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EFFECTS OF PROTEIN AND AMINO-ACID SUPPLEMENTATION ON ATHLETIC PERFORMANCE
Richard B Kreider PhD
Exercise & Sport Nutrition Laboratory, Department of Human Movement Sciences & Education, The University of Memphis, Memphis, Tennessee 38152. Email: kreider.richard=AT=coe.memphis.edu
Sportscience 3(1), sportsci.org/jour/9901/rbk.html, 1999 (5579 words)
Reviewed by Brian Leutholtz PhD, Department of Exercise Science, Physical Education, and Recreation, Old Dominion University, Norfolk, Virginia
BACKGROUND. Protein and amino acids are among the most common nutritional supplements taken by athletes. This review evaluates the theoretical rationale and potential effects on athletic performance of protein, purported anabolic amino acids, branched-chain amino acids, glutamine, creatine, and hydroxymethylbutyrate (HMB). LITERATURE. Two books, 61 research articles, 10 published abstracts, and 19 review articles or book chapters. FINDINGS. Dietary supplementation of protein beyond that necessary to maintain nitrogen balance does not provide additional benefits for athletes. Ingesting carbohydrate with protein prior to or following exercise may reduce catabolism, promote glycogen resynthesis, or promote a more anabolic hormonal environment. Whether employing these strategies during training enhances performance is not yet clear. There is some evidence from clinical studies that certain amino acids (e.g., arginine, histidine, lysine, methionine, ornithine, and phenylalanine) have anabolic effects by stimulating the release of growth hormone, insulin, and/or glucocorticoids, but there is little evidence that supplementation of these amino acids enhances athletic performance. Branched-chain amino acids (leucine, isoleucine, and valine) and glutamine may be involved in exercise-induced central fatigue and immune suppression, but their ergogenic value as supplements is equivocal at present. Most studies indicate that creatine supplementation may be an effective and safe way to enhance performance of intermittent high-intensity exercise and to enhance adaptations to training. Supplementation with hydroxymethylbutyrate appears to reduce catabolism and increase gains in strength and fat-free mass in untrained individuals initiating training; as yet, limited data are available to decide how it affects training adaptations in athletes. CONCLUSIONS. Of the nutrients reviewed, creatine appears to have the greatest ergogenic potential for athletes involved in intense training. FURTHER RESEARCH. All supplements reviewed here need more evaluation for safety and effects on athletic performance. Reprint · Reference List · Help
KEYWORDS: anabolic, BCAA, branched-chain amino acids, creatine, ergogenic, glutamine, HMB, hydroxymethylbutyrate, training
BACKGROUND
Amino acids are the building blocks of protein in the body; as such they are essential for the synthesis of structural proteins, enzymes, and some hormones and neurotransmitters. Amino acids are also involved in numerous metabolic pathways that affect exercise metabolism. Consequently, it has been suggested that athletes involved in intense training require additional protein in the diet or that they should supplement their diet with specific amino acids. I review here the rationale and the evidence for the potential ergogenic effect of short-term supplementation with protein and amino acids and the evidence for the potential anabolic effect of longer-term use when supplementation is combined with training. I deal first with protein, then with the amino acids under the following headings: the potentially anabolic amino acids; the branched-chain amino acids, which have a somewhat different role in metabolism and in their potential effect on performance; glutamine, which is in a class of its own for its effects on the immune system; creatine, an amino acid that is not one of the building blocks of protein but is involved in short-term energy production in muscle; and hydroxymethylbutyrate (HMB), a potentially anabolic metabolite of the amino acid leucine.
LITERATURE
This review is an update rather than an exhaustive account of all published works on the topic. I have cited two books, 60 research articles, 10 published abstracts, and 18 review articles/book chapters from my own database of references. In my database there are a further 97 research articles, 78 abstracts, and 38 review articles/book chapters on the topic. These additional references are reviewed elsewhere (Kreider, 1999; Kreider, 1998; Williams et al., 1999). Download the complete list as a Word 97 file by clicking on this link.
FINDINGS
Protein
A considerable amount of research has evaluated dietary protein needs of athletes. Although there is some debate, most studies indicate that in order to maintain protein balance during intense resistance and/or endurance training, athletes should ingest approximately 1.3 to 1.8 g protein per kg body mass per day (Butterfield, 1991; Lemon, 1998; Kreider et al., 1993; Kreider, 1999). Athletes training at high-altitude may need as much as 2.2 g protein per kg per day in order to maintain protein balance (Butterfield, 1991). This protein intake is about 1.5 to 2 times the recommended dietary allowance (RDA) for the normal adult. In most instances an iso-energetic diet can provide the required protein, but athletes who maintain hypo-energetic diets, do not ingest enough quality protein in their diet, and/or train at altitude may be susceptible to protein malnutrition (Kreider, 1999). In theory, this state could slow tissue growth and/or recovery from training. On the other hand, ingesting more protein than necessary to maintain protein balance during training (e.g., > 1.8 g/kg/d) does not promote greater gains in strength or fat-free mass (Lemon et al., 1992; Tarnopolsky et al., 1992). These findings indicate that athletes typically do not need to supplement their normal diets with protein, provided they ingest enough quality protein to maintain protein balance.
More recently, there has been interest in determining the effects of pre- and post-exercise carbohydrate and protein feedings on hormonal responses to exercise (Cade et al., 1992; Chandler et al., 1994; Roy and Tarnopolsky, 1998; Tarnopolsky et al., 1997; Zawadzki et al., 1992). Ingestion of protein with carbohydrate has been reported to increase insulin and/or growth hormone levels to a greater degree than ingestion of carbohydrate alone (Chandler et al., 1994; Zawadzki et al., 1992). Consequently, ingesting protein and carbohydrate prior to exercise may serve as an anti-catabolic nutritional strategy (Carli et al., 1992). Further, ingesting carbohydrate and protein following exercise may promote a more anabolic hormonal profile, glycogen resynthesis, and/or hasten recovery from intense exercise (Roy and Tarnopolsky, 1998; Roy et al., 1997). Over time these alterations may allow an athlete to tolerate training to a greater degree and/or promote greater training adaptations, but the evidence is not yet clear.
