 95, 96]. It has not been possible to determine whether there is a relationship between gains in muscle strength due to protein supplementation and muscle hypertrophy in training programs lasting either less than [73–75, 82, 89, 92–94] or more than 8 weeks when training sessions per week were limited [73, 87, 89, 90]. However, substantially increasing the frequency or duration of exercise, or manipulating both in combination with protein supplementation does appear to enhance gains in muscle mass and measures of muscle strength in untrained individuals [76, 82, 85, 91, 93, 94], a view that is consistent with findings from a recent metaanalysis [24]. Assuming the training program is of sufficient intensity, frequency, and duration [93, 94], and dietary protein intake is more than adequate for physically active adults [76, 82], protein supplementation may enhance gains in muscle mass, strength, and myofibrillar protein synthesis regardless of protein source (e.g., whey, soy, and milk) [82, 93, 94]. 3.1.2 Resistance-Trained Participants Athletes who regularly engage in strength and explosive power exercise are experienced with resistance-training exercise. Therefore, in training studies, changes in outcome

measures of muscle mass and strength exhibited by these individuals are less likely to reflect the confounding effects of neural adaptations evident during early stages of training with untrained individuals [97, 98]. As a consequence, the modulating effects of dietary protein supplementation on adaptations to training may be more readily apparent with trained participants than in studies of untrained individuals. In addition, resistance training attenuates the duration of the increased rate of muscle protein synthesis following exercise [88, 99] but the increase in muscle protein synthesis is most evident in the myofibrillar component [100]. As a result, one might expect protein supplementation to provide greater changes in muscle mass and strength for resistance-trained participants when provided in close proximity to the training stimulus. 3.1.2.1 Studies of Protein Supplementation During Overtraining Alternating periods of high-volume (i.e., overtraining) and low-volume training is a common practice among trained individuals attempting to enhance performance. Fry et al. [101] examined whether daily protein supplementation impacted performance more than a placebo in elite weightlifters during 1 week of high-volume training. No effects of protein supplementation were observed. However, both the protein (mean 2.4 gkg-1) and placebo (mean 2.2 gkg-1) groups consumed high protein diets. Ratamess et al. [102], and a subsequent paper by Kraemer et al. [103], investigated the use of an amino acid supplement during 4 weeks of high-volume (10–12

Table 1 An overview of the effects of protein supplementation for the different subsections reviewed for untrained participants Type of study

Effect of protein supplementation

Note

PRO versus PLA

$ mass, strength (2)

Low training volume, small muscle mass recruited and lack of strength metric

: mass (2) $ strength (1) PRO versus CHO

: CSA, $ strength (1) : CSA, strength (1)

Change in normal dietary protein during training, lack of proper placebo control

$ mass, : strength (1) $ mass, strength (2) PRO type versus CHO

: mass, strength (1) : mass, $ strength (1)

High normal dietary protein intake indicated timing of supplement ingestion was important

$ mass, strength (1) PRO ? CHO


High normal dietary protein, carry-over effects to non-supplemented limb

: mass, $ strength (1) : myonuclei, $ strength (1) Milk products

$ mass, strength (2) : mass, strength (2)

Low PRO dose, greater frequency and duration of training associated with improvements with milk

The numbers in parentheses refer to the number of studies. The arrows indicate whether the protein supplement increased (:) or had no additional ($) effect CHO carbohydrate, CSA cross-sectional area, PLA placebo, PRO protein

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3–4 sets, 4–15 reps, 4 leg exercises

3 sets 6–12 reps, 6–7 exercises

3 sets, 8–10 reps @ 75 % 1RM for 8 exercises

4–5 sets 6–12 reps @ 60–90 % 1 RM, 6–9 exercises

3–5 sets, 6 reps @ 80 % 1 RM leg extension

2–3 sets, 8–10 reps, 2 arm curl exercises

Specific details not stated

2–4 sets, 6–12 reps @ 80 % 1RM for 4–5 exercises

1–5 sets, 6 reps at 80 % 1 RM for bench and leg press

2–5 sets, 6–20 reps, 40–85 % 1 RM for 12–14 exercises

2–4 sets, 6–12 reps @ 80 % 1 RM for 4–5 exercises

1 set 8 reps @ 80 % 1 RM for leg, chest, PLA 2 sets

Antonio et al. [70]

Bird et al. [89]

Candow et al. [82]

Coburn et al. [75]

Erskine et al. [71]

Fern et al. [72]

Hartman et al. [93]

Herda et al. [86]

Hulmi et al. [73]

Josse et al. [94]

Mielke et al. [81]

Intensity

Training program

Andersen et al. [74]

Study

3

5

2

3

5

3

3

3

6

2

3

3

Frequency (9/week)

8

12

21

8

12

4

12

8

6

12

6

14

Duration (weeks)

20 g whey ? 6.2 g leucine pre/posttraining ? non-training AM/PM, isocaloric CHO

2 9 500 mL milk or CHO 0 and 1 h postexercise

15 g whey pre/post-training

Specifically modified whey (20 g ? 7 g leucine), standard whey (20 g), PLA (27 g CHO) pre/post-training and 1 dose AM nontraining

2 9 500 mL milk, soy or PLA 0 and 1 h postexercise

2 g/kg PRO daily, PLA

20 g whey pre/post-training

20 g whey ? 6.2 g leucine pre/posttraining ? non-training AM, isocaloric CHO

0.4 g/kg PRO ? 0.1 g/kg CHO, 0.5 g/kg CHO for PLA pre/post-training ? PM, non-training AM/29 PM

