The organic compound citrulline is an α-amino acid was first isolated in 1914. Its name is derived from the Latin word for watermelon – citrullus. A cup of watermelon is about 154 grams or 290 mg of Citrulline.
The key effect of L-citrulline is that in the body it converted to L-arginine, thus it works as a donor for the L-arginine/nitric oxide pathway. Supplementation with the NO-boosting substances, L-citrulline and L-arginine, along with antioxidants, has been demonstrated to delay endothelial senescence despite a high-cholesterol diet. This resistance to endothelial weakening through Nitric Oxide production can help the prevention and treatment of atherosclerosis associated with menopause, diabetes and other aging related diseases.
There are two main forms of citrulline being used in products today: L-citrulline and citrulline malate. L-citrulline is the free-form version of the amino acid citrulline and citrulline malate is the amino acid citrulline attached to a molecule of malic acid (malate).
Synergy of L-citrulline/L-arginine combination
L-arginine supplementation improves nitric oxide-mediated vasodilation and endothelial function. However, L-arginine is also a precursor for the synthesis of proteins, urea, creatine, vasopressin and agmatine. This fact makes L-arginine highly consumable bio-substance. In contrast, L-citrulline escapes intestinal or liver metabolism (while L-arginine cannot do the same) and enters the kidneys where it is rapidly converted into L-arginine. Studies have shown that supplementation with L-citrulline/L-arginine combination is effective in boosting plasma L-arginine levels enhancing nitric oxide bioavailability. Placebo-controlled cross-over research analysing how to increase the plasma L-arginine concentration and NO production confirmed that oral L-citrulline supplementation is much more effectively than a pure oral L-arginine intake. Evidence has shown that oral L-citrulline supplementation can reverse the endothelial dysfunction associated with sickle cell disease and pulmonary hypertension as well as that L-citrullline effectively activates the L-arginine and NO pathway.
L-citrulline vs Erectile Dysfunction
Additional benefits of oral L-citrulline supplementation is that it improves erection hardness in men with mild erectile dysfunction. A study (2011) found that a 1.5 grams of Citrulline increased the hardness factor of a significant number of men.