L-Ornithine is a nonprotein amino acid which means that it does not occur in proteins. The body uses it in the biosynthesis of L-arginine, L-proline and polyamines. The liver and immune system require ornithine to function properly. It assists in ammonia detoxification. L-Ornithine may promote muscle-building activity in the body by increasing levels of anabolic (growth-promoting) hormones such as insulin and growth hormone. L-ornithine is also said to increase the metabolism of fat and may therefore be significant in weight regulation and loss.