Smaller studies have concluded that eating dark chocolate aids in weight loss, improves blood pressure, cholesterol, and the body's use of insulin, among other health factors.
The study is sponsored by the National Heart, Lung and Blood Institute and Mars Inc., maker of M&M's and Snickers bars.
The candy company has patented a process to extract flavanols from cocoa in high concentration and put them in capsules.
Mars Inc. currently sells cocoa extract capsules, but with a less active ingredient than those that will be tested in the study.
During the study 18,000 men and women across the Unites States will be either given dummy pills or two capsules daily of cocoa flavanols for four years.
Neither the participants nor the study leaders will know who is taking what during the study.
"You're not going to get these protective flavanols in most of the candy on the market. Cocoa flavanols are often destroyed by the processing," said Dr. JoAnn Manson, preventive medicine chief at Harvard-affiliated Brigham and Women's Hospital in Boston.
Manson will lead the study with Howard Sesso at Brigham and others at Fred Hutchinson Cancer Research Center in Seattle.