Eating dark chocolate helps blood vessels function more effectively, researchers have claimed.
The health claims of chocolate are not accepted by all
Writing in the Journal of the American College of Nutrition, University of California researchers found a chemical in chocolate helped vessels expand.
Narrowed blood vessels increase the risk of heart attacks, strokes and diseases linked to poor circulation.
But UK heart experts said chocolate was more likely to be part of the problem, rather than part of the solution.
The research looked at flavonoids, a group of chemical compounds with antioxidant properties, which are derived from a variety of plants.
Previous research has suggested they could have beneficial effects on the body's cardiovascular system.
Eleven people were given 46 grams (1.6 ounces) of dark, flavonoid-rich chocolate every day for two weeks, while 10 others received dark chocolate with low-flavonoid content.
At the end of the trial, the researchers used ultrasound to measure how well blood vessels are able to relax if blood flow increases - called flow-mediated dilation.
They examined the brachial artery in the arm. How well this artery dilates indicates how coronary arteries are behaving.
In the group eating flavonoid-rich chocolate, blood vessel dilation increased by a tenth, while it fell by as much in the other group.
The researchers also found that concentrations of the cocoa flavonoid epicatechin, thought to be beneficial for blood vessel function, was higher in the high-flavonoid group.
Mary Engler, professor of physiological nursing, who led the study, said: "Improvements in endothelial function - the ability of the artery to dilate - are indicative of improved vascular health and a lower risk for heart disease.
"Arteries that are able to dilate more have increased blood flow, and this is especially important for the heart."
She added: "It is likely that the elevated blood levels of epicatechin triggered the release of active substances that vasodilate, or increase, blood flow in the artery. Better blood flow is good for your heart."
Belinda Linden, head of Medical Information at the British Heart Foundation, said: "Whilst this research portrays chocolate as part of the solution to heart disease, the reality is it is more often part of the problem.
"There has been a great deal of research covering the benefits of flavonoids found in cocoa beans.
"Previous studies suggest that the constituents of cocoa beans can influence the dilation or widening of the arteries and help improve blood vessels. However, no study has yet investigated long-term clinical effects."
She added: "With the rising tide of obesity, we must be clear on the significant drawbacks of eating too much chocolate: it is very high in calories (about 500 calories per 100g) and it contains an average 30% total fat, of which at least 18% is saturated fat.
"We are not saying never eat chocolate - everyone enjoys a treat from time to time - but there are certainly much better ways of improving your blood vessels, such as eating a varied diet, including at least five portions of fruit and vegetables a day."
The study was funded by the UCSF School of Nursing. Chocolate for the study was provided by the American Cocoa Research Institute.