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“In short-term clinical trials looking at changes in blood pressure and blood vessel dilation, cocoa flavanols looked promising, and reductions in cholesterol and inflammatory markers were also seen in some trials,” says JoAnn Manson, chief of preventive medicine at Brigham and Women’s Hospital and professor of medicine at Harvard Medical School.


“So we were interested in launching a large-scale randomized trial that would look at heart attacks, strokes, cardiovascular deaths, and similar clinical events.”

In 2014, Manson and Howard Sesso—her colleague at Brigham and Harvard—started the COcoa Supplement and Multivitamin Outcomes Study (COSMOS).

COSMOS randomly assigned 21,442 men 60 or older and women 65 or older to take a cocoa flavanol supplement (500 milligrams a day) or a placebo for roughly 3½ years.

(The study was an academic-private-public partnership funded by grants from Mars Edge, a research segment of Mars, Inc., and the National Institutes of Health.)

The primary results: Compared to the placebo group, the cocoa flavanol group did not have a statistically significant drop in total cardiovascular events: heart attacks, strokes, cardiovascular deaths, coronary bypasses, angioplasties, unstable angina, and carotid or peripheral artery surgery.

End of story? Not to Manson and her colleagues.

Signals that warrant more studies 

“We saw several signals that pointed to cardiovascular protection,” she says. For example, cardiovascular deaths were 27 percent lower in the flavanol group. (Put another way, 110 people would have to take cocoa flavanols for 3½ years to prevent one cardiovascular death.)

“And when we looked at the participants who were taking the flavanol or the placebo pills regularly, total cardiovascular events were 15 percent lower, and cardiovascular deaths 39 percent lower, in the flavanol group,” notes Manson.

Her takeaway: “We’re not recommending that everyone start taking a cocoa flavanol supplement. But these promising signals for cardiovascular protection warrant further trials.”

flavanols ad
Despite this ad for the flavanols in CocoaVia’s Cardio Health supplement, more trials are needed to know if flavanols help protect the heart.

In the meantime, says Manson, “people could consider a heart healthy diet that’s rich in berries, grapes, tea, and other foods that are high in flavanols.”

What’s not on that list? Chocolate.

Chocolate vs. supplements

“This was not a chocolate trial,” notes Manson. “You would need about 4,000 calories a day of milk chocolate—or maybe 600 calories a day of dark chocolate—to get the 500 milligrams of cocoa flavanols we used in the trial.”

What’s more, “the flavanol content of chocolate is not reliable,” she adds. “It varies tremendously depending on harvesting and processing.”

“Chocolate is a wonderful treat, but it is not a health food.”