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The Practical Scientist – Part V
by: Jamie Hale

In general, athletes have high intakes of branched-chain amino acids (BCAAs) because of their high energy and protein intakes. Many athletes have even higher intakes due to BCAA supplementation. How does supplementation affect performance? If you ingest a sufficient amount of BCAAs from food and protein supplements is it necessary to supplement with them? Michael Gleeson answered those questions at the 4th Amino Acid Workshop. Key points from: 4th Amino Acid Workshop Interrelationship between Physical Activity and Branched-Chain Amino Acids – Michael Gleeson Higher total energy intakes and therefore higher protein intakes. The BCAA’s, leucine, isoleucine, and valine represent 3 of the 30 amino acids that are used in the formation of proteins. Thus, on average, the BCAA content of food proteins is about 15% of the total amino acid content.

Deliberate consumption of high-protein diets and protein supplements. Debate has always raged over how much dietary protein is required for optimal athletic performance, partly because muscle contains a large proportion of the protein in a human body (about 40%). Muscle also accounts for 25% to 35% of all protein turnover in the body. Both the structural proteins that make up the myofibrils and the proteins that act as enzymes within a muscle cell can change as an adaptation to exercise training. Consumption of protein hydrolysates or mixtures of essential amino acids during and after exercise. Protein hydrolysates are produced from purified protein sources (e.g. casein) by heating with acid or, more usually, by addition of proteolytic enzymes followed by purification procedures. Such hydrolysates contain peptides of which up to about 40% may be dipeptides and tripeptides. Consumption of amino acids as dipeptides and tripeptides results in faster absorption into the blood stream compared to the ingestion of whole proteins or single amino acids. This is a desirable characteristic for athletes who wish to maximize amino acid delivery to muscles, although whether this has a practical effect of improving muscle protein synthesis, accretion of muscle mass, or improved recovery from exercise, has not yet been established. In the late 1970’s, BCAAs were suggested to be the third fuel for skeletal muscle after carbohydrate and fat. BCAAs are sometimes supplied to athletes in energy drinks to provide extra fuel. Claims have also been made that BCAA supplementation can reduce net protein breakdown in muscle during exercise, reduce fatigue, and enhance performance via effects on the brain.

BCAAs as a fuel for exercise

Although early studies suggested that BCAAs could act as a fuel during exercise in addition to carbohydrate and fat, it has been shown that the activities of the enzymes involved in the oxidation of BCAAs are too low to allow a major contribution of BCAAs to energy expenditure. Detailed studies with a C-labeled BCAA (C-leucine) showed that the oxidation of BCAAs only increase 2 to 3-fold during exercise, whereas the oxidation of carbohydrate and fat increases 10 to 20-fold. Also, carbohydrate ingestion during exercise can prevent the increase in BCAA oxidation. BCAAs, therefore, do not seem to play a major role as a fuel during exercise, and from this point of view, the supplementation of BCAAs during exercise is unnecessary.

BCAAs and protein turnover

The claims that BCAAs reduce protein breakdown were initially based on early in vitro studies, which showed that adding BCAAs to an incubation or perfusion medium stimulated tissue protein synthesis and inhibited protein degradation. Several in vivo studies in health individuals failed to confirm the positive affect on protein balance that had been observed in vitro. However, several studies in recent years have inferred an anabolic effect of leucine or the BCAAs on muscle protein breakdown and a stimulatory effect on muscle protein synthesis (see the article by Dwight Matthews in this supplement for a review). Very recent work suggests the leucine itself, not its metabolites, acts as a signal to stimulate protein synthesis. ”

BCAAs and Central Fatigue

“The ‘Central Fatigue Hypothesis’ was proposed in 1987 as an important mechanism contributing to the development of fatigue during prolonged exercise. This hypothesis predicts that during exercise, FFAs are mobilized from adipose tissue and are transported via the blood to the muscles to serve as fuel. Because the rate of mobilization is greater than the rate of uptake by the muscle, the blood FFA concentration increases. Both FFAs and the amino acid tryptophan bind to albumin and compete for the same binding sites. Tryptophan is prevented from binding to albumin by the increasing FFA concentration, and, therefore, the free tryptophan concentration and the fTRP; BCAA ratio in the blood rises. Experimental studies in humans have confirmed that these events occur…..The Central Fatigue Hypothesis also predicts that ingestion of BCAAs will raise the plasma BCAA concentration, and hence, reduce transport of fTRP into the brain. Subsequent reduced formation of serotonin may alleviate sensation of fatigue and in turn improve endurance exercise performance.

