So far, only two commercially available chocolate products - Dove dark chocolate bars and M&M Baking Bits - are known to contain high levels of flavanol, the substance linked to heart health, researchers said.

Other products are untested or lack high flavanol levels, such as milk chocolate bars.

Flavanols are naturally occurring compounds found in many plants or plant-based foods such as vegetables, fruit, red wine and cocoa. They are thought to have an effect on vascular dilation, or a relaxing of the muscles around blood vessels, which helps keep blood flowing through the vessels.

Research also suggests flavanols enhance nitric oxide, which causes arteries to dilate and increases blood flow, keeping potentially dangerous deposits from adhering to artery walls.

Flavanols may have an aspirin-like effect on platelets, reducing the blood-clotting linked to heart attacks.

Marguerite Engler, professor and vice-chairwoman of the Department of Physiological Nursing at the University of California at San Francisco, was an investigator in a study that compared Dove dark chocolate with another dark chocolate bar and with aspirin.

The study, presented at the American Heart Association meeting this week in Chicago, was independently funded.

In the study, 21 people ate 46 grams - an average-sized chocolate bar - of either chocolate each day for two weeks.

The researchers found that the subjects who ate the flavanol-rich Dove dark chocolate showed blood-vessel dilation two hours after eating the candy, as measured by an ultrasound of a main artery.

"It's a great alternative, but people still need to be aware there are calories in chocolate," Engler said in an interview. "People should realize that you still should be eating healthy and exercising too."

Engler also said that "the response was more robust" when people took a low dose of aspirin instead of chocolate.

Dove dark chocolate bars and M&M Baking Bits are made by the privately held Mars company.

(Agencies November 22, 2002)