Citrulline is a non-essential amino acid which the body produces from other nutrients under the right conditions. Together with ornithine and arginine, citrulline plays a particularly important role in what is called the urea cycle, in the liver. In the course of this cycle, the toxic ammonium ion is converted into urea. Any disturbance to this cycle - which is essential for eliminating toxic metabolites of nitrogen – can result in a lethal build-up of proteins such as ammonia in the blood. Citrulline or arginine supplements are often recommended to treat such problems. Citrulline is also a precursor of the arginine produced during this cycle.

Studies have shown that citrulline supplementation:

• increases levels of arginine more effectively than supplementation with arginine itself and improves nitrogen balance. Citrulline is not taken up by the liver but passes freely to the kidneys where it is metabolized into arginine;
• appears to have a beneficial effect on sickle cell anaemia;
• has a vasodilatory effect and increases nitric oxide production;
• decreases vascular smooth muscle cell proliferation and retards the progression of atherosclerotic lesions;
• increases the body’s metabolic rate and aerobic performance and reduces muscle fatigue;
• stimulates the immune system;
• speeds up recovery from fatigue.

French researchers have recently shown that citrulline modulates muscle protein metabolism in old, undernourished rats. Preliminary studies suggest that it may directly stimulate protein synthesis.