The study, published by the American Heart Association, joins a growing body of research that show compounds found in chocolate called flavonoids can help the blood vessels work more smoothly, perhaps reducing the risk of heart disease.
"Previous studies suggest flavonoid-rich foods, including fruits, vegetables, tea, red wine and chocolate, might offer cardiovascular benefits, but this is one of the first clinical trials to look specifically at dark chocolate's effect on lowering blood pressure among people with hypertension," said Jeffrey Blumberg of Tufts University in Boston, who led the study.
"This study is not about eating more chocolate," Blumberg added. "It suggests that cocoa flavonoids appear to have benefits on vascular function and glucose sensitivity."
Scientists are far from being able to make specific recommendations for patients based on their research on chocolate, and nutritionists have urged people to be cautious because chocolate is high in fat, sugar and calories.
Blumberg and colleagues at the University of L'Aquila in Italy studied 10 men and 10 women with high blood pressure.
For 15 days, half ate a daily 3.5 ounce (100 gram) bar of specially formulated, flavonoid-rich dark chocolate, while the other half ate the same amount of white chocolate.
Then each group "crossed over" and ate the other chocolate.
"White chocolate, which has no flavonoids, was the perfect control food because it contains all the other ingredients and calories found in dark chocolate," Blumberg said.
"It's important to note that the dark chocolate we used had a high level of flavonoids, giving it a slightly bittersweet taste. Most Americans eat milk chocolate, which has a low amount of these compounds."
Writing in the journal Hypertension, Blumberg's team said when the volunteers ate the special dark chocolate, they had a 12 mm Hg decrease in systolic blood pressure (the top number in a blood pressure reading) and a 9 mm Hg decrease in diastolic blood pressure (the bottom number) on average.
Blood pressure did not change when the volunteers ate white chocolate.
"This is not only a statistically significant effect, but it's also a clinically meaningful decline," Blumberg said. "This is the kind of reduction in blood pressure often found with other healthful dietary interventions."
Eating dark chocolate also seemed to improve how the body used insulin, and reduced low density lipoprotein (LDL) or "bad" cholesterol by about 10 percent on average.
"The findings do not suggest that people with high blood pressure should eat lots of dark chocolate in lieu of other important blood pressure-reduction methods, such as medication and exercise," Blumberg said. "Rather, we are identifying specific flavonoids that can have a benefit on blood pressure and insulin sensitivity."
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