Citrulline is an amino-acid almost exclusively released by the small intestine after its synthesis from glutamine by the OTC in enterocytes. Citrulline from the small intestine is released into the portal vena and, because it does not enter hepatocytes, it reaches the systemic circulation to be metabolized in arginine by kidneys. By this way, is an important source of endogenous ARG. Moreover, evidence suggests that circulating citrulline could have a direct action on the regulation of muscle protein synthesis. Therefore, the administration of citrulline might be an interesting nutritional strategy to preserve or restore muscle mass and function. Muscle mass is a determinant of respiratory function and muscle weakness explains for a large part morbidity and mortality in patients suffering from Chronic Obstructive Pulmonary Disease (COPD).