6 g EAA, 6 % CHO, 8.5 mL/kg divided into 25 servings throughout exercise

12.8 g EAA pre/post-training ?12.8 g nontraining AM

25 g PRO or CHO pre/post-training and nontraining AM

Supplementation

PRO = CHO = PLA for LBM, 1 RM leg, chest

Milk [ CHO for LBM and 1 RM bench press

PRO = PLA for 1 RM leg press and isometric leg and bench press

PRO [ PLA for CSA of vastus lateralis, molecular signals of cell growth

Modified whey = whey = PLA for gains in lean mass, improvements in 1 RM leg and bench press, and reps to failure at 80 % 1 RM

Milk [ [soy = PLA] (p \ 0.1) for 1 RM of leg exercises Milk [ [soy = PLA] for type II of CSA [milk = soy] [ PLA for type I CSA

Milk [ [soy = PLA] for LBM

PRO [ PLA for gains in lean mass

PRO = PLA for 1 RM, isometric force, angle of pennation, muscle size, EMG agonist/antagonist activation

[PRO = CHO] [ CON for CSA

PRO = CHO = CON for LBM

PRO [ CHO [ CON for 1 RM

[Whey = soy] [ PLA for LBM and 1 RM for both squat and bench press

All groups = for IKST

CHO ? EAA [ other groups for type II CSA CHO ? EAA [ PLA for leg press

CHO ? EAA [ PLA for LBM

EAA = PLA for # reps bench press

PRO = CHO for IKST

PRO [ CHO for CSA, vertical jump

Performance outcomes

Table 2 A summary of the performance outcomes for lean mass and muscle strength for those studies with protein and/or carbohydrate supplements that recruited untrained participants

118 S. M. Pasiakos et al.

Several sets combined for 50 reps at 70–85 % 1 RM for 5 exercises

3–5 sets, 3–15 reps for 11 exercises

3–5 sets, 3–12 reps @ 55–97 % 1 RM for 7 exercises

3 sets, 6–10 reps for 9 exercises

Sets and reps not stated for 9 exercises

4 sets, 10 max reps, unilateral leg extension

3 sets, 6–8 reps @ 85 % 1 RM for 4–6 exercises

Vieillevoye et al. [90]

Volek et al. [85]

Walberg-Rankin et al. [92]

Weisgarber et al. [77]

White et al. [95]

Williams et al. [87]

Willoughby et al. [76]

4

5 (alternating leg)

3

4

3

3

2

3

Frequency (9/week)

10

10

8

8

10

36

12

16

Duration (weeks)

14 g PRO ? 6 g AA or 20 g CHO pre/postexercise, 40 g non-training AM

0.2 g/kg PRO ? 0.8 g/kg CHO post-training

3 9 6 oz/day yogurt with 1 serving post-exercise (20 g CHO, 5 g PRO), PRO ? CHO, PLA (CHO) post-exercise only

0.3 g/kg PRO, PLA (0.3 g/kg CHO)

Milk (0.21 g/kg PRO ? 0.92 g/kg CHO), 1.25 g.kg CHO post-training

Whey (21.6 g ? CHO), soy (20 g ? CHO), PLA (CHO) pre/post-training and AM on nontraining

15 g EAA ? 15 g CHO, PLA (30 g CHO) pre/ post-training or twice in AM non-training

20 g PRO ? 60 g CHO, PLA (80 g CHO) pre/ post-training, CON (no training)

Supplementation

PRO [ CHO for LBM, 1 RM leg and bench press, myofibrillar PRO, myosin heavy chain

CHO ? PRO = PLA = CON for LBM

[CHO ? PRO = PLA] [ CON for IMST, IKST, 1 RM

Yogurt = PRO ? CHO = PLA for gains in LBM and increases in 1 RM bench and leg press

PRO = PLA for LBM and gains in 1 RM bench press

Milk = CHO for LBM and all 1 RM exercises

Whey = soy = PLA for 1 RM squat and bench press

Whey [ [soy = PLA] for gains in lean mass

EAA ? CHO [ CHO for muscle thickness and angle of pennation

EAA ? CHO = PLA for gains in lean mass or increases in maximal isokinetic force during squat or bench press

[PRO ? CHO = PLA] [ CON for isometric MVC

PRO ? CHO [ PLA for satellite cells and myonuclei per fiber


AA amino acid, AM morning, CHO carbohydrate, CON control, CSA cross-sectional area, EAA essential amino acid, EMG electromyogram, IKST isokinetic strength, IMST isometric strength, LBM lean body mass, max maximum, MVC maximal voluntary contraction, PLA placebo, PM afternoon, PRO protein, reps repetitions, RM repetition maximum

3–5 sets, 6–12 reps of 3 leg exercises

Intensity

Training program

Olsen et al. [91]

