Another useful supplement is L-arginine, an amino acid that is the primary precursor of nitric oxide (NO)—one of several biochemical pathways that are powered up by exercise. In addition to its protective effects on the mitochondria, NO is a very powerful vasodilator. It relaxes the arteries, enhances vascular health, improves blood flow, and even boosts sexual function. The suggested dose of L-arginine is 1,000–2,000 mg a day.