Where it comes from: Nitric Oxide (NO) is a gas that’s naturally produced in the body; it's used to communicate between cells. “To make nitric oxide, enzymes in the body break down the amino acid, arginine,” explains registered dietitian and American Dietetic Association spokesperson Jim White. Nitric Oxide supplements actually include arginine—not nitric oxide. Arginine is naturally found in foods such as spinach, sesame seeds, crab, shrimp and white meat turkey. What it’ll do for you: Nitric Oxide’s main job is to deliver messages between the body’s cells. It also plays a key role in controlling the circulation of blood and regulating activities of the brain, lungs, liver, kidneys, stomach and other organs. But from a muscle-building prospective, NO affects the release of hormones and adrenaline. It’s also said to speed growth and recovery time as well as increase blood flow, thus delivering more nutrients to muscles, helping them grow. Many athletes take NO supplements because they believe they make them workout harder and for longer—even though there’s no real evidence supporting the theory. However, a 2010 study supports NO use for older men. A researcher at the University of California at Los Angeles examined 16 male cyclists ages 50 to 73. The men who were given powdered supplements (containing arginine and antioxidants) showed a 16.7 percent increase in their anaerobic threshold—the point at which lactic acid starts to accumulate in the muscles—after three weeks. The men given the placebo did not see any increase in their anaerobic thresholds. Suggested intake: “Clear dosing guidelines have not been established,” says White. In the UCLA study mentioned above, the powder (Niteworks, made by Herbalife International) contained 5.2 grams of L-arginine and L-citrulline, 300 milligrams of L-taurine, 500 milligrams of vitamin C, 400 international units (IU) of vitamin E, 400 micrograms of folic acid, 10 milligrams of alpha lipoic acid, and 50 milligrams of lemon balm extract. “Powder is usually mixed with a liquid and then these liquids are absorbed by the body faster and more efficiently than capsules, tablets or pills,” White says. Associated risks/scrutiny: “With any amino acid-containing product, overdose is a possibility,” warns White. Too much arginine can lead to diarrhea, weakness and nausea. Consult your doctor before taking this—and any other—supplement. Find all your favorite supplements at the Muscle & Fitness Store.