Study

Table 2 continued

Protein Supplements and Exercise Performance 119

123

120

repetitions) training and a subsequent 2-week low-volume (3–5 repetitions) training period. Protein supplementation attenuated the initial declines in muscle strength and power observed after the first week of high-volume training. However, changes in strength and power during the remaining 3 weeks of the high-volume training period were not different between the protein and placebo groups. During the subsequent 2-week period of low-volume training, increases in bench press strength for placebo were actually greater than protein. The authors concluded that protein supplementation was beneficial because it limited reductions in muscle strength and power that occurred during the early stages of adaptation to the overtraining program. However, during the later stages of the overtraining program, and subsequently, during the period of reduced training volume, protein supplementation had no influence on gains in muscle strength and power. 3.1.2.2 Studies of Protein Versus Isocaloric Carbohydrate Burke et al. [104] examined changes in body composition and muscle strength over a period of 6 weeks of resistance training, during which whey protein or carbohydrate supplementation was provided. Training consisted of high-volume and high-resistance 3-day blocks followed by 1 day of rest. Change in lean mass during the training and supplementation period was greater for protein than carbohydrate. However, both bench press and leg squat increased similarly for protein and carbohydrate during the training and supplementation period. Protein was associated with greater gains in isokinetic knee extension but not flexion torque. These data suggest whey protein supplementation may accelerate gains in lean mass during resistance training. However, these data also show that during the initial weeks of training, changes in lean mass are not necessarily reflected in appreciable gains in muscle strength. The effects of whey protein or carbohydrate supplementation during 11 weeks of resistance exercise were reported by Cribb et al. [105]. Lean mass increased in both the whey protein (mean ?2.3 kg) and carbohydrate (mean ?0.7 kg) groups during training. However, the increase in lean mass for protein was not statistically different than for carbohydrate, possibly because sample size of the groups (n = 5) was too small. However, in contrast to the findings above by Burke et al. [104], improvements in muscle strength were greater for individuals consuming protein versus carbohydrate. Changes in muscle cross-sectional area also tended (p \ 0.08) to be greater with protein, which was positively associated (0.8 \ r B 0.85 for the different fiber types) with muscle strength. 3.1.2.3 Studies of Protein Versus Protein and Carbohydrate A subsequent study by Cribb et al. [106] compared

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the effects of a combined protein ? carbohydrate supplement with protein alone on lean mass and strength changes during resistance training. The supplements were energymatched but provided different levels of protein (0.6 vs 1.3 gkg-1 for the protein ? carbohydrate and protein supplements, respectively). Increases in lean mass and muscle strength (e.g., bench press, squats, and pull-downs) were similar between groups. There were also no differences between groups in the increase in muscle fiber type cross-sectional area or total muscle contractile protein content. Consistent with earlier findings [105], these data also revealed a positive relationship between changes in cross-sectional area and gains in muscle strength. Collectively, the reports by Cribb et al. [105, 106] suggest that protein not carbohydrate provides the stimulus for muscle fiber hypertrophy during resistance training. These data also suggest that daily protein supplementation above 0.6 gkg-1 may not be necessary, especially when mean daily intake of dietary protein without supplementation was already at or above 1.6 gkg-1 [105, 106]. 3.1.2.4 Studies of Protein Source and Protein Combinations Whey and casein protein have differences in digestion rates and absorption kinetics that differentially affect muscle protein synthesis [83]. As a result, Cribb et al. [107] compared the effects of supplemental whey or casein protein on body composition and muscle strength in response to a 10-week training program. Individuals consuming whey protein increased lean mass and decreased fat mass more than those who consumed casein. In addition, gains in muscle strength were also significantly greater for those receiving whey. Comparisons of whey and casein protein supplementation for resistance-trained female athletes were also studied by Wilborn et al. [108]. Supplementation was provided before and after training, which consisted of whole-body resistance training 4 days per week and skills training three times per week for 8 weeks. However, in contrast to the findings above [107], gains in lean mass and improvements in muscle strength and power were similar between groups, regardless of the type of protein provided with training [108]. Isonitrogenous animal (whey) versus plant (rice) protein sources have also been studied as supplements provided immediately following strength training sessions conducted four times per week for 8 weeks in participants already engaged in resistance training [109]. Both groups demonstrated equal gains in lean mass and improvements in muscle strength and power. Although the amino acid profile varied somewhat between supplements, the authors argued that the high dose of rice protein (48 g) provided sufficient EAA to stimulate muscle protein synthesis equally to the whey supplement, even though the latter had


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Table 3 An overview of the effects of protein supplementation for the different subsections reviewed for resistance-trained participants Type of study

Effect of protein supplementation

Note

PRO versus PLA


Increased training intensity only 1–4 weeks’ duration

$ strength (1) PRO versus CHO

: mass, $ strength (1)

Low participant numbers

$ mass, : strength (1) PRO type

: mass, strength (2)

No placebo control, high supplemental dose of leucine regardless of source

: mass, $ strength (1) $ mass, strength (2) PRO ? CHO versus PRO


High normal dietary protein, no placebo control

PRO timing


High normal dietary protein

The numbers in parentheses refer to the number of studies. The arrows indicate whether the protein supplement increased (:) or had no additional ($) effect CHO carbohydrate, PLA placebo, PRO protein

a greater EAA content. However, these statements were not supported with direct measurements of muscle protein synthesis. Colker et al. [110] compared the effects of a daily whey supplement alone or in combination with branched chain amino acids (BCAA; leucine, isoleucine, and valine) and glutamine. They observed greater increases in muscle strength and endurance and greater increases in lean mass over 10 weeks of 3 daysweek-1 training when supplements included BCAA and glutamine, improvements the authors attributed to the additional leucine content of the combined supplement. Unfortunately, muscle endurance was higher at baseline for those receiving the additional BCAA and glutamine, which confounds the changes that were noted in response to training. It is important to note that none of the studies above [107–110] included a placebo or control group in their experimental design. Thus, their findings whether supportive [107, 110] or not [108, 109] for a particular source of protein are qualitatively less conclusive because of this design limitation. Other studies have assessed whether manipulating the source of high-quality protein with additional BCAA and glutamine altered body composition and muscle strength responses to training. For example, Kerksick et al. [111] assessed the effectiveness of a mixture of whey protein together with either casein or glutamine and BCAA versus an energy-matched carbohydrate supplement on changes in body composition and muscle strength following a 10-week training program. Lean mass increased more for those supplementing with whey and casein but there were no differences among groups in the increases in bench or leg press strength resulting from training. These findings suggest that type of protein supplement consumed may impact changes in lean mass by providing sufficient amino acids necessary to maximize muscle protein synthesis. However, dietary energy and protein

