Mary Engler of the University of California at San Francisco and her colleagues studied 21 people for two weeks in a randomized study in which some subjects received dark chocolate rich in compounds known as flavonoids while others ate placebo chocolate with a low flavonoid content. The researchers used ultrasound to measure blood flow through the brachial artery (the main artery in the arm) and the vessel's ability to expand, or dilate. After two weeks, the arteries of the flavonoid-rich chocolate eaters dilated by about 13 percent more than they had at the beginning of the study period.
The scientists also collected blood samples from the subjects and found that levels of the flavonoid epicatechin, which comes from the cocoa plant, rose significantly among those people who consumed flavonoid-rich chocolate. "It is likely that the elevated blood levels of epicatechin triggered the release of active substances that vasodilate, or increase, blood flow in the artery," Engler explains. "Better blood flow is good for your heart." Because standard processing methods can destroy flavonoids, not all chocolate is created equal when it comes to potentially beneficial effects.