Ornithine is a nonessential amino acid and is manufactured by the body. The amino acid, arginine, is metabolized during urea production and is required by the body as it acts as a precursor of citrulline, proline and glutamic acid.

Ornithine plays an important role in the urea cycle and is the precursor of the amino acids citrulline, glutamic acid, and proline. Another primary role of ornithine is being an intermediate in arginine biosynthesis, although this is due to its participation in the urea cycle (responsible for the production of urea). Ornithine is not directly incorporated into proteins and enzymes and does not have a codon in the genetic code.

Animal research has suggested that ornithine, along with arginine, may promote muscle-building activity in the body by increasing levels of growth-promoting (anabolic) hormones such as insulin and growth hormone. However, most human research does not support these claims at reasonable intake levels. One study that did demonstrate increased growth hormone with oral ornithine used very high amounts (an average of 13 grams per day) and reported many gastrointestinal side effects. One controlled study reported greater increases in lean body mass and strength after five weeks of intensive strength training in athletes taking 1 gram per day each of arginine and ornithine, compared with a group doing the exercise but taking a placebo. These findings require independent confirmation.

In clinical studies on people hospitalized for surgery, generalized infections, cancer, trauma, or burns, supplementation with ornithine alpha-ketoglutarate (OKG) has been reported to produce several beneficial effects. A double-blind trial evaluated the effects of OKG supplementation in elderly people recovering from acute illnesses. Those who took 10 grams of OKG per day for two months had marked improvement in appetite, weight gain, and quality of life compared with those taking the placebo. They also had shorter recovery periods and required fewer home visits by physicians and nurses and needed fewer medications.

Ornithine aspartate has been shown to be beneficial in people with brain abnormalities (hepatic encephalopathy) due to liver cirrhosis. In a double-blind trial, people with cirrhosis and hepatic encephalopathy received either 18 grams per day of L-ornithine-L-aspartate or a placebo for two weeks. Those taking the ornithine had significant improvements in liver function and blood tests compared with those taking the placebo.

Preliminary and controlled studies of people with severe burns showed that supplementation with 10–30 grams of ornithine alpha-ketoglutarate per day significantly improved wound healing and decreased the length of hospital stays.

Since ornithine is produced by the body, a deficiency of this nonessential amino acid is unlikely, though depletion can occur during growth or pregnancy, and after severe trauma or malnutrition.

No side effects have been reported with the use of ornithine, except for gastrointestinal distress with intakes over 10 grams per day. The presence of arginine is needed to produce ornithine in the body, so higher levels of this amino acid should increase ornithine production.

Ornithine is predominantly found in meat, fish, dairy, and eggs. Western diets typically provide 5 grams per day. The body also produces ornithine.