intake varied between groups. Thus, the effects of protein supplementation versus overall dietary protein intake could not be determined. 3.1.2.5 Studies of Protein Timing Although the importance of protein supplementation in close proximity to the exercise stimulus for enhancing muscle protein synthesis in trained participants has been well documented [88, 99], there has been only one study that has examined the timing of protein ingestion together with metrics of muscle strength for resistance-trained participants. Hoffman et al. [112] compared the effects of protein supplementation provided in the morning and evening or immediately before and after workouts, which occurred four times weekly for 10 weeks. Supplementation also occurred at the same time of day during the three non-training days and there was a control group that did not receive supplementation. There were no differences among the groups, regardless of supplementation or its timing, on changes in muscle strength or power. The authors suggested that supplementation did not enhance performance gains since the athletes were in positive nitrogen balance at the beginning of the study and consumed high daily dietary protein ([1.6 gkg-1). 3.1.2.6 Summary: Resistance-Trained Participants An overview of the findings from the subsections on resistance-trained participants is presented in Table 3 and a subsequent detailed list of the findings for protein supplements is provided in Table 4. Protein supplements had little or no effect on measures of strength and body composition when programs were 4 weeks or less [101–103], whereas positive effects of protein supplements have been observed on changes in lean mass and/or muscle strength when training programs were 8 weeks or longer [105–111]. The addition of carbohydrate to protein does not appear advantageous [106], whereas the addition of casein or

123

123

8–12 reps/set for several exercises

2–4 sets, 4–10 reps, 70–95 % 1 RM

2–4 sets, 4–10 reps, 70–95 % 1 RM

2–4 sets, 4–10 reps, 70–95 % 1 RM

70–100 % 1RM for several exercises

3–4 sets, 6–10 reps for several exercises


3 sets, 6–10 reps to fatigue, 7–8 exercises

3–5 sets, 3–12 reps for 5–8 exercises

3–5 sets, 3–12 reps for 5–8 exercises


Colker et al. [110]

Cribb et al. [107]

Cribb et al. [105]

Cribb et al. [106]

Fry et al. [101]

Hoffman et al. [112]

Joy et al. [109]

Kerksick et al. [111]

Kraemer et al. [103]

Ratamess et al. [102]

Wilborn et al. [108]

49/week

49/week

49/week

49/week

49/week

49/week

79/week, 39/day

3x/week

39/week

39/week

39/week

3 days on/1 off

Frequency

8

4

4

10

8

10

1

10

10

10

10

6

Duration (weeks)

24 g whey or casein pre/post-exercise

0.4 g/kg/day AA in 3 doses

0.4 g/kg/day AA in 3 doses

40 g whey ? 5 g glutamine ? 3 g BCAA, 40 g whey ? 8 g casein, 48 g CHO within 2 h post-exercise, nontraining AM

48 g whey or rice PRO post-exercise

42 g collagen, whey and casein mix AM/ PM, pre/post-exercise or CON

2.4 g AA 39/day with meals ? 2.1 g BCAA pre-training

1.3 g/kg/day PRO, 0.6 g/kg/day PRO ? 0.7 g/kg/day CHO divided in 3 doses

1.3 g/kg/day PRO or CHO divided in 3 doses

1.5 g/kg/day PRO in 3–4 servings

40 g/day whey, 3 g/day BCAA ? 5 g/ day glutamine

1.2 g/kg/day whey or CHO divided in 4 doses

Supplementation

Whey = casein for gains in LBM, 1 RM and reps to failure for bench and leg press

PRO = PLA for LBM

PRO = PLA for 1 RM bench and squat at 4 weeks

PRO = PLA for 1 RM bench and squat at 4 weeks

Whey ? casein = whey ? BCAA ? glutamine = CHO for 1 RM and # reps

Whey ? casein [ [whey ? BCAA ? glutamine = CHO] for LBM

Whey = rice for gains in lean mass, 1 RM leg and bench press, and leg power

AM/PM = pre/post = CON for change in 1 RM and power for squat and bench, and LBM

PRO = PLA for vertical jump and 1 RM for snatch

PRO [ CHO for 1 RM PRO = PRO ? CHO for LBM and 1 RM

PRO = CHO for LBM

Whey [ casein for LBM and all 1 RM

Whey ? BCAA ? glutamine [ whey for LBM, bench press reps

Whey [ CHO for knee extension

Whey = CHO for 1 RM bench press and squat

Whey [ CHO for LBM


AA amino acid, AM morning, BCAA branched chain amino acid, CON control, CHO carbohydrate, LBM lean body mass, PLA placebo, PM evening, pre before exercise, post immediately after exercise, PRO protein, reps repetitions, RM repetition maximum

6–12 reps/4–5 sets for several exercises

Intensity

Training program

Burke et al. [104]

Study

Table 4 A summary of the performance outcomes for lean mass and muscle strength for those studies with protein and/or carbohydrate supplements that recruited resistance-trained participants