Anabolic Amino Acids
One of the commonly purported benefits of amino acid supplementation is that certain amino acids (e.g., arginine, histidine, lysine, methionine, ornithine, and phenylalanine) may stimulate the release of growth hormone, insulin, and/or glucocorticoids, thereby promoting anabolic processes (Kreider, 1993). There is some clinical evidence that amino acid supplementation may stimulate growth hormone releasing factors and/or growth hormone release (Carlson, et al., 1989;; Garlick and Grant, 1988; Iwasaki et al., 1987; Merimee et al., 1969). For example, intravenous arginine and ornithine infusion have been used clinically for stimulating growth hormone release (Carlson et al., 1989; Iwasaki et al., 1987). In addition, preliminary clinical studies indicated that protein (20 to 60 g); arginine and lysine (1.2 g); and ornithine (70 mg/kg) increased growth hormone and somatomedin concentrations in the blood (Bucci et al., 1990; Jackson et al., 1968; Isidori et al., 1981). However, other researchers have not replicated these findings, particularly in healthy individuals (Lemon, 1991). There is also little evidence that supplementation of these amino acids during training significantly affects body composition, strength, and/or muscle hypertrophy (Kreider, 1999). Consequently, the effects of amino acid supplementation on growth-hormone release and training adaptations are as yet unclear.
Branched-Chain Amino Acids
Researchers have expended a considerable amount of effort on evaluating the effects of supplementation of branched-chain amino acids (BCAAs: leucine, isoleucine, and valine) on physiological and psychological responses to exercise (Blomstrand et al., 1991; Kreider, 1998; Wagenmakers, 1998). There are two primary hypotheses regarding the ergogenic value of supplementation with these amino acids.
First, BCAA supplementation has been reported to decrease exercise-induced protein degradation and/or muscle enzyme release (an indicator of muscle damage) possibly by promoting an anti-catabolic hormonal profile (Carli et al., 1992; Coombes and McNaughton, 1995). Theoretically, BCAA supplementation during intense training may help minimize protein degradation and thereby lead to greater gains in fat-free mass. Although several studies support this hypothesis, additional research is necessary to determine the long-term effects of BCAA supplementation during training on markers of catabolism, body composition, and strength (Kreider, 1998).
Second, the availability of BCAA during exercise has been theorized to contribute to central fatigue (Newsholme et al., 1991). During endurance exercise, BCAAs are taken up by the muscles rather than the liver in order to contribute to oxidative metabolism. The source of BCAAs for muscular oxidative metabolism during exercise is the plasma BCAA pool, which is replenished through the catabolism of whole body proteins during endurance exercise (Davis, 1995; Kreider, 1998; Newsholme et al., 1991). However, the oxidation of BCAAs in the muscle during prolonged exercise may exceed the catabolic capacity to increase BCAA availability, so plasma BCAA concentration may decline during prolonged endurance exercise (Blomstrand et al., 1988; Blomstrand et al., 1991). The decline in plasma BCAAs during endurance exercise can result in an increase in the ratio of free tryptophan to BCAAs. Free tryptophan and BCAAs compete for entry into the brain via the same amino-acid carrier (Newsholme et al., 1991). Therefore, a decrease in BCAAs in the blood facilitates entry of tryptophan into the brain. Moreover, most tryptophan in the blood is bound to albumin, and the proportion of tryptophan bound to albumin is influenced by the availability of long-chained fatty acids (Davis et al., 1992; Newsholme et al., 1991). In endurance exercise free fatty-acid concentration rises, so the amount of tryptophan bound to albumin falls, increasing the concentration of free tryptophan in the blood (Davis, 1995).
Collectively, the decline in plasma BCAAs and increase in free tryptophan during prolonged endurance exercise alters the ratio of free tryptophan to BCAAs and increases the entry of tryptophan into the brain (Newsholme et al., 1991). An increased concentration of tryptophan in the brain promotes the formation of the neurotransmitter 5-hydroxytryptamine (5-HT). 5-HT has been shown to induce sleep, depress motor neuron excitability, influence autonomic and endocrine function, and suppress appetite in animal and human studies. An exercise-induced imbalance in the ratio of free tryptophan to BCAAs has been implicated as a possible cause of acute physiological and psychological fatigue (central fatigue). It has also been hypothesized that chronic elevations in 5-HT concentration, which may occur in athletes maintaining high-volume training, explains some of the reported signs and symptoms of the overtraining syndrome: postural hypotension, anemia, amenorrhea, immunosuppression, appetite suppression, weight loss, depression, and decreased performance (Newsholme et al., 1991; Gastmann and Lehmann, 1998; Kreider, 1998).
A number of studies have recently been conducted to evaluate whether carbohydrate and/or BCAA supplementation affects central fatigue during exercise and/or signs and symptoms of overtraining. Analysis of this literature indicates that carbohydrate and/or BCAA supplementation during exercise can affect the ratio of free tryptophan to BCAA. For example, carbohydrate administration during exercise has been reported to attenuate FFA release and minimize increases in the free tryptophan:BCAA ratio (Davis et al., 1992). In addition, BCAA supplementation has been reported to increase plasma BCAA concentration and minimize and/or prevent increases in the ratio of free tryptophan to BCAAs (Blomstrand et al., 1991). Studies also indicate that BCAA administration with or without carbohydrate prior to and during exercise can affect physiological and psychological responses to exercise (Coombes and McNaughton, 1995; Hefler et al., 1993; Kreider et al., 1992; Kreider and Jackson, 1994).
Nevertheless, the effect of these nutritionally-induced alterations in the free tryptophan to BCAA ratio on physical performance is still not clear. Most studies indicate that BCAA supplementation does not improve single-bout endurance performance, but these studies almost certainly lacked power to delimit small but useful enhancements of performance (Davis, 1995; Gastmann and Lehmann, 1998; Kreider, 1998). Additional research is also necessary to determine the effect of long-term BCAA supplementation on training adaptations and the signs and symptoms of overtraining (Kreider, 1998).
Glutamine
Rennie and colleagues have suggested glutamine supplementation as a strategy to promote muscle growth (Rennie et al., 1994; Rennie, 1996). They based the suggestion on animal and human studies of the effect of glutamine on protein synthesis, cell volume, and glycogen synthesis (Rennie et al., 1994; Varnier et al., 1995; Rennie, 1996; Low et al., 1996). Glutamine is also an important fuel for white blood cells, so reductions in blood glutamine concentration following intense exercise may contribute to immune suppression in overtrained athletes (Parry-Billings et al., 1990a; Parry-Billings et al., 1990b; Parry-Billings et al., 1992; Kargotich et al., 1996; Newsholme and Calder, 1997).
Preliminary studies indicate that supplementation with branched-chain amino acids (4 to 16 g) and/or glutamine (4 to 12 g) can prevent the decline or even increase glutamine concentration during exercise (Kreider, 1998). In theory these changes in glutamine concentration could have beneficial effects on protein synthesis and immune function. However, in the few studies of increased glutamine availability, there was little or no effect on performance or immune status (Rohde et al., 1998; Nieman and Pedersen, 1999). It is also unclear whether long-term supplementation of glutamine affects protein synthesis, body composition, or the incidence of upper respiratory-tract infections during training.
