L-Glutamine can be converted to glutamic acid, which is both a precursor to the important inhibitory neurotransmitter GABA (gamma amino butyric acid) and an excitatory neurotransmitter in its own right. Athletes on high-protein diets will appreciate the fact that glutamine transports ammonia, the toxic metabolic by-product of protein breakdown, to the liver, where it is converted into less toxic urea and then excreted by the kidneys. Finally, glutamine can convert into alanine, an amino acid that the liver converts into glucose, supplying additional fuel to muscles during a prolonged workout. L-Glutamine is energy sources utilized by rapidly proliferating cells. It is a major fuel source for the brain and together with its relatives the stimulant neurotransmitter. Glutamine is a supplement that is used in weightlifting and bodybuilding, as well as those who suffer for muscular cramps or pain - particularly elderly people. The main use of glutamine within the diet of either groups is as a means of replenishing the body's stores of amino acids that have been used during exercise or everyday activities.

Very good post on L-glutamine.Also worth looking at supplementing with:

1) Branched chain amino acids (BCAA's). The release of these by the muscle gives the body the general signal to stop the conversion of alanine into glucose (gluconeogenesis) . Ideally you would not want gluconeogenesis to occur for an extended period of time because this is the process of breaking down muscle tissue and unless you immediately supplement with a high bioavailable protein and carbs post workout you lose muscle. Lost muscle slows your metabolism. Of course supplementing with l-glutamine will aid against this. Back to BCAA's there are three: l-leucine, l-isoleucine and l-valine. When working out intensely BCAA's work to spare muscle tissue by becoming a fuel source. So with L-glutamine and BCAA's, having a high quality protein diet that supply both can help with supplying fuel to working muscles.

2) Beta Alanine. When you exercise at high intensity levels, your blood pH levels fall (your blood becomes more acidic). When this happens you feel fatigued, get that burning sensation and begin to feel like quitting. Beta alanine is another amino acid that helps to increase your blood's pH making it less acidic and thereby allowing you to workout a little longer and make things a little easier.

3) Glycogen (carb) loading. Good if your an athlete or workout in high intensity zones (like doing intervals, high set and high repetition weight training etc.). Would probably want to skip though if you're seeking to lose bodyfat.