Researchers found that obese(肥満の)people have higher levels of unusually efficient bacteria in their guts(腸、消化管)than lean(やせた)people do. Humans need bacteria to help convert foods into a form that is digestible(消化できる), and the bacteria in obese people are better at the process, causing obese people to obtain more energy, or calories, from food. The same disparity(違い)was found in mice; giving lean mice the bacteria from fat animals caused them to gain weight.