Creatine
Creatine is a naturally occurring amino acid derived from the amino acids glycine, arginine, and methionine (Balsom et al., 1994; Williams et al., 1999). Most creatine is stored in skeletal muscle, primarily as phosphocreatine; the rest is found in the heart, brain, and testes (Balsom et al., 1994; Kreider, 1998). The daily requirement of creatine is approximately 2 to 3 g; half is obtained from the diet, primarily from meat and fish, while the remainder is synthesized (Williams et al., 1999). Creatine supplementation has been proposed as a means to "load" muscle with creatine and phosphocreatine (PCr). In theory, an increased store of creatine or phosphocreatine would improve the ability to produce energy during high intensity exercise as well as improve the speed of recovery from high-intensity exercise.
A number of studies have been conducted to determine the effects of creatine supplementation on muscle concentrations and performance. Creatine supplementation (20 g per day or 0.3 g per kg body mass per day for 4 to 7 days) has been reported to increase intramuscular creatine and phosphocreatine content by 10 to 30% (Casey et al., 1996; Febbraio et al, 1995; Green et al., 1996a; Green et al., 1996b; Greenhaff et al., 1993a; Hultman et al., 1996; Smith et al., 1998b, Vandenberghe et al., 1997). There is also evidence that creatine supplementation enhances the rate of PCr resynthesis following intense exercise (Greenhaff et al., 1993b; Greenhaff et al., 1994a; Greenhaff et al., 1994b). Most studies indicate that short-term creatine supplementation increases total body mass (Hultman et al., 1996; Williams et al., 1999), work performed during multiple sets of maximal effort muscle contractions (Greenhaff et al., 1993a; Volek et al., 1997), and single and/or repetitive sprint capacity (Birch et al., 1994; Grindstaff et al., 1997; Prevost et al., 1997). In addition, long-term creatine supplementation during training has been reported to promote greater gains in strength (Earnest et al., 1995; Peeters et al., 1999; Stone et al., 1999; Vandenberghe et al., 1997), fat-free mass (Kreider et al., 1998; Stone et al., 1999; Stout et al., 1999; Vandenberghe et al., 1997), and sprint performance (Kreider et al., 1998; Peyreburne et al., 1998; Stout et al., 1999). However, it should be noted that not all studies report ergogenic benefit (Burke et al., 1996; Redondo et al., 1996; Terrillion et al., 1997) and that caffeine has been reported to counteract the potential ergogenic value of creatine supplementation (Vanakoski et al., 1998; Vandenburghe et al., 1996). Although more research is needed, creatine supplementation appears to be a safe and effective nutritional strategy to enhance high intensity exercise performance and improve training adaptations (Williams et al., 1999).
Hydroxymethylbutyrate (HMB)
The leucine metabolite hydroxymethylbutyrate (more exactly the calcium salt of b -hydroxy-b -methylbutyric acid) has recently become a popular dietary supplement purported to promote gains in fat-free mass and strength during resistance training (Kreider, 1999). The rationale is that leucine and its metabolite a -ketoisocaproate (KIC) appear to inhibit protein degradation (Nair et al., 1992; Nissen et al., 1996), and this anti-proteolytic effect may be mediated by HMB. Animal studies indicate that approximately 5% of oxidized leucine is converted to HMB via KIC (Nissen et al., 1994; Van Koevering et al., 1994). The addition of HMB to dietary feed improved colostral milk fat and growth of sows (Nissen et al., 1994), tended to improve the carcass quality of steers (Van Koevering et al., 1994), and decreased markers of catabolism during training in horses (Miller et al., 1997). Supplementing with leucine and/or HMB may therefore inhibit protein degradation during periods associated with increased proteolysis, such as resistance training.
Although much of the available literature on HMB supplementation in humans is preliminary in nature, several recently published articles and abstracts support this hypothesis. Leucine infusion appears to decrease protein degradation in humans (Nair et al., 1992). HMB supplementation during 3 to 8 weeks of training has been reported to promote significantly greater gains of fat-free mass and strength in untrained men and women initiating resistance training (Nissen et al., 1996; Nissen et al., 1997; Vukovich et al., 1997). In some instances these gains were associated with signs of significantly less muscle damage (efflux of muscle enzymes and urinary 3-methylhistidine excretion) (Nissen et al., 1996). Although these findings suggest that HMB supplementation during training may enhance training adaptations in untrained individuals initiating training, it is less clear whether HMB supplementation reduces markers of catabolism or promotes greater gains in fat-free mass and strength during resistance training in well-trained athletes. Indeed, there are several reports of no significant effects of HMB supplementation (3 to 6 g per day) in well-trained athletes (Almada et al., 1997; Kreider et al., 1997; Kreider et al., 1999). More research is needed (Kreider, 1999).
CONCLUSIONS
Dietary supplementation of protein beyond that necessary to maintain nitrogen balance does not provide additional ergogenic benefit.
Ingesting carbohydrate/protein prior to exercise may reduce catabolism whereas ingesting carbohydrate/protein following exercise may promote glycogen resynthesis, a more anabolic hormonal environment, and recovery. The extent to which these strategies affect training adaptations is unknown.
There is some evidence from clinical populations that certain amino acids (e.g., arginine, histidine, lysine, methionine, ornithine, and phenylalanine) may stimulate the release of growth hormone, insulin, and/or glucocorticoids and thereby promote anabolic processes. However, there is little evidence that supplementation of these amino acids provides ergogenic benefit for athletes.
Branched-chain amino acids and glutamine have been hypothesized to affect central fatigue and exercise-induced immune suppression, but their ergogenic value during prolonged exercise is equivocal at present.
Most studies indicate that creatine supplementation may be an effective and safe means to enhance intermittent high-intensity exercise performance as well as training adaptations. Of the nutrients evaluated, creatine appears to have the greatest ergogenic potential for athletes involved in intense training.
Hydroxymethylbutyrate supplementation has been reported to reduce catabolism and promote greater gains in strength and fat-free mass in untrained individuals initiating training. Limited data are available on the effects of HMB supplementation on training adaptations in athletes.
FURTHER RESEARCH
Over the last few decades researchers have found that amino acids play multiple roles in metabolism. For this reason, researchers and athletes are interested in the effects of amino-acid supplementation on exercise metabolism, exercise performance, and training adaptations. Although significant advances have been made, much remains to be learned about these effects. Researchers should also evaluate the long-term safety of amino-acid supplementation, as well as the potential medical value in the treatment of various diseases.
