Arginine is an amino acid. The enzyme nitric oxide synthase (NOS) uses it to make nitric oxide (NO), and the enzyme arginase uses it to make urea and ornithine in a process that also disposes of excess ammonia.
Stress, illness, trauma, premature birth, hypertension, type 2 diabetes, and other health conditions may increase arginase production, and thus increase ornithine levels and decrease arginine and NO levels. This can lead to hypoxia, vascular and neurological complications, and increases in inflammation and reactive oxygen species.
Because the body cannot always make enough arginine to compensate for an increase in arginase, arginine is considered a conditionally essential amino acid — meaning that, under certain conditions, you need to get some from your diet. Fortunately, nuts, seeds, dairy products, and meat (particularly white meat) are rich in arginine.
Although arginine is marketed for athletic performance, its supplementation in healthy adults has unreliable effects on NO production. This may be due to arginine having poor bioavailability. The amino acid citrulline is more bioavailable and is converted into arginine in the kidneys; oral citrulline is better than oral arginine at increasing blood levels of arginine.