What is L-Citrulline? L-Citrulline is an amino acid that plays an important role in nitric oxide metabolism and regulation. In the body, L-Citrulline is converted to L-Arginine to support L-Arginine and nitric oxide levels. Increased production of nitric oxide promotes vascular dilation which improves oxygen and blood circulation throughout the body. Research has shown that L-Citrulline raises plasma L-Arginine, thereby allowing higher protein synthesis and protein content in muscle during exercise.