122 S. M. Pasiakos et al.


BCAA to whey protein may lead to greater gains in lean mass and strength [110, 111], but there may be a ceiling to the beneficial effect from the additional source of protein [109]. 3.2 Concurrent Training for Enhancement of Aerobic and Anaerobic Power This section of the review addresses the issue of whether protein supplementation reliably enhances aerobic or anaerobic performance when training is being conducted concurrently to enhance both parameters. Success in many sports requires that athletes not restrict their training to resistance exercise but instead train to augment both aerobic and anaerobic power as well as muscular strength. In theory, the use of protein supplements could assist with the diverse training adaptations required to be successful in various sporting activities. For example, Wilkinson et al. [113] demonstrated, with use of a one-legged resistance or endurance training model, that increases in mitochondrial protein synthesis were confined to the endurance-trained leg following 10 weeks of a regular training program. Since protein supplementation augments rates of muscle protein synthesis following endurance exercise [114], it is conceivable supplementation during repeated training sessions could further stimulate mitochondrial adaptations [113]. However, potential benefits of protein supplements following endurance exercise might only be evident during the initial period of training as Breen et al. [115] reported no effect of supplementation in well trained cyclists on rates of mitochondrial protein synthesis following 90 minutes of exercise. To our knowledge, studies have not considered whether repeated use of protein supplements following sprint or high-intensity anaerobic training might increase muscle buffering capacity or synthesis of key glycolytic enzymes. 3.2.1 Untrained Participants As mentioned earlier (Sect. 3.1.1.1), Antonio et al. [70] examined the effects of EAA supplementation during 6 weeks of combined aerobic and resistance training in sedentary, untrained individuals. Time to exhaustion during an incremental treadmill test increased more for those individuals consuming EAA compared with placebo. Unfortunately, oxygen uptake was not measured to verify whether this improvement reflected increased oxidative potential of skeletal muscle or was simply a change in running economy due to training. To examine whether protein supplementation enhances oxidative capacity, Ferguson-Stegall et al. [116] examined the effects of a combined carbohydrate and protein supplement provided as low-fat chocolate milk versus an energy-matched carbohydrate supplement or placebo

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on changes in muscle oxidative enzymes and maximal aer_ 2max Þ in recreationally active, untrained obic power ðVO adults. The training program was 4.5 weeks of 5 daysweek-1 cycling exercise that progressed in duration from _ 2max . 30 to 60 minutes and intensity from 75 to 80 % VO _ Mean changes in VO2max expressed in both absolute (Lmin-1) and relative (mLkg-1min-1) terms were more than double for milk compared with the improvements produced by carbohydrate or placebo. However, there were no differences between groups in muscle oxidative enzyme activity or markers of muscle mitochondrial biogenesis. Additional research is clearly warranted to clarify the potential mechanisms that might account for faster gains in aerobic power with milk during these initial phases of endurance training. 3.2.2 Sports-Specific and Military Training The use of protein supplements on development of aerobic and anaerobic power during the early training phase of elite junior judoists was examined by Laskowski and Antosiewicz [117]. Both protein and placebo groups increased aerobic and anaerobic power over an initial 4-week period of exercise and supplementation but the changes were greater for those consuming protein. However, after an additional 3 months of training without supplementation, _ 2max was similar between groups. VO Because leucine is an independent stimulator of muscle protein synthesis [56], Crowe et al. [118] studied the effects of 6 weeks of daily leucine supplementation on aerobic and anaerobic power of outrigger canoeists who maintained their normal 8 hoursweek-1 of training throughout the study. Peak power during arm-cranking and _ 2max increased more rowing time to exhaustion at 75 % VO with leucine than placebo. The authors suggested leucine supplementation enhanced recovery and repair of muscle damage following training and thereby maximized muscle adaptations during the 6-week period. Using a repeated measures crossover design, the effects of a carbohydrate ? protein supplement provided immediately following a training session were studied during the indoor season using national competitive badminton players [119]. Participants consumed carbohydrate ? protein or a lowcalorie carbohydrate supplement throughout 14–15 weeks of training. Although players reported feeling more alert and stronger with the carbohydrate ? protein supplement, there _ 2max , leg were no differences between supplements on VO strength, lean mass or body fatness, grip strength, or 20-m shuttle-run performance. These data suggest that although players may report they feel better with ingestion of a carbohydrate ? protein recovery supplement, this does not translate into improvements in physical function.

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124


Table 5 A summary of the performance outcomes for the effects of protein and/or carbohydrate supplements on aerobic and anaerobic power Study

Training program

Supplementation


Intensity

Frequency

Duration (weeks)

Antonio et al. [70]

20 min @ 70 % HRmax

39/week

6

12.8 g EAA pre/posttraining ? 12.8 g non-training AM

PRO [ PLA for incremental treadmill time to maximum

Crowe et al. [118]

Outrigging canoe, cycling, jogging and resistance training

8 h/week

6

45 mg/kg/day leucine

PRO [ PLA for peak armcranking power, TTE at 75 % _ 2max rowing VO

Fahlstro¨m et al. [119]

Elite badminton club training

49/week

15

0.62 g/kg CHO ? 0.22 g/kg PRO post-training

CHO ? PRO = PLA for LBM, _ 2max leg IKST, VO

FergusonStegall et al. [116]

_ 2max 60 min @ 75–80 % VO

59/week

4.5

4 mL/kg milk, CHO or PLA at 0 and 1 h post-training

Milk [ [CHO = PLA] for _ 2max VO

Laskowski and Antosiewicz [117]

