l-Ornithine is one of the products of the action of the enzyme arginase on l-arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excess nitrogen. Ornithine is recycled and, in a manner, is a catalyst. First, ammonia is converted into carbamoyl phosphate . Ornithine is converted into a urea derivative at the ? (terminal) nitrogen by carbamoyl phosphate synthetase. Another nitrogen is added from aspartate, producing the denitrogenated fumarate, and the resulting arginine (a guanidinium compound) 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 not an amino acid coded for by DNA, that is, not proteinogenic. However, in mammalian non-hepatic tissues, the main use of the urea cycle is in arginine biosynthesis, so, as an intermediate in metabolic processes, ornithine is quite important.
l-Ornithine supplementation attenuated fatigue in subjects in a placebo-controlled study using a cycle ergometer. The results suggested that l-ornithine has an antifatigue effect in increasing the efficiency of energy consumption and promoting the excretion of ammonia.
Amino acid supplements, including l-ornithine, are frequently marketed to bodybuilders and weightlifters with claims for increasing levels of human growth hormone (HGH), muscle mass and strength. A clinical study reported that L-ornithine at 2 g/d did not increase HGH. A review on the topic concluded "The use of specific amino acids to stimulate GH release by athletes is not recommended."
^Sugino T, Shirai T, Kajimoto Y, Kajimoto O (November 2008). "L-ornithine supplementation attenuates physical fatigue in healthy volunteers by modulating lipid and amino acid metabolism". Nutrition Research. 28 (11): 738-43. doi:10.1016/j.nutres.2008.08.008. PMID19083482.
^Demura S, Yamada T, Yamaji S, Komatsu M, Morishita K (October 2010). "The effect of L-ornithine hydrochloride ingestion on performance during incremental exhaustive ergometer bicycle exercise and ammonia metabolism during and after exercise". European Journal of Clinical Nutrition. 64 (10): 1166-71. doi:10.1038/ejcn.2010.149. PMID20717126.
^Fogelholm GM, Näveri HK, Kiilavuori KT, Härkönen MH (September 1993). "Low-dose amino acid supplementation: no effects on serum human growth hormone and insulin in male weightlifters". International Journal of Sport Nutrition. 3 (3): 290-7. doi:10.1123/ijsn.3.3.290. PMID8220394.