Altering the bacteria in your gut could lead to major changes in your weight. A study from Washington University in St. Louis is among the latest looking into the increasingly popular subject of how gut microbes can affect the body at large, and it found that weight gain may be preventable by swapping microbes associated with obesity for microbes associated with a slim figure. In tests performed on mice, researchers were able to make just that happen.

The research team transplanted gut microbes from lean and obese people into mice. Microbes from the lean people kept the mice lean, while microbes from the obese people made the mice gain weight. But when the two microbe types were allowed to mix, those associated with lean bodies were able to overtake those associated with obesity — so long as the mice ate right too.

"Eating a healthy diet encourages microbes associated with leanness to quickly become incorporated into the gut," Jeffrey Gordon, a co-author of the paper on the group's research, says in a statement. "But a diet high in saturated fat and low in fruits and vegetables thwarts the invasion of microbes associated with leanness."

Gordon believes that distinction is an important one. "Our results underscore the strong interactions between gut microbes and diet," he says. This suggests that neither gut microbes nor diet alone may be the key factor in obesity — instead, it could be the interplay between them. Those findings are described this week by the group's paper, which has been published in Science.

While the tests have only been performed on mice so far, they work toward what Gordon calls an "intense interest" by researchers right now to use microbes to treat diseases. What the Washington University research team found could help to develop probiotics that promote microbes associated with leanness. That may still be a ways out, but researchers will now know not to look at microbes alone, but to the food they interact with as well.