After just one day of switching from a plant-based diet to a high-fat-and-sugar diet, mice with human intestinal bacteria developed bacteria associated with obesity in humans, and soon became grossly obese (Science Translational Medicine, November 11, 2009).Dr. Jeffrey Gordon of Washington University in St Louis first showed that certain types of bacteria in the human intestinal tract can break down food more efficiently and help you absorb a greater percentage of calories from the food that you eat. He also showed that humans whose intestinal tracts are dominated by these bacteria tend to be overweight.In this new study, Dr. Gordon created germ-free mice and fed them a low-fat, plant-rich diet. Then he fed them bacteria extracted from human...