People who like milk chocolate have slightly different microbes in their intestines than those who prefer their chocolate dark, although researchers do not know why. Significant differences in the so-called microbiome are also found in individuals based on whether or not they eat a lot of fiber or take certain medications—such as the diabetes drug metformin, female hormones or antihistamines.

Source: Findings from the Gut–New Insights into the Human Microbiome – Scientific American