47 % endurance, 35 % speed and 18 % resistance training

119/week

4

0.5 g/kg/day PRO after AM training

Walker et al. [120]

Self-regulated running and resistance training

C39/ week

8

19.7 g whey ? 6.2 g leucine pre/post-training, 29 AM nontraining

PRO = PLA for BM

Milk = CHO = PLA for oxidative enzymes _ 2max , peak PRO [ PLA for VO power and total work during 30-s Wingate PRO = CHO for 3-mile run and sprint PRO [ CHO for 1 RM bench press

AM morning, BM body mass, CHO carbohydrate, EAA essential amino acid, HRmax maximal heart rate, IKST isokinetic strength, LBM lean body _ 2max maximal aerobic power mass, PLA placebo, PRO protein, RM repetition maximum, TTE time to exhaustion, VO

Walker et al. [120] examined the effects of a whey protein and leucine supplement on physical performance for U.S. Air Force personnel conducting their normal training over a period of 8 weeks, which included a minimum of 3 daysweek-1 of aerobic and muscle endurance exercise. Before and following each training session participants consumed whey protein ? leucine or an isocaloric carbohydrate supplement. Multiple measures of performance improved with training but there was no difference between groups. However, since the training programs were not standardized, the interpretation of these findings is not straightforward. Further, some of the participants reported they were not consuming the supplements every day. This study highlights the problems associated with attempting to transition the use of protein supplements to the general population based on studies conducted with elite athletes. Most members of the general population do not participate in homogenous training regimens and may not be as motivated as elite athletes to adhere to strict supplementation schedules. 3.2.3 Summary: Aerobic and Anaerobic Power A detailed summary of the effects of protein supplements on aerobic and anaerobic power is provided in Table 3. The effects of protein supplementation during aerobic training

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or during sporting activities that require a combination of aerobic and anaerobic power for success have not been definitively established. The limited information available indicates that when protein supplements are consumed _ 2max following training sessions, there are faster gains in VO during the first several weeks of aerobic training for previously untrained individuals [116] or during the beginning of the training cycle for athletes [117], as well as greater gains in anaerobic (Table 5) power during normal training periods for competitors [117, 118].

4 Discussion 4.1 General Study Limitations of Papers Reviewed Examining changes in caloric and macronutrient intake of the diet throughout the course of a study is critical for isolating the effects of protein supplementation. The validity of a study is reduced if total caloric and protein intake are not adequately reported [89, 91, 119] or in cases where changes in diet are not consistent or do not reflect the additional supplements (i.e., protein or carbohydrate) provided to different groups of participants [81, 104, 106, 107, 111]. It should also be noted that in the studies reviewed above, mean normal daily protein intake for


participants varied considerably, from slightly less than 1 to well over 2 gkg-1 per day but few studies reported the nitrogen balance of their participants [72, 90, 96, 112]. Since protein supplements may be more effective when participants are in an otherwise negative nitrogen balance due to their normal diet [121, 122], the benefits of a diet that is typically high in protein may offset any further advantage additional protein supplements might provide. In these situations, ingestion of the supplement in close proximity to the beginning or immediately following exercise may be more critical than the total dose provided throughout the day [57, 73, 123]. However, the only study that was conducted with resistance-trained participants and included metrics of performance failed to support this premise [112]. The mean total dose of protein supplement provided varied considerably among the studies reviewed from a low of 3 gday-1 of leucine [118] to over 100 gday-1 of whey or casein [107]. As little as 6 g EAA consumed within 1 hour of resistance training is known to stimulate muscle protein synthesis [64] with maximal stimulation obtained with 20 g of intact whole-egg protein ingestion following exercise [65]. Thus, although the protein dose provided in some studies may not have been optimal, there were no apparent relationships between the dose provided and the gains in muscle strength or aerobic and anaerobic power. Many studies that were reviewed justified selection of their sample size with a priori calculations using sample variances and expected effect sizes from their previous research and other publications. However, this a priori justification was not always described [70, 72–74, 77, 82, 86, 87, 89–96, 101, 104, 107–110, 112, 117, 119]. Studies with small numbers of participants in some or all of their experimental groups [72, 87, 91, 96, 105, 107, 112, 117, 118], which results in low statistical power and therefore increased risk of a type II error, should be replicated to verify their findings. In addition, some authors reported, for various reasons, loss of data due to withdrawal or exclusion of participants after the study began [77, 85, 91, 95, 96, 104, 105, 107], which may indicate the need to conduct additional testing to validate their results. The reliability, reproducibility, and sensitivity of performance test metrics selected to evaluate changes in performance with protein supplements were also of concern across the papers reviewed. It is unclear whether the sensitivity analyses conducted by Amann et al. [124] for cycling time-trial and time-to-exhaustion tests have been performed for measures of muscle strength using isokinetic, isometric or 1 RM techniques. Since many investigations incorporated more than one method to evaluate changes in muscle strength and reported consistent [71, 81, 87] or inconsistent [73, 74, 89, 102, 104] findings for the

