Corti R, Flammer AJ, Hollenberg NK, Lüscher TF. Cocoa and cardiovascular health. Circulation. 2009;119: 1433-1441.

The proposed health benefits of cocoa (Theobroma cacao) have been known for centuries. The Incas considered cocoa to be the drink of the gods, hence its scientific name originating from the Greek words theo (god) and broma (drink). Cocoa consumption dates back to 1600 BCE and was introduced from the New World to the Europeans in the 16^th century. Since this time, the chocolate industry has grown drastically, and cocoa is processed in many different ways. Several commonly purported health benefits of cocoa consumption include improvements in heart function, digestion, and kidney and bowel function. It is important to distinguish between the natural product cocoa and the processed product chocolate. The authors note that the effects of cocoa reported in this paper may not apply to chocolate. The objective was to review the "clinically relevant cardiovascular effects of cocoa" with a focus on the potential mechanisms responsible for the responses to cocoa and the potential clinical implications of its consumption.

The first epidemiologic evidence of the cardiovascular health benefits of cocoa was observed in the Kuna Indians, indigenous to islands off the coast of Panama. The Kunas consume "enormous" amounts of cocoa, sometimes with added salt, yet have "markedly lower" cardiovascular mortality than other Pan-American citizens and no age-dependent increase in blood pressure or decline in kidney function. The mechanism is environmental and not genetic, because these benefits are lost after migration to urban Panama City where cocoa is replaced with lower flavanol content foods. The Iowa Women's Health Study (34,489 postmenopausal women with a 16-year follow-up) found that consumption of high flavonoid foods was associated with a decrease in coronary disease. In the Dutch Zutphen Study, cocoa intake specifically was associated with a 50% reduced risk of cardiovascular mortality between the highest and lowest quintile of intake in 470 elderly men. These epidemiologic data led to the hypothesis that the health benefits of cocoa may be linked to its constituent flavonoids (e.g., flavanols and procyanidins)—a subgroup of polyphenols. Both the flavanol content and the total antioxidant capacity have been shown to increase in plasma after cocoa consumption. 

Cocoa flavanols (also known as flavan-3-ols) can be found in high concentrations in grape juice, wine, various berries, and especially cocoa. In cocoa, the flavanols consist of monomers like epicatechin, dimmers, and oligomers; the latter are called procyanidins and are responsible for the bitter flavor of cocoa. Conventional chocolate processing can markedly reduce the levels of flavanols. Detrimental effects to flavanol content include fermentation and roasting. The location where the cacao beans are grown also determines the flavanol content.

Functional impairment of the endothelium, the layer of cells lining the blood vessels and heart, is a first step in atherosclerotic changes and is positively affected by nitric oxide.  Cocoa consumption has been shown to affect nitric oxide levels, increasing its production and metabolites in humans. Endothelial function and circulation were concurrently improved in a number of studies. Cocoa flavanols also improved circulation to the brain in elderly subjects, suggesting protection against dementia and stroke. Platelet dysfunction is another indicator of atherosclerotic disease, and several in vivo studies showed that cocoa inhibits platelet aggregation and adhesion. Cocoa was also correlated with lowered blood pressure in a number of studies, with some evidence for reduced insulin resistance. It also displayed antioxidant properties, increasing overall antioxidant capacity and decreasing several markers of oxidation. Cocoa butter, the fat portion of cocoa, appears not to adversely affect cholesterol.

Many positive effects of cocoa intake on the cardiovascular system have been documented, including a reduction in platelet aggregation and improvements in endothelial function, blood pressure, insulin resistance, and blood lipids. Current evidence suggests that these beneficial health effects are attributed to the flavanol content of cocoa, particularly epicatechin, which increases the bioavailability of nitric oxide. However, it should be noted that cocoa contains many other active substances, including theobromine and magnesium. In studies, the exact content of flavanols and plasma flavanol concentration should be reported. Furthermore, wholesale recommendations to consume flavanol supplements are premature because of the potential pro-oxidant effects of large quantity intake. Commercially available chocolate can have a high sugar and fat content, which can induce weight gain and dental caries, whereas cocoa has a low sugar and fat content and is thus preferred.