Leucine supplementation does not enhance acute strength or running performance but affects serum amino acid concentration. 
This study described the effect of leucine supplementation on serum amino acid concentration during two different exercise sessions in competitive male power athletes.  The subjects performed a strength exercise session (SES; n = 16; 26 +/- 4 years) or a maximal anaerobic running exercise session (MARE; n = 12; 27 +/- 5 years) until exhaustion twice at a 7-day interval.  The randomized subjects consumed drinks containing leucine (100 mg x kg/body weight before and during SES or 200 mg x kg/body weight before MARE) or placebo.  Blood specimens taken 10 min before (B) and after (A) the sessions were analyzed for serum amino acids.  In SES the concentration of leucine was distinctly higher in the leucine supplemented group than in the placebo group in both B (p < 0.001) and A (p < 0.001) samples.  The leucine concentration decreased in placebo but not in the leucine supplemented group following the exercise session.  Isoleucine (p = 0.017) and valine (p = 0.006) concentration decreased more in the leucine supplemented group than in placebo in A samples.  In MARE the concentration of leucine was higher in the leucine supplemented group than in placebo in both B (p < 0.001) and A (p < 0.001) samples and increased (p < 0.001) in the supplemented group following the session.  Isoleucine (p = 0.020) and valine (p = 0.006) concentration decreased in the supplemented group in A samples.  There were no differences in a counter movement jump after SES or in the running performance in MARE between the leucine supplemented group and placebo.  These findings indicate that consuming leucine before or before and during exercise sessions results in changes in blood amino acid concentration.  However, the supplementation does not affect an acute physical performance.