The effect of BCAA ingestion on physical performance was investigated for the first time in a field test by Blomstrand et al. A total of 193 male subjects were studied during a marathon in Stockholm. The subjects were randomly divided into an experimental group receiving BCAAs in plain water and a placebo group receiving flavored water. The subjects also had free access to carbohydrate-containing drinks. No difference was observed in the marathon time of the two groups. However, when the original subject group was divided into faster and slower runners, a significant reduction in marathon time was observed in subjects given BCAAs in the slower runners only. This study has since been criticized for its design and statistical analysis. A study that examined the effect of BCAA ingestion during exercise in the heat (ambient temperature of 34°C) has provided some further evidence in support of these early findings. A 14% increase in the capacity to perform relatively low intensity exercise (40% VO2max) was reported after BCAA supplementation compared with placebo. No difference in peripheral markers of fatigue was reported between the BCAAs and placebo treatments and the BCAA supplementation (which began 1 h before the start of exercise) resulted in a 2 to 3-fold reduction in the plasma ratio of fTRP to BCAAs. Indeed, the majority of studies using various exercise and treatment designs and several forms of the administration of BCAAs (infusion, oral, and with and without carbohydrates), have failed to find a performance-enhancing effect. Van Hall et al. studied time-trial performance in trained cyclists consuming carbohydrate during exercise with and without BCAAs. A high and a low dose of BCAAs were given, but no differences were seen in time-trial performance. If the Central Fatigue Hypothesis is correct and the ingestion of BCAAs reduces the exercise-induced increase of brain fTRP uptake and thereby delays fatigue, the opposite must also be true; that is, ingestion of tryptophan before exercise should reduce the time to exhaustion. A few studies have included supplemental tryptophan in human subjects before or during exercise and from these studies the conclusion must be drawn that tryptophan has no effects on exercise performance. The effect of chronic administration of BCAAs on exercise performance has also been examined. After two weeks of BCAA supplementation (16 g/d), performance of a 40-km cycling time trial in temperate ambient conditions was improved by 12% compared to placebo. However, the data from this study are still not published as a full paper, precluding any definitive conclusions on those results. Acute intakes of BCAA supplements of about 10–30 g/d seem to be without ill effect. However, intake of individual amino acids has no added nutritional value compared with the intake of proteins containing these amino acids. Despite the lack of strong evidence for the efficacy of BCAA supplements, athletes continue to use them. However, normal food alternatives are available and are almost certainly cheaper. For example, a typical BCAA supplement sold in tablet form contains 100 mg valine, 50 mg isoleucine, and 100 mg leucine. A chicken breast (100 g) contains 470 mg valine, 375 g isoleucine, and 656 mg leucine, the equivalent of about 7 BCAA tablets. One quarter of a cup of peanuts (60 g) contains even more BCAA and is equivalent to 11 tablets.”

My thoughts

If you are consuming a sufficient quantity of protein there is no need for BCAA supplementation (from a physiological standpoint). It is hard to get this point across to athletes who have been supplementing with BCAAs for the past 5-6 years (or longer). Dietary Supplementation seems to offer a psychological benefit to some athletes (because that’s what they have always done and many don’t like change). If this is the case BCAAs may be beneficial. Read the recommendations for BCAA supplementation offered by six different advisors in my new book Knowledge and Nonsense: the science of nutrition and exercise. The advisors include Dan Moore, Martin Berkhan, Layne Norton, Alan Aragon, Bryan Haycock and Justin Harris. The answers given by the advisors are diverse and detailed.


Gleeson M. 4th Amino Acid Workshop Interrelationship between Physical Activity and Branched-Chain Amino Acids. The American Society for Nutritional Sciences J. Nutr. 135: 1591S-1595S, June 2005.

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