It may take a village to make you overweight. New evidence suggests that those bacterial hangers-on living in your gastrointestinal tract may be partly to blame for the state of your waist.

Microbiologists at Washington University in St. Louis checked out the gut bugs in mice of various sizes. The docs found that the resident GI fauna differed predictably based on the size of the host.

Furthermore, when the researchers analyzed the intestinal microbes of obese people, they found the population skewed heavily towards Firmicutes and short on Bacteroidetes compared with normal weight controls. After weight loss, the little Firmicutes lost ground to a growing group of Bacteroidetes, assuming a pattern more consistent with that of their leaner colleagues.

If mousy microbes are at all indicative of those that we carry through life, here's what your little gut friends may be doing to you:

Microbes from the obese mice had more genes for processing starches and complex sugars and produced more simple sugars and fatty acids--that is, calories--for the gut to absorb.