REFERENCES
Protein
Butterfield G (1991). Amino acids and high protein diets. In Lamb D, Williams M (editors), Perspectives in exercise science and sports medicine, Vol. 4; Ergogenics, enhancement of performance in exercise and sport (pages 87-122). Indianapolis, Indiana: Brown & Benchmark
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Carli G, Bonifazi M, Lodi L et al (1992). Changes in exercise-induced hormone response to branched chain amino acid administration. European Journal of Applied Physiology 64, 272-7
Chandler RM, Byrne HK, Patterson JG et al (1994). Dietary supplements affect the anabolic hormones after weight-training exercise. Journal of Applied Physiology 76, 839-45
Kreider RB (1999). Dietary supplements and the promotion of muscle growth with resistance training. Sports Medicine 27, 97-110
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Lemon PW, Tarnopolsky MA, MacDougall JD et al (1992). Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders. Journal of Applied Physiology 73, 767-75
Lemon PWR (1998). Effects of exercise on dietary protein requirements. International Journal of Sport Nutrition 8, 426-47
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Anabolic Amino Acids
Bucci L, Hickson JF, Pivarnik JM et al (1990). Ornithine ingestion and growth hormone release in bodybuilders. Nutrition Research 10, 239-45
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Jackson D, Grant DB, Clayton B (1968). A simple oral test of growth hormone secretion in children. Lancet 2, 373-5
Kreider RB (1999). Dietary supplements and the promotion of muscle growth with resistance training. Sports Medicine 27, 97-110
Kreider RB, Miriel V, Bertun E (1993). Amino acid supplementation and exercise performance: proposed ergogenic value. Sports Medicine 16, 190-209
Lemon PWR (1991). Protein and amino acid needs of the strength athlete. International Journal of Sport Nutrition. 1, 127-145
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Branched-Chain Amino Acids
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Glutamine
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Rennie MJ (1996). Glutamine metabolism and transport in skeletal muscle and heart and their clinical relevance. Journal of Nutrition 126(4), 1142S-9S
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Rohde T, Asp S, MacLean DA et al (1998). Competitive sustained exercise in humans, lymphokine activated killer cell activity, and glutamine--an intervention study. European Journal of Applied Physiology 78, 448-53
Varnier M, Leese GP, Thompson J et al (1995). Stimulatory effect of glutamine on glycogen accumulation in human skeletal muscle. American Journal of Physiology 269, E309-15
Creatine
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Febbraio MA, Flanagan TR, Snow R et al (1995). Effect of creatine supplementation on intramuscular TCr metabolism and performance during intermittent supramaximal exercise in humans. Acta Physiologica Scandinavica 155, 387-95
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Greenhaff PL, Bodin K, Harris R et al (1993b). The influence of oral creatine supplementation on muscle phosphocreatine resynthesis following intense contraction in man. Journal of Physiology 467, 75P (abstract)
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Greenhaff PL, Casey A, Short A et al (1993a). Influence of oral creatine supplementation on muscle torque during repeated bouts of maximal voluntary exercise in man. Clinical Science 84, 565-71
Greenhaff PL, Constantin-Teodosiu D, Casey A et al (1994b). The effect of oral creatine supplementation on skeletal muscle ATP degradation during repeated bouts of maximal voluntary exercise in man. Journal of Physiology 476, 84P (abstract)
Grindstaff PD, Kreider R, Bishop R et al (1997). Effects of creatine supplementation on repetitive sprint performance and body composition in competitive swimmers. International Journal of Sports Nutrition 7, 330-46
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Kreider R, Ferreira M, Wilson M et al (1998). Effects of creatine supplementation on body composition, strength, and sprint performance. Medicine and Science in Sports and Exercise 30, 73-82
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Peeters BM, Lantz CD, Mayhew JL (1999). Effect of oral creatine monohydrate and creatine phosphate supplementation on maximal strength indices, body composition, and blood pressure. Journal of Strength and Conditioning Research (in press)
Peyrebrune MC, Nevill ME, Donaldson FJ et al (1998). The effects of oral creatine supplementation on performance in single and repeated sprint swimming. Journal of Sports Sciences 16, 271-9
Prevost MC, Nelson AG, Morris GS (1997). Creatine supplementation enhances intermittent work performance. Research Quarterly for Exercise and Sport 68, 233-40
Redondo D, Dowling EA, Graham BL et al (1996). The effect of oral creatine monohydrate supplementation on running velocity. International Journal of Sports Nutrition 6, 213-21
Smith SA, Montain SJ, Matott RP et al (1998). Creatine supplementation and age influence muscle metabolism during exercise. Journal of Applied Physiology 85, 1349-56
Stone MH, Sanborn K, Smith L et al (1999). Effects of in-season (5 weeks) creatine and pyruvate supplementation on anaerobic performance and body composition in American football players. International Journal of Sports Nutrition (in press)
Stout JR, Echerson J, Noonan D et al (1999). The effects of a supplement designed to augment creatine uptake on exercise performance and fat free mass in football players. Nutrition Research 19, 217-25
Terrillion KA, Kolkhorst FW, Dolgener FA et al (1997). The effect of creatine supplementation on two 700-m maximal running bouts. International Journal of Sports Nutrition 7, 138-43
Vanakoski J, Kosunen V, Meririnne E et al (1998). Creatine and caffeine in anaerobic and aerobic exercise, effects on physical performance and pharmacokinetic considerations. International Journal of Clinical Pharmacology and Therapeutics 36, 258-62
Vandenberghe K, Gillis N, Van Leemputte M et al (1996A). Caffeine counteracts the ergogenic action of muscle creatine loading. Journal of Applied Physiology 80, 452-7
Vandenberghe K, Goris M, Van Hecke P et al (1997). Long-term creatine intake is beneficial to muscle performance during resistance training. Journal of Applied Physiology 83, 2055-63
Volek JS, Kraemer WJ, Bush JA et al (1997). Creatine supplementation enhances muscular performance during high-intensity resistance exercise. Journal of the American Dietetic Association 97, 765-70
Williams MH, Kreider RB, Branch JD (1999). Creatine: The Power Supplement. Champaign, Illinois: Human Kinetics
Hydroxymethylbutyrate (HMB)
Almada A, Kreider R, Ferreira M et al (1997). Effects of calcium b -HMB supplementation with or without creatine during training on strength and sprint capacity FASEB Journal 11, A374 (abstract)
Kreider R, Ferreira M, Wilson M et al (1997). Effects of calcium b -HMB supplementation with or without creatine during training on body composition alterations. FASEB Journal 11, A374 (abstract)
Kreider RB (1999). Dietary supplements and the promotion of muscle growth with resistance training. Sports Medicine 27, 97-110
Kreider RB, Ferreira M, Wilson M et al (1999). Effects of calcium b -hydroxy b -methylbutyrate (HMB) supplementation during resistance-training on markers of catabolism, body composition and strength. International Journal of Sports Medicine (in press)
Miller P, Sandberg L, Fuller JC (1997). The effect of intensive training and b -hydroxy-b -methylbutyrate (HMB) on the physiological response to exercise in horses. FASEB Journal 11, A1683 (abstract)
Nair KS, Schwartz RG, Welle S (1992). Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans. American Journal of Physiology 263, E928-34
Nissen S, Faidley TD, Zimmerman DR et al (1994). Colostral milk fat and pig performance are enhanced by feeding the leucine metabolite b -hydroxy-b -methylbutyrate to sows. Journal of Animal Science 72, 2331-7
Nissen S, Panton L, Fuller J et al (1997). Effect of feeding b -hydroxy-b -methylbutyrate (HMB) on body composition and strength of women. FASEB Journal 11, A150 (abstract)
Nissen S, Sharp R, Ray M et al (1996). Effect of leucine metabolite beta-hydroxy-beta-methylbutyrate on muscle metabolism during resistance-exercise training. Journal of Applied Physiology 81, 2095-104
Van Koevering MT, Dolezal HG, Gill DR et al (1994). Effects of b -hydroxy-b -methylbutyrate on performance and carcass quality of feedlot steers. Journal of Animal Science 72, 1927-35
Vukovich MD, Stubbs NB, Bohlken RM et al (1997). The effect of dietary b -hydroxy-b -methylbutyrate (HMB) on strength gains and body composition changes in older adults. FASEB Journal 11, A376 (abstract)
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Published March 1999.
