Faculty Peer Reviewed
I was recently counseling an overweight patient about nutrition and exercise when he asked, “Doc, is it true what they say about dark chocolate being good for you?” I told him that although I had heard about this, I was actually not too sure about the evidence behind this. As a strong supporter of the “I wish that the best tasting foods were good for you” club, I decided this was the type of research that warranted my immediate professional evaluation. How could a food so rich in fats and sugars possibly be good for you?
The secret lies in one special ingredient of dark chocolate—catechins. Catechins are a subgroup within the flavanoid family. Flavanoids, in general, are a polyphenol that contribute to the aroma, color and taste in foods such as tea, wine, and chocolate. But it is the catechins that make chocolate, especially dark chocolate, so special. They have strong antioxidant properties and have been shown to improve endothelial function by inducing nitric oxide synthase. In vitro and in animals, they have been shown to decrease the formation of blood clots by reducing platelet activation . In a study comparing the catechin content within dark chocolate, milk chocolate, and tea, dark chocolate contained the highest total catechin concentration. Per 100 grams, dark chocolate had 53.5 mg of cachetin, milk chocolate had 15.9 mg, and black tea had 13.9 mg.  Given the strong amount of antioxidants within dark chocolate, several studies have been undertaken to prove what type of cardioprotective effects it may have.
Flammer et al studied the possible antiatherogenic properties of dark chocolate in 22 heart transplant recipients with a double-blinded randomized control trial. Transplant recipients were chosen as an experimental group because high oxidative stress and reduced antioxidant defense play a critical role in the development and progression of transplant-associated atherosclerosis . Baseline coronary angiography was done before the chocolate intervention. Eleven participants then received 40g (approximately 1 Hershey’s bar) of flavanol-free control chocolate while the other eleven received flavanol-rich dark chocolate. Repeat coronary angiography was performed two hours later. Results showed that both coronary artery diameter and endothelium dependent vasomotion improved significantly in the dark chocolate group, but not in the control chocolate group. The group also found that shear-stress related platelet adhesion was significantly reduced by dark chocolate. These results suggest a theoretical cardioprotective effect of dark chocolate, particularly in preventing the development of atherothrombosis. However, given the short-term follow up, no conclusion can be made in terms of how often patients should consume chocolate or of its long-term effects on coronary artery diameter.
Ried et al conducted a meta-analysis involving 15 trials to study the effects of flavanol-rich cocoa products on blood pressure. All trials had a duration of at least 14 days. Nine out of fifteen used dark chocolate as the treatment group while the other six used high flavanol chocolate or cocoa. Results showed no significant change in blood pressure in the baseline normotensive patients on therapy. However, in hypertensive patients, there was an approximate 5mm Hg reduction in systolic blood pressure in the patients taking flavolol rich cocoa foods. This is equivalent to a 20% reduction in cardiovascular risk over 5 years—similar to the reduction in cardiovascular risk over 5 years by exercising 30 min/day ! Although this is good news for those of us with a sweet tooth, I am not sure that I would feel comfortable telling my patient about the results of this study—at least not without knowing the possible metabolic effects of consuming this amount of chocolate.
To explore this further, I looked into the maximum amount of chocolate one could eat before the metabolic risks started to outweigh the cardioprotective benefits. Desch et al  explored just that in a randomized control trial involving ninety-one patients. Half of the patients were given 6g of chocolate (approximately 3 M&Ms) and the other half received 25g every day for 3 months to determine if blood pressure reduction had a dose-dependent relationship with dark chocolate consumption. Results showed a reduction in mean arterial pressure in both groups, but no significant difference in blood pressure changebetween the two groups. However, the group consuming 25g did experience a slight increase in body weight.
Overall, I found that dark chocolate does have cardioprotective effects. The drawback of these studies was their short duration and poor long-term follow-up. We still need more long term studies to quantify how much or how long we can consume dark chocolate before its detrimental metabolic effects develop. When my patient returns, I will tell him that although it seems like chocolate may be good for you, there is not enough data out for me to wholeheartedly recommend it to my patients. Like so many good things in life, chocolate should be used in moderation.
Dr. Parikh is a first year resident at NYU Langone Medical Center
Peer reviewed by Barbara Porter, MD, editor myths and realities section, Clinical Correlations
Image courtesy of Wikimedia Commons
1. Arts, Ilja W., Peter Hollman, and Daan Kromhout. “Chocolate as a Source of Tea Flavanoids.” The Lancet 354 (1999): 488. Print.
2. Desch, Steffen, Daniela Kobler, and Johanna Schmidt. “Low vs. Higher-dose Dark Choclate and Blood Pressure in Cardiovascular High-Risk Patients.” American Journal of Hypertension 23.6 (2010): 694-700. Print.
3. Faridi, Zubaida, Valentine Njike, and Suparna Dutta. “Acute Dark Chocolate and Cocoa Ingestion and Endothelial Function: a Randomized Controlled Crossover Trial.” American Journal of Clnical Nutrition 88.1 (2008): 58-63. Print.
4. Flammer, Andreas, Frank Hermann, and Isabella Sudano. “Dark Chocolate Improves Coronary Vasomotion and Reduces Platelet Reactivity.” Circulation 116: 2376-382. Print.
5. Ried, Karin, Thomas Sullivan, and Peter Fakler. “Does Chocolate Reduce Blood Pressure? A Meta Analysis.” BMC Medicine 8 (2010). Print.