The size of many people's bellies may be determined by some of the billions of bacteria living in their guts, according to new research. Scientists took pairs of human twins—one twin obese, the other thin—and transplanted some of their gut bacteria into young mice that had been raised microbe-free, the BBC reports. When they were fed the same diet, the mice with the obese twin's bacteria grew fat while those with the lean twin's bacteria did not. And when the mice were exposed to each other's bacteria in a "battle of the microbes," the obese mice grew lean but the thin mice did not gain weight.
Much more research remains to be done, but experts say the "weird and wonderful" finding that gut bacteria can cause obesity could lead to new therapies in the not-too-distant future, reports the New York Times. Since people, unlike mice, don't tend to eat each other's droppings, a fecal transplant would be the simplest way of transferring thin people's bacteria to obese people. Such transplants are already used to treat a persistent gut infection, CBS notes, but the lead researcher says there is a less nauseating alternative: Tailoring combinations of bacteria to suit patients, then growing them in a lab and turning them into pills. And getting the right combination of bacteria could be very important—another group of researchers working with microbes in mice guts found that transplants could result in major personality changes. (Read more gut stories.)