Arginine is an amino acid that increases your body's production of nitric oxide, which is thought to increase blood flow to the muscles for a greater exchange of nutrients. It's a popular addition to many supplements thought to increase performance. "Even though there's a tiny bit of supportive research for arginine's increasing performance, it's not overwhelmingly positive," says Chad M. Kerksick, ., assistant professor of exercise physiology at the University of Oklahoma. He took part in a study at Baylor University, in Texas, that found a small increase in bench-press strength with arginine. But a 2006 study published in the Journal of the American Medical Association found that taking a daily supplement with 9 grams of arginine may cause heart attack or even death in men with known—and unknown—heart disease. That study was forced to end early.