During a long bout of exercise such as a marathon, your muscles use BCAAs for fuel. This lowers the amount of circulating BCAAs, which means that more tryptophan can get into the brain, as it doesn't have to compete with as many BCAAs to do so. The result: more serotonin is produced, and fatigue sets in.
- Mike Reynolds
Bodybuilding.com - The Effects Of Glutamine Supplementation On Athletic Performance! - Chris Goulet - http://www.bodybuilding.com/fun...