The health benefits of eating dark chocolate have been extolled for centuries, but the exact reason has remained a mystery.
Researchers from Louisiana State University have now found that bacteria in our stomach ferment chocolate into useful anti-inflammatory compounds that are good for the heart.
"We found that there are two kinds of microbes in the gut: the 'good' ones and the 'bad' ones," said Maria Moore, one of the study's researchers.
"The good microbes, such as Bifidobacterium and lactic acid bacteria, feast on chocolate. When you eat dark chocolate, they grow and ferment it, producing compounds that are anti-inflammatory," she said.
The other bacteria in the gut are associated with inflammation and can cause gas, bloating, diarrhea and constipation. These include some Clostridia and some E coli.
"When these compounds are absorbed by the body, they lessen the inflammation of cardiovascular tissue, reducing the long-term risk of stroke," said John Finley, who led the study.
He said that the study is the first to look at the effects of dark chocolate on the various types of bacteria in the stomach.
The team tested three cocoa powders using a model digestive tract, composed of a series of modified test tubes, to simulate normal digestion.
They subjected the non-digestible materials to anaerobic fermentation using human fecal bacteria.
Finley explained that cocoa powder, an ingredient in chocolate, contains several polyphenolic, or antioxidant, compounds such as catechin and epicatechin, and a small amount of dietary fibre.
Both components are poorly digested and absorbed, but when they reach the colon, the desirable microbes take over.
"In our study we found that the fibre is fermented and the large polyphenolic polymers are metabolised to smaller molecules, which are more easily absorbed. These smaller polymers exhibit anti-inflammatory activity," Finley said.
Finley also noted that combining the fibre in cocoa with prebiotics is likely to improve a person's overall health and help convert polyphenolics in the stomach into anti-inflammatory compounds.
Prebiotics are carbohydrates found in foods like raw garlic and cooked whole wheat flour that humans can't digest but that good bacteria like to eat.
Finley said that people could experience even more health benefits when dark chocolate is combined with solid fruits like pomegranates and acai.
The findings were presented at a meeting of the American Chemical Society (ACS) in Dallas.