"This is the longest clinical trial to date to show improvement in blood vessel function from consuming flavonoid-rich dark chocolate daily over an extended period of time," says lead author Mary Engler, PhD RN, professor of physiological nursing in the UCSF School of Nursing. "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." The study appears in the June issue of the Journal of the American College of Nutrition.
Previous clinical studies have shown the beneficial effects of chocolate on the function of blood vessel endothelium [the inner lining of blood vessels] after either a single dose or several doses of chocolate over a few days.
Flavonoids, a group of chemical compounds with antioxidant properties, are derived from a variety of plants. They have been shown to promote several beneficial effects in the cardiovascular system, including decreasing oxidation of LDL cholesterol (a harmful process that allows cholesterol to accumulate in blood vessels); inhibiting aggregation of blood platelets (which contributes to the risk of blood clots that produce stroke and heart attack); and decreasing the body's inflammatory immune responses (which contribute to atherosclerosis).
In the randomized, double-blind, placebo-controlled study, eleven people received 46 grams (1.6 ounces) of dark, flavonoid-rich chocolate every day for two weeks, while ten others received dark chocolate with low-flavonoid content. At the end of the two-week trial, Engler and her team recorded the ability of the principal artery in the arm, the brachial artery, to expand. The brachial artery's dilation measurements correlate well with those of the coronary arteries that supply the heart.
The team measured the artery's "flow-mediated dilation" using ultrasound to obtain the brachial artery's diameter immediately after deflating a blood pressure cuff that had been inflated for five minutes on study participants' forearms. "This measurement actually gives us a "videoclip" of an individual's vascular health, and can be helpful in determining whether one is at risk for heart disease," says Engler. The induced increase in blood flow after the cuff is deflated causes release of many dilator substances, such as nitric oxide and prostanoids, which relax, the artery. Flow-mediated dilation is expressed as the percentage maximum change in vessel diameter from baseline.
In the high-flavonoid group, flow-mediated dilation increased from an initial 10.2 percent at the beginning of the study to 11.5 percent at the end of the study, while in the low-flavonoid group dilation decreased from 10.7 percent at the beginning of the study to 9.74 percent at the end of the study. The mean increase in flow-mediated dilation between the two groups showed a statistically significant difference, says Engler.
"Improvements in endothelial function [the ability of the artery to dilate] are indicative of improved vascular health and a lower risk for heart disease," Engler says. "Arteries that are able to dilate more have increased blood flow, and this is especially important for the heart."
Engler and her group also found that concentrations of the cocoa flavonoid epicatechin soared in blood samples taken from the group that received the high-flavonoid chocolate, rising from a baseline of 25.6 nmol/L to 204.4 nmol/L. In the group that received the low-flavonoid chocolate, concentrations of epicatechin decreased slightly, from a baseline of 17.9 nmol/L to 17.5 nmol/L.
The authors also found that the participants did not have increased blood cholesterol levels after these daily snacks of chocolate for two weeks.
The study was funded by the UCSF School of Nursing. Chocolate for the study was provided by the American Cocoa Research Institute, Vienna, VA.
In the past five years, scientists have developed increasingly accurate methods of detecting flavonoids. Only a few years ago, dark chocolate was found to contain more flavonoids than any other food that's been tested so far, including such flavonoid-rich foods as green and black tea, red wine and blueberries, says Engler. "Many people don't realize that chocolate is plant-derived, as are the fruits and vegetables recommended for a healthy heart. Chocolate is made from the cacao bean found in the fruit pod of the cacao tree (Theobroma cacao)."
Standard manufacturing of chocolate destroys about a quarter to half of its flavonoids. Now, some companies are using processing methods with reduced heat and alkalization, which can preserve as much as 70 percent to 95 percent of the chocolate flavonoids.
The UCSF study suggests that the beneficial effects of eating small doses of chocolate can be found in eating chocolate with higher amounts of cocoa , i.e., at least seventy percent cocoa content, says Engler. Additional information on current chocolate research can be found at the Chocolate Information Center website (http://www.chocolateinfo.com) sponsored by Mars, Inc, she says.
The current study supports many other studies that have shown benefits from cocoa flavonoids, Engler says. "Even though we still have a long way to go before we understand all of chocolate's effects, for now, there's little doubt that in moderation and in conjunction with a healthy, balanced diet and exercise we can enjoy -- and even benefit from -- moderate amounts of high-flavonoid dark chocolate."
Engler's co-author, Marguerite Engler, PhD, RN, presented an abstract of this research at the American Heart Association's Scientific Sessions in New Orleans in November 2002. Mary Engler and her team also presented the work at the Experimental Biology 2003 meeting in San Diego in April 2003. The abstract was published in The FASEB Journal in conjunction with that meeting.
Other researchers who participated in the study are Amanda Browne, RN, BS, Elisa Chiu, RN, MS, Michele Mietus-Snyder, MD, all from the Department of Physiological Nursing, UCSF School of Nursing; Chung Y. Chen, PhD, Ho-Kyung Kwak, PhD, Paul Milbury, MS, and Jeffrey Blumberg, PhD, FACN, all from the Jean Mayer USDA Human Nutrition Research Center on Aging, Antioxidants Research Laboratory at Tufts University, Boston, MA; Mary J. Malloy, MD, Cardiovascular Research Institute at UCSF, and Steven M. Paul, PhD, Office of Research, UCSF School of Nursing.