EFFECTS OF PROTEIN AND AMINO-ACID SUPPLEMENTATION ON ATHLETIC PERFORMANCE
Richard B Kreider PhD
Exercise & Sport Nutrition Laboratory, Department of Human Movement Sciences & Education, The University of Memphis, Memphis, Tennessee 38152. Email: kreider.richard=AT=coe.memphis.edu
Sportscience 3(1), sportsci.org/jour/9901/rbk.html, 1999 (5579 words)
Reviewed by Brian Leutholtz PhD, Department of Exercise Science, Physical Education, and Recreation, Old Dominion University, Norfolk, Virginia
BACKGROUND. Protein and amino acids are among the most common nutritional supplements taken by athletes. This review evaluates the theoretical rationale and potential effects on athletic performance of protein, purported anabolic amino acids, branched-chain amino acids, glutamine, creatine, and hydroxymethylbutyrate (HMB). LITERATURE. Two books, 61 research articles, 10 published abstracts, and 19 review articles or book chapters. FINDINGS. Dietary supplementation of protein beyond that necessary to maintain nitrogen balance does not provide additional benefits for athletes. Ingesting carbohydrate with protein prior to or following exercise may reduce catabolism, promote glycogen resynthesis, or promote a more anabolic hormonal environment. Whether employing these strategies during training enhances performance is not yet clear. There is some evidence from clinical studies that certain amino acids (e.g., arginine, histidine, lysine, methionine, ornithine, and phenylalanine) have anabolic effects by stimulating the release of growth hormone, insulin, and/or glucocorticoids, but there is little evidence that supplementation of these amino acids enhances athletic performance. Branched-chain amino acids (leucine, isoleucine, and valine) and glutamine may be involved in exercise-induced central fatigue and immune suppression, but their ergogenic value as supplements is equivocal at present. Most studies indicate that creatine supplementation may be an effective and safe way to enhance performance of intermittent high-intensity exercise and to enhance adaptations to training. Supplementation with hydroxymethylbutyrate appears to reduce catabolism and increase gains in strength and fat-free mass in untrained individuals initiating training; as yet, limited data are available to decide how it affects training adaptations in athletes. CONCLUSIONS. Of the nutrients reviewed, creatine appears to have the greatest ergogenic potential for athletes involved in intense training. FURTHER RESEARCH. All supplements reviewed here need more evaluation for safety and effects on athletic performance. Reprint · Reference List · Help
KEYWORDS: anabolic, BCAA, branched-chain amino acids, creatine, ergogenic, glutamine, HMB, hydroxymethylbutyrate, training
BACKGROUND
Amino acids are the building blocks of protein in the body; as such they are essential for the synthesis of structural proteins, enzymes, and some hormones and neurotransmitters. Amino acids are also involved in numerous metabolic pathways that affect exercise metabolism. Consequently, it has been suggested that athletes involved in intense training require additional protein in the diet or that they should supplement their diet with specific amino acids. I review here the rationale and the evidence for the potential ergogenic effect of short-term supplementation with protein and amino acids and the evidence for the potential anabolic effect of longer-term use when supplementation is combined with training. I deal first with protein, then with the amino acids under the following headings: the potentially anabolic amino acids; the branched-chain amino acids, which have a somewhat different role in metabolism and in their potential effect on performance; glutamine, which is in a class of its own for its effects on the immune system; creatine, an amino acid that is not one of the building blocks of protein but is involved in short-term energy production in muscle; and hydroxymethylbutyrate (HMB), a potentially anabolic metabolite of the amino acid leucine.
LITERATURE
This review is an update rather than an exhaustive account of all published works on the topic. I have cited two books, 60 research articles, 10 published abstracts, and 18 review articles/book chapters from my own database of references. In my database there are a further 97 research articles, 78 abstracts, and 38 review articles/book chapters on the topic. These additional references are reviewed elsewhere (Kreider, 1999; Kreider, 1998; Williams et al., 1999). Download the complete list as a Word 97 file by clicking on this link.
FINDINGS
Protein
A considerable amount of research has evaluated dietary protein needs of athletes. Although there is some debate, most studies indicate that in order to maintain protein balance during intense resistance and/or endurance training, athletes should ingest approximately 1.3 to 1.8 g protein per kg body mass per day (Butterfield, 1991; Lemon, 1998; Kreider et al., 1993; Kreider, 1999). Athletes training at high-altitude may need as much as 2.2 g protein per kg per day in order to maintain protein balance (Butterfield, 1991). This protein intake is about 1.5 to 2 times the recommended dietary allowance (RDA) for the normal adult. In most instances an iso-energetic diet can provide the required protein, but athletes who maintain hypo-energetic diets, do not ingest enough quality protein in their diet, and/or train at altitude may be susceptible to protein malnutrition (Kreider, 1999). In theory, this state could slow tissue growth and/or recovery from training. On the other hand, ingesting more protein than necessary to maintain protein balance during training (e.g., > 1.8 g/kg/d) does not promote greater gains in strength or fat-free mass (Lemon et al., 1992; Tarnopolsky et al., 1992). These findings indicate that athletes typically do not need to supplement their normal diets with protein, provided they ingest enough quality protein to maintain protein balance.
