Don't know if it's true. I just found this quote and footnote. That's my story and I'm sticking to it. Bring on the dark chocolate.
Exercisers can improve their performance by as much as 50 percent if they eat dark chocolate regularly.At least they can if they're mice. Researchers fed mice the flavonol epicatechin, a key ingredient in dark chocolate, then put them through two weeks of treadmill tests and observed "an integrated response that includes structural and metabolic changes in skeletal and cardiac muscles resulting in greater endurance capacity."