Ornithine releases growth hormone and helps maintain the health of the immune system and the liver as it is a key component in the urea cycle. It is a central part of the urea cycle, as one of the products of the action of the enzyme arginase on L-arginine, which creates urea. Ornithine is recycled and, in a manner, is a catalyst. First, ammonia is converted into carbamoyl phosphate and ornithine is then converted into a urea derivative at the nitrogen terminal by carbamoyl phosphate. Another nitrogen is added from aspartate, which produces denitrogenated fumarate, and the resulting arginine is hydrolysed back to ornithine, producing urea. The nitrogens of urea come from the ammonia and aspartate, and the nitrogen in ornithine remains intact. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency (OTCD). OTCD is a genetic disorder resulting in a mutated and ineffective form of the enzyme ornithine transcarbamylase. It is a common urea cycle disorder, but a rare metabolic disorder, occurring in one out of every 80,000 births.
Amino acids are also used by the body to build proteins. Proteins are essential for the formation of cells and are also involved in the biochemical structure of hormones, enzymes, neurotransmitters and antibodies. L-ornithine is frequently used by bodybuilders and weightlifters under the belief that it will increase levels of human growth hormone (HGH). However, clinical study has shown has since disproved this theory.