Do the bugs in your gut make you fat?

Obese people have a distinctive mix of bacteria in their digestive systems that seems to make them prone to gaining weight, a startling discovery that could lead to new ways to fight the obesity epidemic, researchers reported yesterday.

Obese people have more digestive microbes that are especially efficient at extracting calories from food, the researchers said, and the proportion of these super-digesting organisms ebbs as the people lose weight. Moreover, when the scientists transplanted these bacteria from obese mice into lean mice, the thin animals start getting fat. This provides more support for the provocative theory that the bacteria that populate the intestine play an important role in regulating weight.