A recent study examining the role of gut bacteria in obesity has reported that germ-free mice transplanted with human fecal microbiota either gained weight or stayed lean depending on the body profile of the human donor. Vanessa Ridaura, et al., “Gut Microbiota from Twins Discordant for Obesity Modulate Metabolism in Mice,” Science, September 2013. Using mice with no gut microbiota of their own, researchers with the Washington University School of Medicine apparently conducted two separate experiments, the first of which involved transplanting fecal microbiota from one lean twin and one obese twin into mice that were then kept in separate cages and fed a diet low in fat and high in plant polysaccharides. After 15 days, the mice that received bacteria from the lean twin reportedly stayed lean while the mice that received bacteria from the obese twin gained weight and fat in addition to developing signs of insulin resistance.

In the second experiment, the study’s authors co-housed the mice with “lean microbes” with those that received “obese microbes.” When the mice were both fed a healthy diet, the lean twin’s microbes were purportedly able to colonize the guts of mice with the obese twin’s microbes, thus preventing these animals from gaining weight. The researchers noted, however, that a diet high in saturated fats and low in fruits and vegetables effectively prevented the lean microbes from colonizing the obese-microbe mice, which continued to gain weight and fat as before.

According to a September 5, 2013, press release, the authors purportedly associated these observations “with an invasion of a group of bacteria called Bacteroidetes” that are “efficient at harvesting calories and nutrients from food and have been associated with leanness.” As Jeffery Gordon, director of the Center for Genome Sciences & Systems Biology at Washington University, explained, “Eating a healthy diet encourages microbes associated with leanness to quickly become incorporated into the gut. But a diet high in saturated fat and low in fruits and vegetables thwarts the invasion of microbes associated with leanness. This is important as we look to develop next-generation probiotics as a treatment for obesity.” See The New York Times, September 5, 2013.