Surprisingly, gut bacteria may also control appetite
, nudging you toward the oozy, creamy, crunchy foods they
like. Fact is, the little buggers have species-specific appetites (a microbiologist can tell what types of food you eat and predict whether you're slender or not just from the variety of microbes in your stool sample). "Bad guy" bacteria may be partly to blame for your junk-food cravings—by, for instance, sending out sugar-and-fat-jonesing signals via the gut-to-brain vagus nerve. Plus, some microbes may help you extract more calories from food and increase fat storage
—effectively making you put on weight.