A recent study in Tuesdays New York Times science section demonstrated epicatechin, an antioxidant found in the cocoa plant, can vasodilate our arteries, particularly to our heart. This is significant because the consequence is increased blood flow to our heart and thus more oxygen delivery. This study revealed that the highest amount of epicatechin is in dark chocolate, which also will have the least amount of sugar.
The mechanism of this action is related to increased nitrous oxide production. A study was performed on 8 male recreational cyclists who, after eating the dark chocolate every day for two weeks, showed increased distances attained after the two-mile time trial, even though all their other diet and routine remained the same. This trial was published in the International Society of Sports Medicine.
This reminds me of a very dear friend and colleague in Japan who is approaching 100 years old, still practicing medicine and every day eats a small cube of dark chocolate. He also continues to run every day for about two miles.