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 diet to meet their body's requirements, healthy adults can produce enough for their daily needs. 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. Bodybuilders and other athletes seeking to increase their muscle mass often combine L-arginine and L-lysine in an effort to increase growth hormone levels. However, a December 2005 review article published in the "Journal of the International Society of Sports Nutrition" found there in no research evidence of increased growth hormone levels when taking L-arginine and L-lysine.
Unlike L-arginine, L-lysine cannot be produced in your body, so it must be acquired from your diet. L-lysine is needed for the manufacture of collagen, the most abundant structural protein in your body. It is also needed for the production of carnitine, which is required for converting fatty acids into energy in your cells. A February 2001 review article in the "American Journal of Health-System Pharmacy" reported that in daily doses of 1 to 2 g, there is some evidence L-lysine may help prevent outbreaks of cold sores. When used for this purpose, L-lysine should not be combined with L-arginine, as L-arginine supplementation may 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, the cells responsible for increasing your bone mass. In this laboratory experiment, L-arginine’s effects on nitric oxide-induced cell signaling and L-lysine’s contributions to collagen synthesis triggered increased bone production in the test tube when both amino acids were applied to the osteoblasts. However, there have been no studies in people to see whether taking supplemental L-arginine and L-lysine actually improves bone health.
L-arginine and L-lysine are both needed for optimal health. Their combined effects are desirable in some situations, such as potentially increasing growth hormone levels or strengthening your bones. In other cases -- recurrent cold sores, 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 people. Ask your doctor if amino acid supplements are safe and appropriate for you.