Although the thought of chocolate as a health food has captured public attention, not much research on the issue has been done, said a team from the University Hospital of Cologne.
Their report covered exactly 10 studies on cocoa with a total of 173 participants and five tea studies with 343 participants.
The benefits are believed to come from compounds known as polyphenols (or flavonoids), explained Dr. Dirk Taubert, senior lecturer in pharmacology and toxicology at Cologne and lead author of the report.
He leavened his support of chocolate with a bit of caution.
"Based on our analysis, regular consumption of polyphenol-rich cocoa products like dark chocolate may be considered a part of a blood pressure-lowering diet, provided there is no total gain in calorie intake," Taubert said. "However, in the studies we reviewed, the blood pressure results occurred with cocoa doses above the habitual intake and were observed only in the setting of short-term interventions."
In other words, for the average chocolate nibbler, the jury is still out on the sweet's health effects, Taubert said. "To date, it is not known whether long-term intake of small habitual amounts of cocoa, such as a small bar or piece of chocolate per day, may also cause significant blood pressure effects," he said.
The cocoa studies lasted an average of two weeks, with four out of five trials reporting a reduction in both systolic blood pressure (the top number in a blood pressure reading, when the heart contracts) and diastolic number, when the heart relaxes.
The average reduction was 4 to 5 millimeters of mercury (mm/Hg) in systolic pressure and 2 to 3 millimeters in diastolic pressure -- enough to reduce the risk of stroke by 20 percent and of coronary heart disease by 10 percent.
No such reduction in blood pressure was noted in any of the tea trials, which lasted an average of four weeks.
Tea and cocoa contain different kinds of polyphenols -- flavan-3-ols in tea, procyanids in cocoa, the researchers said.
"We do not know exactly which are the active blood pressure-lowering ingredients in cocoa," Taubert said. "There is evidence that the cocoa polyphenols are responsible, but there are several hundreds of phenols in cocoa."
Whichever are responsible, studies of cell cultures in his laboratory have also suggested that polyphenols can stop the oxidation of beta-amyloid protein, the process that leads to formation of plaque in the brains of Alzheimer's disease patients, said Chang Y. Lee, chairman of the department of food science and technology at Cornell University in Ithaca, N.Y.
In addition, "reports from many laboratories clearly show that the ingestion of flavonoid-rich foods enhance circulation," Lee said.
Much remains to be learned, Lee added. "Many people have been talking about different kinds of flavonoids," he said. But we do not know exactly how much of these compounds are absorbed in the body and also when they are absorbed, how much is distributed to different sites."
Lee said he can "happily recommend" the occasional cup of cocoa. "But I am cautious about people taking chocolate milk, because it is high in sugar and high in fat," he said. "Dark chocolate may be all right, but I do not recommend cocoa preparations that contain high sugar."
Drug treatment is the basis of blood pressure control, Taubert said, and it should always be accompanied by lifestyle measures such as exercise and proper diet. "Rationally applied, cocoa products may be part of such an antihypertensive diet," he said.
More information
There's much more on flavonoids and cocoa is available at the Linus Pauling Institute.
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