Scientists at UC San Diego and other institutions gave middle-aged, sedentary male mice a purified form of cacao's primary nutritional ingredient, epicatechin, twice daily for 15 days. (Epicatechin, a flavanol, has been shown to have multiple health benefits in humans, according to the study's authors.) The mice then ran on a treadmill. A control group of mice received only water and also was forced to exercise. The rodents given the supplement were able to cover about 50 percent more distance than the control animals. The findings were published last week in the Journal of Physiology.
Researchers then biopsied the leg and heart muscles of the mice (try not to think about what that means). The muscles of all of the animals that had been given epicatechin contained new capillaries, as well as indications that their cells were making new mitochondria, structures in cells that produce energy. The more functioning mitochondria a muscle contains, the healthier and more fatigue-resistant it is.
"It seems likely that muscle cells contain specific receptors for epicatechin," Dr. Francisco Villarreal, a professor of medicine at UC San Diego and one of the authors of the study, told the Times. Epicatechin binds to the receptors and "induces an integrated response that includes structural and metabolic changes in skeletal and cardiac muscles resulting in greater endurance capacity," the study concluded.
Milk chocolate doesn't contain epicatechin because the flavanol is destroyed by processing. Although it is still present in dark chocolate, eating more dark chocolate isn't better and may in fact be worse. "A very small amount is probably enough," Dr. Villarreal told the Times, estimating that 5 grams of dark chocolate daily -- about half of one square of a typical chocolate bar -- is probably a reasonable amount if your goal is to intensify the effects of a workout. More than that could lessen or even undo any benefits by overloading the muscles' receptors.
In other words, eating a bowl of Ben & Jerry's Chocolate Fudge Brownie before a workout will not build stronger muscles. Ain't that a bitch.