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same or different muscle groups, sensitivity of these various test metrics may not be similar. Other factors such as specificity of adaptation to the training velocity of contraction may explain disparate findings reported with slow versus fast isokinetic tests of muscle strength [74], which speaks to the advantage of using 1 RM tests for exercises performed during the training programs to evaluate changes in muscle strength with supplements [75, 76, 81, 82, 92–94, 96, 102–107, 111]. In addition, several studies assessed various functions of muscle performance including tests of aerobic and anaerobic power alone [117, 118] or in combination with tests of muscle strength [70, 108, 109, 112, 119, 120] and it is unclear whether sensitivity of these various test metrics are similar. Likewise, the sensitivity and accuracy of the various methods used by studies to estimate muscle or lean mass is noteworthy. The use of magnetic resonance imaging provides the most accurate estimate of muscle mass [125] but this methodology is expensive and was usually not used in the studies reviewed [69, 71, 73, 75]. Dual X-ray absorbance technology was consequently used [77, 82, 85, 89, 92–94, 103–109, 111, 112] but this method can overestimate muscle mass due to its inclusion of organ tissue mass and body water [125]. Finally, estimates of fat-free mass were made from measures of body density with hydrostatic weighing [72, 76, 81, 86, 95] and/or from skinfold and limb circumference measures [72, 76, 87, 90], but these methodologies cannot determine accurate changes in the muscle mass component. 4.2 Muscle Mass and Strength A topic of concern across studies that incorporate resistance-training programs is whether protein supplements would affect performance differently for untrained individuals versus individuals who are regularly active, or elite athletes. The well documented occurrence of neural adaptation during the early period of resistance training [97, 126, 127] raises doubts about whether protein supplements are themselves beneficial, especially among untrained individuals. In fact, when the confounding influence of neural adaptations during the early training period was removed, no effect of protein supplementation on measures of muscle strength was observed [71]. Another important unresolved issue is the minimum duration of training required to induce measurable phenotypic changes in muscle and observe benefits of protein supplementation. Increases in muscle cross-sectional area have been observed in as few as nine sessions over 3 weeks of resistance training [128], although it is uncommon to see measureable changes in less than 6–8 weeks [70, 75, 81, 96]. Willoughby et al. [76] reported greater gains in lean mass, strength, myofibrillar contractile protein and myosin

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RNA expression with protein supplementation for untrained volunteers following 10 weeks of alternating upper and lower body training 4 daysweek-1. Others also reported increases in muscle cross-sectional area determined from tomography [86] or ultrasound [90], angle of pennation [90], and proliferation of satellite cells and myonuclei [91] with training programs of at least 8 weeks in duration. In addition, even with trained participants, changes in muscle cross-sectional area and muscle contractile protein that were related to gains in muscle strength were observed after 8–11 weeks of supplementation that involved resistance training 3–4 timesweek-1 [105, 106, 109]. These data suggest that a training stimulus of at least 8 weeks in duration with appropriate progressions in intensity, frequency, and duration is necessary before measurable changes in phenotype and muscle function occur and reflect altered rates of protein signaling and expression. This interpretation is supported by a recent meta-analysis that concluded protein supplementation resulted in greater gains in lean mass, muscle cross-sectional area and 1 RM strength for both trained and untrained participants during training that averaged 12 weeks’ duration and was conducted at least three times weekly [24]. Typically, the majority of studies reviewed used wholebody resistance training programs that would be expected to distribute increases in lean mass throughout the body. However, measures of strength were typically restricted to one upper (bench press) and/or lower body (leg press) exercise. Protein supplementation often led to greater gains in lean mass with training but these changes were not associated with similar improvements in metrics of muscle strength [73, 85, 89, 90, 92, 93, 104, 111]. As noted by Volek et al. [85], restricting the measure of strength to a specific movement may not be the best way to represent the functional gains in lean mass distributed throughout the body. From a research perspective, one way to circumvent this issue is to use a single-limb resistance-training protocol [71, 75, 87], but the findings from these studies may be limited by the small muscle mass recruited during training [71] or carry-over effects to the non-supplemented and/or untrained limb [75, 87]. Another unresolved issue is the optimal composition of the supplements provided. Several studies examined effectiveness of the type of protein supplement provided. These studies noted similar improvements in lean mass and/or strength with whey and soy [82, 85], whey and casein [108], specifically modified whey and standard whey [86], or whey and rice [109] supplements, greater increases with ingestion of whey isolate versus casein [107], and greater improvements when casein [111] or BCAA [110] were combined with whey. However, dietary records were not collected, not reported or changed

123


throughout training in some of these studies, which may compromise their validity [107, 108, 111]. Comparisons were also made between energy-matched carbohydrate and carbohydrate ? protein supplements, with protein delivered as EAA [90], low-fat milk [92–94], milk hydrolysate [91], or yogurt [95], or between energy-matched whey protein isolate and protein ? carbohydrate [106]. When the resistance-training stimulus was of sufficient duration and frequency, addition of protein with carbohydrate in the form of milk led to greater gains in lean mass and/or strength in untrained participants than energy-matched carbohydrate supplementation [93, 94], but improvements in strength and muscle cross-sectional area were no different when protein was provided alone or in combination with carbohydrate [106]. Collectively these findings demonstrate carbohydrate has little effect on changes in muscle protein accretion following resistance training lasting several weeks, in agreement with results of isotope tracer studies conducted following a single bout of resistance exercise [67]. Recently, it has also been shown that differences among participants in microRNAs involved in post-translational control of genes coding for skeletal muscle protein growth are related to the variability observed for changes in lean mass and muscle strength that follow resistance training [129]. Thus, despite random allocation prior to the initiation of resistance training, it is still conceivable that some or all of the differences observed between groups may be a reflection of those characterized as high or low responders due to differences in microRNA expression [129] rather than to any effect related to protein supplementation. 4.3 Aerobic and Anaerobic Power There are also various unresolved issues with respect to aerobic and anaerobic power and protein supplements. For example, the relationships between training, protein supplementation and mitochondrial protein synthesis are not understood. There is evidence in support of a preferential stimulus for mitochondrial protein synthesis during the early stages of aerobic training for untrained individuals _ 2max [113], which appears consistent with faster gains in VO or treadmill running time to exhaustion that have been attributed to protein supplementation [70, 116]. In addition, mean increases of 20–50 % in mitochondrial enzyme activity, such as cytochrome oxidase, have been reported in as little as 6–10 training sessions [130, 131], supporting the potential for rapid changes in protein synthesis to be augmented with supplementation during early phases of training. Unfortunately, the only study that compared changes in mitochondrial enzyme activity failed to demonstrate faster gains during the initial weeks of training with protein supplementation [116]. Further, as reported by Breen et al.


