"Athletes also rely heavily on nutritional supplements, which are legal. “They’re 98.5% hype,” says Conrad Earnest, an exercise physiologist at the University of Bath, UK. But one supplement that does work for some athletes is creatine, which contributes to the synthesis of the energy carrier molecule ATP during exercise. Earnest estimates that athletes taking creatine could see their performance improve by as much as 8%. Another effective supplement is beetroot juice. Researchers at the University of Essex have found that the nitrate present in the juice increases nitric oxide levels in the body, allowing muscles to use oxygen more efficiently."