An expanding waistline may have less to with what a person eats than what?s already inside, say microbiologists Jeffrey Gordon and Fredrik Backhed at the Washington University School of Medicine in Saint Louis. Variations in the population of bacteria living in the gut may explain why some people pack on extra pounds while others stay slim.

Gordon and Backhed base their claim on a study of two groups of mice, one exposed to normal intestinal microbes and another raised in a germ-free bubble. The germ-free mice had 42 percent less body fat, even though they were fed one-third more calories. When the animals were inoculated with bacteria from their normal counterparts, the bubble mice increased their body fat by 57 percent in just two weeks.

?We know that gut microbes have ways of breaking down otherwise indigestible carbohydrates, increasing the calories available to the animal, but we thought something else must be at work,? Gordon says. His team therefore also looked at a hormone that limits fat storage in the body. They found that the gut bacteria secrete a substance that interferes with the hormone, causing even more of the calories to be stored as fat than would happen normally. The result is that microbe-containing mice pork up, even on a moderate diet.

?Having or not having certain species in our intestinal bacterial communities may have a profound effect on how efficiently we harvest and store energy from our food,? Gordon concludes. Killing off the gut bacteria is not a viable option?it would trigger opportunistic infections long before it would yield meaningful weight loss?but Gordon is targeting the fat-promoting hormone itself in hopes of developing a better diet drug.