L-Glutamine, an amino acid, is the most abundant amino acid in the body and the preferred fuel for the cells of the gastrointestinal tract. It 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. There 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. Glutamine may also play a role in supporting the immune system.