There's new evidence that bacteria play a role in weight gain. Scientists transferred intestinal microbes from obesity-prone or obesity-resistant rats into germ-free mice (bred to lack innate microbes) and then gave them unlimited access to high-fat foods. The mice that received bacteria from obese donors gained more weight and nearly twice as much body fat as the other group. Certain bacteria may activate enzymes that promote fat storage and reduce hormones that control appetite, explains lead author Frank Duca of the National Institute for Agricultural Research in Jouy-en-Josas, France. Obese people tend to have different types of gut bacteria than thinner people do, he says. He and his colleagues are optimistic that scientists will one day be able to shift a person's mix of gastrointestinal bacteria—via probiotics, prebiotics, and changes in diet—to enhance weight loss.