For the most part we can obtain the nutrients we need to through a well balanced daily diet, however there are some unique circumstances in which the nutrition we need to achieve optimal performance can only be harnessed through supplementation. BCAA's (Branch Chain Amino Acids) are one example of peri workout nutrition (before, during and after working out) that may only be obtained through supplementation.
BCAA's are comprised of the three amino acids Leucine, Iso-leucine and Valine. Most products sell BCAA's in a 2:1:1 Ratio with leucine being the most abundant.
BCAA's have been researched extensively and have time and again proven their effectiveness, recently Shimomura et al (2004) proved that BCAA supplementation reduced muscular breakdown during your workout and greatly increasing muscular repair and recovery afterwards. Extrapolated this means that BCAA's can help in maintaining muscle mass whilst trying to lose weight and also aid in creating more muscle mass when that is your primary goal.This makes BCAA's a vital part of an complete supplementation stack.
For the best results take BCAA's before during and immediately after your workout.
Yoshiharu Shimomura, Taro Murakami, Naoya Naka Masaru Nagasaki, and Robert A. Harris(2004),Exercise Promotes BCAA Catabolism: Effects of BCAA Supplementation on Skeletal Muscle during Exercise, J. Nutr. June 1, 2004 vol. 134 no. 6 1583S-1587S