Branched-chain amino acid supplementation lowers perceived exertion but does not affect performance in untrained males

Journal Title (Medline/Pubmed accepted abbreviation): J. Strength Cond. Res.
Year: 2011
Volume: 25
Page numbers: 539-544

Summary of Background and Research Design

Background:Serotonin (5-hydroxytryptophan) is a hormone that may contribute to fatigue during endurance events. BCAAs (including valine, leucine, and isoleucine), due their similar structure, may be able to compete with tryptophan for entry into the brain, which would reduce the amount of serotonin produced and therefore delay fatigue (i.e., the central fatigue hypothesis). In addition, BCAA, expecially leucine, are also a potential energy source during exercise, mainly under conditions of low carbohydrate availability.

Respiratory exchange ratio (RER)= (carbon dioxide exhaled)/(oxygen inhaled). From this ratio one can estimate the amount of carbohydrates vs. amino acids vs. fat is being oxidized for energy because the efficiency of energy production from these macromolecules differs.

Research question: How does a BCAA supplement compare to an isocaloric carbohydrate supplement in regard to performance and perceived exertion during endurance exercise?

Subjects:9 untrained males, age 21.6 ± 3.2 y

Experimental design:randomized, single-blinded (participants were blinded), placebo-controlled, repeated measures design

Treatments:All beverages contained salts, artificial sweetener, and lemon flavor to mask the identity of the treatment. The BCAA beverage contained 4.8 g isoleucine, 12.2 g leucine, and 7.3 g valine (100 kcal) per serving. The carbohydrate beverage was Gatorade and was also 100 kcal per serving. The placebo was a calorie-free, artificially-sweetened beverage. One serving of the beverage was consumed 5 min before exercise and one at 60 min so that 200 kcal of BCAA or carbohydrates was consumed per trial.

Protocol:Endurance training was accomplished on a cycle ergometer. Subjects warmed up for 5 min and then resistance was adjusted to equal 55% of their previously determined VO2 peak. Subjects cycled for 90 min. RER was monitored every 15 min by analyzing expired air. Rate of perceived exertion (RPE) was determined every 15 min using a 6-20 point Borg scale. To assess performance, after the steady-state ride, participants cycled as far (fast) as possible in a 15 min time trial. Blood samples were acquired immediately pre- and post-exercise and analyzed for glucose and BCAA concentrations. At least 8 wks separated the 3 trials.

Summary of research findings:
  • Performance during the time trial was significantly greater after the carbohydrate beverage than the placebo. There were no significant differences between the BCAA beverage and the placebo or between the BCAA beverage and the carbohydrate beverage.
  • RPE was lower at 75 and 90 min with the BCAA beverage than either of the other beverages. The difference reached statistical significance for the placebo beverage but not the carbohydrate beverage.
  • RER was higher after BCAA ingestion than the other two beverages, indicating that the BCAAs were being oxidized for fuel.
  • Blood BCAA concentrations were lower after exercise with the carbohydrate and placebo supplements. However, individual BCAA concentrations were 4.4-8.7 times higher after exercise with the BCAA supplement.

Interpretation of findings/Key practice applications:

Replacing carbohydrates with BCAAs did not improve performance relative to either the isocaloric carbohydrate or noncaloric placebo, though it did reduce perceived exertion after 75 min of exercise. Previous research has shown that a combination of carbohydrates and protein/amino acids before an endurance event may be beneficial in that it reduces the rate of muscle protein breakdown which prevents soreness. The combination of protein and carbohydrates is further supported, albeit indirectly, by this experiment.


This study used untrained subjects whose bodies have not adapted to aerobic exercise. Trained athletes may utilize substrates differently. In addition, the doses of BCAA (9.6 g isoleucine, 24.4 g leucine, and 14.6 g valine per day) used were very large compared with typical dietary intakes or amounts available in commercial dietary supplements and may be unrealistic for athletes to achieve.
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