[115], preferential effects of protein supplements on rates of mitochondrial protein synthesis decrease as participants become more adapted to the training stimulus. Nevertheless, greater changes in metrics of aerobic performance such _ 2max , treadmill run time, or rowing time to exhaustion as VO have been reported following protein supplementation for several weeks for both untrained [70, 116] and trained participants [117, 118]. Clearly, additional research is warranted to elucidate the mechanism(s) responsible for _ 2max and other metrics of aerobic perimprovements in VO formance observed following only 4–6 weeks of protein supplementation. The mechanisms responsible for faster gains in anaerobic power following protein supplementation have also not been determined. The improvements observed are not simply related to greater gains in muscle mass since both peak power and total work, when expressed per unit of mass, increased following supplementation [117, 118]. As such, further study is required to assess anaerobic-associated protein signaling and effects of protein supplements on muscle buffering capacity.

5 Summary Evidence Statements This paper has reviewed the evidence base that supports use of protein supplements during or following exercise to increase the gains in muscle mass and strength, and aerobic and anaerobic power, which occur as a result of training. Based on our interpretation of the available evidence base, we offer the following evidence statements regarding the use of protein supplements to enhance muscle mass and strength and/or aerobic and anaerobic power. 5.1 Evidence Statement—Ingestion of Protein to Increase Muscle Mass and Strength There is consistent and good quality experimental data that shows, in untrained individuals, changes in lean mass and muscle strength observed during the initial weeks of resistance training are not influenced when protein supplements are provided (evidence category A). There is limited, but quality experimental data to support the statement that as the duration and frequency of the resistance training duration is increased for untrained or trained individuals, ingestion of protein supplements will promote greater gains in lean mass and muscle strength (evidence category B). There is consistent and good quality experimental data that show that addition of carbohydrate to protein supplements will not promote any further changes in lean mass and muscle strength observed during a resistance training program (evidence category A). There is limited, but quality experimental data that show that the

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type or combinations of various types of protein supplements will affect the gains in lean mass and muscle strength observed throughout resistance training (evidence category B). 5.2 Evidence Statement—Ingestion of Protein to Increase Aerobic or Anaerobic Power There is limited, but quality experimental data to support the statement that ingestion of protein supplements following daily training for several weeks will enhance gains _ 2max in previously untrained individuals or athletes at in VO the beginning of their seasonal training programs or result in improvements in tests of aerobic and anaerobic power for athletes during their normal training programs (evidence category B).

6 Future Research It is hoped this review will provide stimulus for additional research that links measures of muscle protein synthesis, and also intracellular signaling following protein supplementation and exercise, to measures of muscle mass and function. Also, it is suggested that future studies that recruit untrained individuals should consider including a baseline resistance training period to accommodate the influence of neural adaptations on performance metrics similar to the design of Erskine et al. [71]. In addition, the confounding effects of an individual’s nitrogen balance before supplementation and variations in dietary protein intake throughout the period of training and supplementation should be repeatedly assessed throughout the investigation and perhaps be considered as a covariate with data analyses. Since experienced resistance-trained individuals routinely consume high-protein diets, additional research is needed to address the importance of the timing and type of protein supplement consumed. In contrast to the volume of research focused on understanding the effects of protein supplementation combined with resistance training on skeletal muscle, there is limited data that relates protein supplementation with longitudinal adaptations in aerobic or anaerobic power. Although there is evidence to support a role for protein supplements for enhancing adaptation during the early phases of aerobic and anaerobic training [116–118], the mechanisms have yet to be fully described and additional research is certainly warranted. Collectively, findings from these studies would provide the necessary evidence base to support or refute the use of protein supplements for greater gains in lean mass that result in improved muscle strength, and aerobic and anaerobic power.

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7 Conclusions This review has assessed the existing evidence base that could support use of protein supplements during resistance training to enhance gains in muscle mass and strength and during an aerobic- or sport-based training program to enhance gains in aerobic and anaerobic power. Although there is overwhelming evidence that supports the rationale for including protein supplements before or immediately following a bout of resistance or aerobic exercise to enhance protein synthesis and anabolic signaling, to date, these acute changes have not been shown to be consistently translated into greater long-term gains in muscle mass or strength and increases in aerobic and anaerobic power. Nevertheless, additional research is warranted that causally relates the time course of changes in protein synthesis, and associated anabolic signaling after exercise, to changes in muscle mass or strength and gains in aerobic and anaerobic power that may result from protein supplementation. Acknowledgments This work was supported by the US Army Medical Research and Materiel Command (USAMRMC) and the Department of Defense Center Alliance for Dietary Supplements Research. The views, opinions and/or findings in this report are those of the authors, and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation. Citation of commercial organization and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations. T.M. McLellan was supported by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USAMRMC. The authors have no potential conflicts of interest that are directly relevant to the content of this review.

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