From Science:
If the scale has tipped too far in the wrong direction, perhaps you should blame the bugs living in your gut. Some microbes are better at wringing calories out of those holiday meals than others, researchers report in two papers in today’s Nature. Transferring such high-octane bugs into lean mice causes the rodents to plump up, suggesting a microbial contribution to obesity.
Genetics certainly play a role in obesity, which is on the rise in many countries. But there’s more to the problem. In 2004, Jeffrey Gordon from Washington University in St. Louis, Missouri, and his colleagues demonstrated that intestinal bacteria could also contribute to weight gain in mice. A year later, microbial ecologist Ruth Ley, a postdoctoral fellow working in Gordon’s lab, discovered that lean and obese mice have different microbial communities in their gut. Now Gordon and his colleagues have shown this difference exists in people as well, and that diets can shift the microbial balance.
More here.