The effects of phenylalanine on exercise-induced fat oxidation: a preliminary, double-blind, placebo-controlled, crossover trial

BACKGROUND: When combined with exercise, dietary amino acid (AA) supplementation is an effective method for accelerating fat mobilization. However, the effects of single AAs combined with exercise on fat oxidation remains unclear. We hypothesized that consumption of a specific amino acid, L- phenylalanine, may result in the secretion of glucagon, and when combined with exercise may promote fat oxidation. METHODS: Six healthy, active male volunteers were randomized in a crossover study to ingest either phenylalanine (3 g/dose) or placebo. Thirty minutes after ingestion each subject performed workload trials on a cycle ergometer for 1 h at 50% of maximal oxygen consumption. RESULTS: Oral intake of phenylalanine caused a significant increase in the concentrations of plasma glycerol and glucagon during exercise. The respiratory exchange ratio was also decreased significantly following ingestion of phenylalanine. CONCLUSION: These results suggested that pre-exercise supplementation of phenylalanine may stimulate whole body fat oxidation. No serious or study-related adverse events were observed. TRIAL REGISTRATION: UMIN000027502 Registered 26 May 2017. Restrospectively registered.