Citruline is synthesized in the body from ornithine by the addition of carbon dioxide and ammonia, and is a precursor of arginine. It is found primarily in the liver and is a major component of the urea cycle. Therapeutically it is used for the detoxification of ammonia, a byproduct of protein breakdown, and in the treatment of fatigue. It is also thought to stimulate the immune system. Onions and garlic contain an abundance of citruline.