L-arginine and L-citrulline are two amino acids that have potential therapeutic uses. A relationship between the two exists in that L-citrulline is converted into L-arginine once inside the body. Therefore, L-citrulline is sometimes supplemented to increase levels of L-arginine. Being a precursor to L-arginine, L-citrulline can potentially be used as treatment for the same conditions as L-arginine, in addition to being used as a sports supplement.
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L-arginine works primarily through increasing nitric oxide levels in the blood. Higher nitric oxide levels relax and dilate blood vessels, thus increasing blood circulation to various organs, including the heart, skin and sexual organs. L-arginine is a conditionally essential amino acid, meaning the body can normally manufacture a sufficient supply, although times of stress or injury can deplete the body's supply. Many foods, including meat, dairy products, fish, poultry, nuts and chocolate, are good sources of L-arginine. Doses of 2 to 8 grams per day are typically used, although higher amounts can be used for certain conditions, according to New York University Langone Medical Center.
Benefits of L-arginine include its ability to treat and prevent a number of conditions. Most commonly, L-arginine is used for its blood-vessel-dilating effects, which can be effective in improving a number of heart-related conditions. High blood pressure, chest pain and coronary heart disease have been treated with L-arginine, according to MedlinePlus. L-arginine is also used to treat male infertility and erectile dysfunction. Benefits of taking L-arginine for healthy people include its potential to improve circulation and stimulate the release of growth hormone.
L-citrulline is a nonessential amino acid, which means the body can synthesize it from other nutrients. Glutamine is an amino acid that can be used to manufacture L-citrulline. Although L-citrulline deficiencies are unlikely to occur, citrulline supplements have been promoted for some uses, most notably enhancing sports performance. Dosage for citrulline typically ranges between 6 and 18 grams per day, and it is often sold in the form of citrulline malate.
NYU Langone Medical Center notes that supplement companies often promote L-citrulline as an aerobic complement (a supplement that improves aerobic performance) to creatine -- a supplement used to enhance anaerobic exercise performance. There is a lack of scientific evidence that L-citrulline can enhance aerobic exercise, however. Still, L-citrulline may be useful in some capacities with regard to exercise. Examine.com has given L-citrulline a "C" rating, meaning single-blind, double-blind or multiple cohort studies support L-citrulline for increasing protein synthesis (muscle building), reducing muscle soreness and fatigue, enhancing immunity and increasing nitric oxide and growth hormone levels.