Glutamine.

Glutamine is the most abundant amino acid (building block of protein) in the bloodstream. It is considered a "conditionally essential amino acid" because it can be manufactured in the body, but under extreme physical stress the demand for glutamine exceeds the body's ability to make it. Most glutamine in the body is stored in muscles followed by the lungs, where much of the glutamine is manufactured. Glutamine is important for removing excess ammonia (a common waste product in the body).

When the body is subjected to strenuous exercise, glutamine levels in muscles drop. In hard training, such as multiple forms of exercise per day, for example lifting and running; it is prudent to supplement glutamine in your diet.