More recently, there has been interest in determining the effects of pre- and post-exercise carbohydrate and protein feedings on hormonal responses to exercise (Cade et al., 1992; Chandler et al., 1994; Roy and Tarnopolsky, 1998; Tarnopolsky et al., 1997; Zawadzki et al., 1992). Ingestion of protein with carbohydrate has been reported to increase insulin and/or growth hormone levels to a greater degree than ingestion of carbohydrate alone (Chandler et al., 1994; Zawadzki et al., 1992). Consequently, ingesting protein and carbohydrate prior to exercise may serve as an anti-catabolic nutritional strategy (Carli et al., 1992). Further, ingesting carbohydrate and protein following exercise may promote a more anabolic hormonal profile, glycogen resynthesis, and/or hasten recovery from intense exercise (Roy and Tarnopolsky, 1998; Roy et al., 1997). Over time these alterations may allow an athlete to tolerate training to a greater degree and/or promote greater training adaptations, but the evidence is not yet clear.
Anabolic Amino Acids
One of the commonly purported benefits of amino acid supplementation is that certain amino acids (e.g., arginine, histidine, lysine, methionine, ornithine, and phenylalanine) may stimulate the release of growth hormone, insulin, and/or glucocorticoids, thereby promoting anabolic processes (Kreider, 1993). There is some clinical evidence that amino acid supplementation may stimulate growth hormone releasing factors and/or growth hormone release (Carlson, et al., 1989;; Garlick and Grant, 1988; Iwasaki et al., 1987; Merimee et al., 1969). For example, intravenous arginine and ornithine infusion have been used clinically for stimulating growth hormone release (Carlson et al., 1989; Iwasaki et al., 1987). In addition, preliminary clinical studies indicated that protein (20 to 60 g); arginine and lysine (1.2 g); and ornithine (70 mg/kg) increased growth hormone and somatomedin concentrations in the blood (Bucci et al., 1990; Jackson et al., 1968; Isidori et al., 1981). However, other researchers have not replicated these findings, particularly in healthy individuals (Lemon, 1991). There is also little evidence that supplementation of these amino acids during training significantly affects body composition, strength, and/or muscle hypertrophy (Kreider, 1999). Consequently, the effects of amino acid supplementation on growth-hormone release and training adaptations are as yet unclear.
Branched-Chain Amino Acids
Researchers have expended a considerable amount of effort on evaluating the effects of supplementation of branched-chain amino acids (BCAAs: leucine, isoleucine, and valine) on physiological and psychological responses to exercise (Blomstrand et al., 1991; Kreider, 1998; Wagenmakers, 1998). There are two primary hypotheses regarding the ergogenic value of supplementation with these amino acids.
First, BCAA supplementation has been reported to decrease exercise-induced protein degradation and/or muscle enzyme release (an indicator of muscle damage) possibly by promoting an anti-catabolic hormonal profile (Carli et al., 1992; Coombes and McNaughton, 1995). Theoretically, BCAA supplementation during intense training may help minimize protein degradation and thereby lead to greater gains in fat-free mass. Although several studies support this hypothesis, additional research is necessary to determine the long-term effects of BCAA supplementation during training on markers of catabolism, body composition, and strength (Kreider, 1998).
Second, the availability of BCAA during exercise has been theorized to contribute to central fatigue (Newsholme et al., 1991). During endurance exercise, BCAAs are taken up by the muscles rather than the liver in order to contribute to oxidative metabolism. The source of BCAAs for muscular oxidative metabolism during exercise is the plasma BCAA pool, which is replenished through the catabolism of whole body proteins during endurance exercise (Davis, 1995; Kreider, 1998; Newsholme et al., 1991). However, the oxidation of BCAAs in the muscle during prolonged exercise may exceed the catabolic capacity to increase BCAA availability, so plasma BCAA concentration may decline during prolonged endurance exercise (Blomstrand et al., 1988; Blomstrand et al., 1991). The decline in plasma BCAAs during endurance exercise can result in an increase in the ratio of free tryptophan to BCAAs. Free tryptophan and BCAAs compete for entry into the brain via the same amino-acid carrier (Newsholme et al., 1991). Therefore, a decrease in BCAAs in the blood facilitates entry of tryptophan into the brain. Moreover, most tryptophan in the blood is bound to albumin, and the proportion of tryptophan bound to albumin is influenced by the availability of long-chained fatty acids (Davis et al., 1992; Newsholme et al., 1991). In endurance exercise free fatty-acid concentration rises, so the amount of tryptophan bound to albumin falls, increasing the concentration of free tryptophan in the blood (Davis, 1995).
Collectively, the decline in plasma BCAAs and increase in free tryptophan during prolonged endurance exercise alters the ratio of free tryptophan to BCAAs and increases the entry of tryptophan into the brain (Newsholme et al., 1991). An increased concentration of tryptophan in the brain promotes the formation of the neurotransmitter 5-hydroxytryptamine (5-HT). 5-HT has been shown to induce sleep, depress motor neuron excitability, influence autonomic and endocrine function, and suppress appetite in animal and human studies. An exercise-induced imbalance in the ratio of free tryptophan to BCAAs has been implicated as a possible cause of acute physiological and psychological fatigue (central fatigue). It has also been hypothesized that chronic elevations in 5-HT concentration, which may occur in athletes maintaining high-volume training, explains some of the reported signs and symptoms of the overtraining syndrome: postural hypotension, anemia, amenorrhea, immunosuppression, appetite suppression, weight loss, depression, and decreased performance (Newsholme et al., 1991; Gastmann and Lehmann, 1998; Kreider, 1998).
A number of studies have recently been conducted to evaluate whether carbohydrate and/or BCAA supplementation affects central fatigue during exercise and/or signs and symptoms of overtraining. Analysis of this literature indicates that carbohydrate and/or BCAA supplementation during exercise can affect the ratio of free tryptophan to BCAA. For example, carbohydrate administration during exercise has been reported to attenuate FFA release and minimize increases in the free tryptophan:BCAA ratio (Davis et al., 1992). In addition, BCAA supplementation has been reported to increase plasma BCAA concentration and minimize and/or prevent increases in the ratio of free tryptophan to BCAAs (Blomstrand et al., 1991). Studies also indicate that BCAA administration with or without carbohydrate prior to and during exercise can affect physiological and psychological responses to exercise (Coombes and McNaughton, 1995; Hefler et al., 1993; Kreider et al., 1992; Kreider and Jackson, 1994).
