New research has pointed to stomach bacteria as an indicator of whether someone is fat or thin. The stomach contains trillions of bacteria, some that make bacteria and some that digest foods. This research was led by an experiment involving human stomach bacteria and mice. The stomach bacteria from obese people was placed in mice, and the mice became fat. When the bacteria from skinny people was placed into the mice, they remained lean. 

Scientists like Dr. Jeffrey I. Gordon of Washington University in St. Louis, want to isolate the bacteria that is responsible for this phenomenon, ideally leading to a method in which the beneficial bacteria is transplanted to obese people. This would hopefully lead to a bacteria that promotes thinness. 

The mice experiment also revealed that proper dieting can lead to a change from the "fat" bacteria to the "lean" stomach bacteria. This suggests that people do not have a predetermined type of bacteria in their stomachs.

Dr. GOrdon believes that the process of isolating the correct bacteria "could take many decades."