Citrulline is a ubiquitous amino acid in mammals, is strongly related to arginine, and has scope beyond that normally associated with an amino acid. Your kidneys change citrulline into another amino acid called arginine and a chemical called nitric oxide. Citrulline has important roles in the body, but unlike some amino acids, it is not used to build proteins. Citrulline is found in several foods, especially watermelon. Most foods have not been analyzed specifically for their citrulline content (Wikipedia).