L-Citrulline is one of the three dietary amino acids in the urea cycle, alongside L-arginine and L-Ornithine. Taking L-Citrulline increases plasma levels of ornithine and arginine and improves the ammonia recycling process and nitric oxide metabolism. Consequently, it is used in areas where nitric oxide is relevant, namely erectile dysfunction caused by high blood pressure, athletic performance, and cardiovascular health. There are very few foods that have notable amounts of citrulline.
Limited research suggests that it results in reduced fatigue and improved endurance for both aerobic and anaerobic exercise. There isn't enough evidence to support the claim that L-citrulline supplementation improves power output during exercise. More research for erectile dysfunction and blood pressure is needed, but a small amount of research is supportive of a beneficial effect.
It's not known to have notable side-effects, though more research is needed to confirm its long-term safety when taken in high doses. Unlike L-arginine and L-ornithine, very high doses don't seem to result in gastrointestinal upset.
A small amount of research suggests that taking citrulline will lead to higher and more consistent arginine levels than taking arginine. Citrulline is very readily converted to arginine as needed, and it is also better absorbed than arginine, which not only makes it a better source of arginine for the body but can mean a lower rate of gastrointestinal upset than arginine when taken in high doses.
Citrulline bound to malate, an organic salt of malic acid, an intermediate in the citric acid cycle. It is the most researched form of citrulline, and there is speculation about an independent role of malate in producing performance benefits, but there's insufficient research to compare citrulline malate to L-citrulline directly. Citrulline malate can be taken in the dose used in studies, but it's important to keep in mind that 1.76 g of citrulline malate is needed to about 1 g of citrulline.