Citrulline is a nonessential amino add made in the liver during the urea cycle (important for eliminating ammonia, a toxic poison), Citrulline may be the next super supplement because it promotes nitric oxide production (important for blood flow control in the body) and stimulates
protein synthesis. It may be particularly valuable as a supplement for older adults who often suffer from malnutrition and muscle wasting. French researchers in a study on malnourished rats- found that citrulline supplementation promoted muscle protein synthesis.
In older adults, amino acids often accumulate in the spleen (blood storage organ), which limits their availability for protein synthesis throughout the body. Citrulline isn't processed by the liver, so its nitrogen can be used to make amino acids that are critical for muscle protein synthesis. It's not known whether these results apply to healthy bodybuilders.