A study in the Italian journal, Medicina Dello Sport, investigated the effect of taking supplemental branched-chain amino acids (leucine, isoleucine, and valine) on bodybuilding progress. The study involved 31 male bodybuilders between ages 18 and 34, all of whom were drug free.
All these athletes had at least two years of training experience, and were divided into two groups: 16 subjects ingested a placebo and 15 others ingested 0.2g per kilogram (2.2 pounds) of a branched-chain amino acid supplement 30 minutes before training and 30 minutes after the workout.
The results showed that while bodyweight increased in both groups, those taking the BCAA supplement showed greater weight gains. Analysis of the composition of this weight increase in the BCAA group showed an increase in muscle mass in both the legs and arms, with no changes in the trunk area of the body. In contrast those taking the placebo showed no muscle mass gains in these areas. The BCAA group also showed strength gains in both the squat and bench press, while those ingesting the placebo got stronger only in the squat exercise.
P.S. About 50% of the protein in whey protein are BCAA's.