Phytochemical components of the cacao bean have been shown to have antioxidant, anti-inflammatory, neuroprotective and antimutagenic actions in the laboratory. The most abundant phenolic compound in cocoa extracts is the flavonoid epicatechin, while the most abundant methylxanthine is theobromine (a stimulant which is similar to caffeine).
Cocoa also contains catechin, chromium, ferulic acid, iron, resveratrol and anthocyanins. Like the coffee bean, the cacao bean has a complex and varied chemical makeup. However, chocolate consumption has not been as well studied as coffee consumption and the information concerning its health effects are limited and confounded by the fact that chocolate normally contains added sugar.
Dark chocolate has been found to significantly improve coronary circulation in healthy adults and moderate consumption of dark chocolate is thought to exert protective effects against the development of cardiovascular disease. Both milk chocolate and dark chocolate consumption have been linked to lower risk of stroke and heart attack.
Cocoa polyphenolic extracts have been shown to have anticancer activity against human prostate, colon and adrenal cancer cells. A cocoa polyphenolic extract was also found to reduce the incidence of carcinogen-induced prostate tumors in laboratory rats. Stearate, a long-chain saturated fatty acid abundant in chocolate, has been found to inhibit tumor growth and reduce proliferation of cancer cells in the laboratory. However, chocolate consumption has been found in population studies to be associated with increased risks of stomach, pancreatic and colorectal cancer.

Breast cancer-related effects of eating chocolate

One study reported that cocoa extract induced cell death in two hormone receptor positive (ER+/PR+) breast cancer cell lines without harming normal cells. In addition, cocoa-derived pentameric procyanidin has been shown to inhibit the proliferation of several types of human breast cancer cells, while sparing normal cells.
The long-chain saturated fatty acid stearate found in chocolate has been shown to inhibit breast cancer cell proliferation, invasion, and migration, and to induce apoptosis in the laboratory. Animal experiments have demonstrated that stearate can decrease mammary tumor incidence and size.
The beneficial effects of individual anticancer compounds in the cacao bean might be difficult to capture by consuming chocolate since its typically high sugar and fat content are not favorable characteristics for breast cancer risk. An Italian study found that consumption of sweet desserts and sugar (including chocolate) was positively associated with the risk of breast cancer, even after adjusting for body mass index. In addition, chocolate bars, hot chocolate and cocoa powder can contain relatively high levels of copper, which could contribute to angiogenesis and metastasis of breast cancer, especialy in women with inflammatory breast cancer (IBC) or triple negative (ER-/PR-/HER2-) disease.

Additional comments

The highest levels of bioactive cocoa compounds are found in products with the highest content of cocoa solids (i.e., cocoa liquor, cocoa powder and dark chocolate with 88% cocoa solids), while the lowest levels are found in milk chocolate, white chocolate and chocolate bars. In fact, the milk proteins in milk chocolate may inhibit the absorption of cocoa flavonoids.
Hot chocolate, with its relatively low cocoa content and high milk and sugar content, is not likely to be beneficial with respect to breast cancer risk. It is also a dietary source of acrylamide, a suspected carcinogen. Numerous studies have examined the association between acrylamide and breast cancer, with mostly inconclusive or negative results. However, a 2012 Danish study reported that pre-diagnostic exposure to acrylamide was associated with increased mortality among breast cancer patients with ER+ breast cancer.
Frequent consumption of chocolate has been found to be associated with lower bone density and bone strength in older women.
Note that while we are continually searching for new evidence specifically concerning this food, there is not much interest in it among breast cancer researchers, so few studies are available.