Scientists reached this conclusion by transferring microbes from bypass - treated obese mice to a group of lean mice raised in sterile conditions that left them with no intestinal bacteria at all. (discovermagazine.com)
What's more, intestinal bacteria from obese mice increased fat gain more than bacteria from lean mice (1). (alivebynature.com)
Other studies have revealed that lean mice didn't gain as much weight when a high fat diet is supplemented with green tea. (ahealthblog.com)