"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."

From "Findings from the Gut—New Insights into the Human Microbiome/A preference for dark versus milk chocolate, among other things, shows up in the kinds of healthy germs found in the gut" (in Scientific American).