L-arginine and L-lysine are amino acids that coexist in many protein-rich foods, such as tuna, watercress, nuts and soy. Your body uses amino acids to manufacture structural proteins, hormones, enzymes, antibodies and other important molecules. However, amino acids also participate in a number of other crucial processes, such as energy production, neurotransmitter synthesis and cell signaling. L-arginine and L-lysine have multiple functions, some of which may be better served when they are consumed together.
Although infants must acquire L-arginine from their diets to meet their physiologic requirements, healthy adults can synthesize enough for their daily needs. Nutritionist Elson Haas states that one of L-arginine's most important functions is to help your body eliminate ammonia, which is the byproduct of protein metabolism. L-arginine can also be converted to citrulline, releasing a highly reactive and metabolically important gas called nitric oxide. When taken in daily doses of at least 1 g each, L-arginine and L-lysine support the production of growth hormone, a benefit that is useful for bodybuilders and other athletes.
Unlike L-arginine, L-lysine cannot be synthesized in your body, so it must be acquired from your diet. According to the University of Maryland Medical Center, L-lysine is needed for the manufacture of collagen, the most abundant structural protein in your body, and it is essential for the synthesis of carnitine, which is required for converting fatty acids into energy in your mitochondria. In daily doses of 1 to 2 g, L-lysine is believed to help prevent outbreaks of facial herpes. When used for herpes, lysine should not be combined with L-arginine, as L-arginine supplementation may actually contribute to outbreaks.
A study published in the December 2002 issue of "Biomedicine and Pharmacotherapy" demonstrated that combining L-arginine and L-lysine supplements stimulated the activity of human osteoblasts, which are the cells responsible for increasing your bone mass. L-arginine's effects on nitric oxide-induced cell signaling and L-lysine's contributions to collagen synthesis triggered increased bone production when the two amino acids were taken together.
L-arginine and L-lysine are both needed for optimal health. Their combined effects are desirable in some situations, such as increasing growth hormone levels or strengthening your bones. In other cases---recurrent herpes, for example---supplementation with only L-lysine is indicated. Daily doses of up to 2 g of each amino acid are usually well tolerated. Even higher doses---3 to 6 grams---are sometimes used, but high doses can cause nausea and diarrhea in some individuals. Ask your doctor if amino acid supplements are appropriate for you.