This year's hot new performance enhancer was quercetin, a flavonoid found in some fruits and teas. In studies of mice, it boosted stamina and increased the development of mitochondria, the energy-processing centers in cells. What's more, as a natural supplement, it's legal for human athletes. Alas, tests don't confirm that it works in us. In a number of studies, people given quercetin drinks did no better on athletic tests than people given a placebo. Want to win the gold? Looks like you'll have to train. It's old-fashioned, but it works.