Nevertheless, the effect of these nutritionally-induced alterations in the free tryptophan to BCAA ratio on physical performance is still not clear. Most studies indicate that BCAA supplementation does not improve single-bout endurance performance, but these studies almost certainly lacked power to delimit small but useful enhancements of performance (Davis, 1995; Gastmann and Lehmann, 1998; Kreider, 1998). Additional research is also necessary to determine the effect of long-term BCAA supplementation on training adaptations and the signs and symptoms of overtraining (Kreider, 1998).
Glutamine
Rennie and colleagues have suggested glutamine supplementation as a strategy to promote muscle growth (Rennie et al., 1994; Rennie, 1996). They based the suggestion on animal and human studies of the effect of glutamine on protein synthesis, cell volume, and glycogen synthesis (Rennie et al., 1994; Varnier et al., 1995; Rennie, 1996; Low et al., 1996). Glutamine is also an important fuel for white blood cells, so reductions in blood glutamine concentration following intense exercise may contribute to immune suppression in overtrained athletes (Parry-Billings et al., 1990a; Parry-Billings et al., 1990b; Parry-Billings et al., 1992; Kargotich et al., 1996; Newsholme and Calder, 1997).
Preliminary studies indicate that supplementation with branched-chain amino acids (4 to 16 g) and/or glutamine (4 to 12 g) can prevent the decline or even increase glutamine concentration during exercise (Kreider, 1998). In theory these changes in glutamine concentration could have beneficial effects on protein synthesis and immune function. However, in the few studies of increased glutamine availability, there was little or no effect on performance or immune status (Rohde et al., 1998; Nieman and Pedersen, 1999). It is also unclear whether long-term supplementation of glutamine affects protein synthesis, body composition, or the incidence of upper respiratory-tract infections during training.
Creatine
Creatine is a naturally occurring amino acid derived from the amino acids glycine, arginine, and methionine (Balsom et al., 1994; Williams et al., 1999). Most creatine is stored in skeletal muscle, primarily as phosphocreatine; the rest is found in the heart, brain, and testes (Balsom et al., 1994; Kreider, 1998). The daily requirement of creatine is approximately 2 to 3 g; half is obtained from the diet, primarily from meat and fish, while the remainder is synthesized (Williams et al., 1999). Creatine supplementation has been proposed as a means to "load" muscle with creatine and phosphocreatine (PCr). In theory, an increased store of creatine or phosphocreatine would improve the ability to produce energy during high intensity exercise as well as improve the speed of recovery from high-intensity exercise.
A number of studies have been conducted to determine the effects of creatine supplementation on muscle concentrations and performance. Creatine supplementation (20 g per day or 0.3 g per kg body mass per day for 4 to 7 days) has been reported to increase intramuscular creatine and phosphocreatine content by 10 to 30% (Casey et al., 1996; Febbraio et al, 1995; Green et al., 1996a; Green et al., 1996b; Greenhaff et al., 1993a; Hultman et al., 1996; Smith et al., 1998b, Vandenberghe et al., 1997). There is also evidence that creatine supplementation enhances the rate of PCr resynthesis following intense exercise (Greenhaff et al., 1993b; Greenhaff et al., 1994a; Greenhaff et al., 1994b). Most studies indicate that short-term creatine supplementation increases total body mass (Hultman et al., 1996; Williams et al., 1999), work performed during multiple sets of maximal effort muscle contractions (Greenhaff et al., 1993a; Volek et al., 1997), and single and/or repetitive sprint capacity (Birch et al., 1994; Grindstaff et al., 1997; Prevost et al., 1997). In addition, long-term creatine supplementation during training has been reported to promote greater gains in strength (Earnest et al., 1995; Peeters et al., 1999; Stone et al., 1999; Vandenberghe et al., 1997), fat-free mass (Kreider et al., 1998; Stone et al., 1999; Stout et al., 1999; Vandenberghe et al., 1997), and sprint performance (Kreider et al., 1998; Peyreburne et al., 1998; Stout et al., 1999). However, it should be noted that not all studies report ergogenic benefit (Burke et al., 1996; Redondo et al., 1996; Terrillion et al., 1997) and that caffeine has been reported to counteract the potential ergogenic value of creatine supplementation (Vanakoski et al., 1998; Vandenburghe et al., 1996). Although more research is needed, creatine supplementation appears to be a safe and effective nutritional strategy to enhance high intensity exercise performance and improve training adaptations (Williams et al., 1999).
Hydroxymethylbutyrate (HMB)
The leucine metabolite hydroxymethylbutyrate (more exactly the calcium salt of b -hydroxy-b -methylbutyric acid) has recently become a popular dietary supplement purported to promote gains in fat-free mass and strength during resistance training (Kreider, 1999). The rationale is that leucine and its metabolite a -ketoisocaproate (KIC) appear to inhibit protein degradation (Nair et al., 1992; Nissen et al., 1996), and this anti-proteolytic effect may be mediated by HMB. Animal studies indicate that approximately 5% of oxidized leucine is converted to HMB via KIC (Nissen et al., 1994; Van Koevering et al., 1994). The addition of HMB to dietary feed improved colostral milk fat and growth of sows (Nissen et al., 1994), tended to improve the carcass quality of steers (Van Koevering et al., 1994), and decreased markers of catabolism during training in horses (Miller et al., 1997). Supplementing with leucine and/or HMB may therefore inhibit protein degradation during periods associated with increased proteolysis, such as resistance training.
Although much of the available literature on HMB supplementation in humans is preliminary in nature, several recently published articles and abstracts support this hypothesis. Leucine infusion appears to decrease protein degradation in humans (Nair et al., 1992). HMB supplementation during 3 to 8 weeks of training has been reported to promote significantly greater gains of fat-free mass and strength in untrained men and women initiating resistance training (Nissen et al., 1996; Nissen et al., 1997; Vukovich et al., 1997). In some instances these gains were associated with signs of significantly less muscle damage (efflux of muscle enzymes and urinary 3-methylhistidine excretion) (Nissen et al., 1996). Although these findings suggest that HMB supplementation during training may enhance training adaptations in untrained individuals initiating training, it is less clear whether HMB supplementation reduces markers of catabolism or promotes greater gains in fat-free mass and strength during resistance training in well-trained athletes. Indeed, there are several reports of no significant effects of HMB supplementation (3 to 6 g per day) in well-trained athletes (Almada et al., 1997; Kreider et al., 1997; Kreider et al., 1999). More research is needed (Kreider, 1999).
