Gretchen Reynolds in The New York Times:
Adding a little dark chocolate to a training diet may effortlessly improve endurance performance, according to a new study of sports nutrition. The findings provide ammunition both for athletes looking for an edge and those hoping for an excuse to indulge. For some time, dark chocolate has been touted as a relatively healthy treat, with studies showing that small amounts may have benefits for the heart and brain. Most of this research has focused on the role of a substance called epicatechin, a plant nutrient found in cocoa. Dark chocolate is generally rich in epicatechin, though levels vary, depending on how the sweet was produced. Levels of epicatechin tend to be much lower in milk chocolate, which contains little cocoa, and white chocolate contains little or none of the nutrient.
Epicatechin is known to prompt cells that line blood vessels to release extra nitric oxide, a substance that has multiple effects in the body. Nitric oxide slightly increases vasodilation, or a widening of the veins and arteries, improving blood flow and cardiac function. It also gooses muscle cells to take in more blood sugar, providing them with more energy, and it enhances the passage of oxygen into cells. Because of its many physiological effects, each of which can aid physical performance, athletes long have looked for ways to increase the amount of nitric oxide in their bloodstreams. Some down supplement pills, although the benefits of nitric oxide supplements are unproven. Others swallow beetroot juice, a beverage that contains a hefty dose of nitrates, which then break down in the body into nitric oxide and other substances. There are questions, however, about the safety of nitrates and also, as anyone who has tried beetroot juice will tell you, the palatability of a beverage that tastes distinctly like liquid dirt.