CONCLUSIONS
Dietary supplementation of protein beyond that necessary to maintain nitrogen balance does not provide additional ergogenic benefit.
Ingesting carbohydrate/protein prior to exercise may reduce catabolism whereas ingesting carbohydrate/protein following exercise may promote glycogen resynthesis, a more anabolic hormonal environment, and recovery. The extent to which these strategies affect training adaptations is unknown.
There is some evidence from clinical populations that certain amino acids (e.g., arginine, histidine, lysine, methionine, ornithine, and phenylalanine) may stimulate the release of growth hormone, insulin, and/or glucocorticoids and thereby promote anabolic processes. However, there is little evidence that supplementation of these amino acids provides ergogenic benefit for athletes.
Branched-chain amino acids and glutamine have been hypothesized to affect central fatigue and exercise-induced immune suppression, but their ergogenic value during prolonged exercise is equivocal at present.
Most studies indicate that creatine supplementation may be an effective and safe means to enhance intermittent high-intensity exercise performance as well as training adaptations. Of the nutrients evaluated, creatine appears to have the greatest ergogenic potential for athletes involved in intense training.
Hydroxymethylbutyrate supplementation has been reported to reduce catabolism and promote greater gains in strength and fat-free mass in untrained individuals initiating training. Limited data are available on the effects of HMB supplementation on training adaptations in athletes.
FURTHER RESEARCH
Over the last few decades researchers have found that amino acids play multiple roles in metabolism. For this reason, researchers and athletes are interested in the effects of amino-acid supplementation on exercise metabolism, exercise performance, and training adaptations. Although significant advances have been made, much remains to be learned about these effects. Researchers should also evaluate the long-term safety of amino-acid supplementation, as well as the potential medical value in the treatment of various diseases.
REFERENCES
Protein
Butterfield G (1991). Amino acids and high protein diets. In Lamb D, Williams M (editors), Perspectives in exercise science and sports medicine, Vol. 4; Ergogenics, enhancement of performance in exercise and sport (pages 87-122). Indianapolis, Indiana: Brown & Benchmark
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Carli G, Bonifazi M, Lodi L et al (1992). Changes in exercise-induced hormone response to branched chain amino acid administration. European Journal of Applied Physiology 64, 272-7
Chandler RM, Byrne HK, Patterson JG et al (1994). Dietary supplements affect the anabolic hormones after weight-training exercise. Journal of Applied Physiology 76, 839-45
Kreider RB (1999). Dietary supplements and the promotion of muscle growth with resistance training. Sports Medicine 27, 97-110
Kreider RB, Miriel V, Bertun E (1993). Amino acid supplementation and exercise performance: proposed ergogenic value. Sports Medicine 16, 190-209
Lemon PW, Tarnopolsky MA, MacDougall JD et al (1992). Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders. Journal of Applied Physiology 73, 767-75
Lemon PWR (1998). Effects of exercise on dietary protein requirements. International Journal of Sport Nutrition 8, 426-47
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Tarnopolsky MA, Bosman M, Macdonald JR et al (1997). Postexercise protein-carbohydrate and carbohydrate supplements increase muscle glycogen in men and women. Journal of Applied Physiology 83, 1877-83
Zawadzki KM, Yaspelkis BB, Ivy JL (1992). Carbohydrate-protein complex increases the rate of muscle glycogen storage after exercise. Journal of Applied Physiology 72, 1854-9
Anabolic Amino Acids
Bucci L, Hickson JF, Pivarnik JM et al (1990). Ornithine ingestion and growth hormone release in bodybuilders. Nutrition Research 10, 239-45
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Jackson D, Grant DB, Clayton B (1968). A simple oral test of growth hormone secretion in children. Lancet 2, 373-5
Kreider RB (1999). Dietary supplements and the promotion of muscle growth with resistance training. Sports Medicine 27, 97-110
Kreider RB, Miriel V, Bertun E (1993). Amino acid supplementation and exercise performance: proposed ergogenic value. Sports Medicine 16, 190-209
Lemon PWR (1991). Protein and amino acid needs of the strength athlete. International Journal of Sport Nutrition. 1, 127-145
Merimee TJ, Rabinowitz D, Fineberg SE (1969). Arginine-initiated release of human growth hormone. New England Journal of Medicine 280, 1434-8
Branched-Chain Amino Acids
Blomstrand E, Celsing F, Newshome EA (1988). Changes in plasma concentrations of aromatic and branch-chain amino acids during sustained exercise in man and their possible role in fatigue. Acta Physiologica Scandinavica 133, 115-21
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Bloomstrand E, Hassmen P, Newsholme E (1991). Effect of branch-chain amino acid supplementation on mental performance. Acta Physiologica Scandinavica 143, 225-6
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Coombes J, McNaughton L (1995). The effects of branched chain amino acid supplementation on indicators of muscle damage after prolonged strenuous exercise. Medicine and Science in Sports and Exercise 27, S149 (abstract)
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Hefler SK, Wildman L, Gaesser GA et al (1993). Branched-chain amino acid (BCAA) supplementation improves endurance performance in competitive cyclists. Medicine and Science in Sports and Exercise 25, S24 (abstract)
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Glutamine
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Parry-Billings M, Blomstrand E, Leighton B et al (1990). Does endurance exercise impair glutamine metabolism? Canadian Journal of Sport Science 13, 13P (abstract)
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Parry-Billings M, Budgett R, Koutedakis K et al (1992). Plasma amino acid concentrations in the overtraining syndrome: Possible effects on the immune system. Medicine and Science in Sports and Exercise 24, 1353-8
Rennie MJ (1996). Glutamine metabolism and transport in skeletal muscle and heart and their clinical relevance. Journal of Nutrition 126(4), 1142S-9S
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Rohde T, Asp S, MacLean DA et al (1998). Competitive sustained exercise in humans, lymphokine activated killer cell activity, and glutamine--an intervention study. European Journal of Applied Physiology 78, 448-53
Varnier M, Leese GP, Thompson J et al (1995). Stimulatory effect of glutamine on glycogen accumulation in human skeletal muscle. American Journal of Physiology 269, E309-15
Creatine
Balsom P, Söderlund K, Ekblom B (1994). Creatine in humans with special reference to creatine supplementation. Sports Medicine 18, 268-80
Birch R, Nobel D, Greenhaff P (1994). The influence of dietary creatine supplementation on performance during repeated bouts of maximal isokinetic cycling in man. European Journal of Applied Physiology 69, 268-76
Burke L, Pyne LD, Telford R (1996). Effect of oral creatine supplementation on single-effort sprint performance in elite swimmers. International Journal of Sports Nutrition 6, 222-33
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Hydroxymethylbutyrate (HMB)
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