They say it probably accounts for 20% of the weight loss. Outline of the experiment: Take a mouse model of human obesity. Divide your sample in three groups:

They say it probably accounts for 20% of the weight loss. Outline of the experiment: Take a mouse model of human obesity. Divide your sample in three groups: * Gastric bypass * Sham surgery (fake bypass, the surgical equivalent of a placebo) * Sham surgery + caloric restriction. Observe gut flora change in the first group only. "Graft" some of the gut bacteria of the three groups in obese mice who didn't undergo surgery. Observe a moderate weight loss, only in the mice who received a stool sample of the first group. Here's the abstract: Roux-en-Y gastric bypass (RYGB) results in rapid weight loss, reduced adiposity, and improved glucose metabolism. These effects are not simply attributable to decreased caloric intake or absorption, but the mechanisms linking rearrangement of the gastrointestinal tract to these metabolic outcomes are largely unknown. Studies in humans and rats have shown that RYGB restructures the gut microbiota, prompting the hypothesis that some of the effects of RYGB are caused by altered host-microbial interactions. To test this hypothesis, we used a mouse model of RYGB that recapitulates many of the metabolic outcomes in humans. 16S ribosomal RNA gene sequencing of murine fecal samples collected after RYGB surgery, sham surgery, or sham surgery coupled to caloric restriction revealed that alterations to the gut microbiota after RYGB are conserved among humans, rats, and mice, resulting in a rapid and sustained increase in the relative abundance of Gammaproteobacteria (Escherichia) and Verrucomicrobia (Akkermansia). These changes were independent of weight change and caloric restriction, were detectable throughout the length of the gastrointestinal tract, and were most evident in the distal gut, downstream of the surgical manipulation site. Transfer of the gut microbiota from RYGB-treated mice to nonoperated, germ-free mice resulted in weight loss and decreased fat mass in the recipient animals relative to recipients of microbiota induced by sham surgery, potentially due to altered microbial production of short-chain fatty acids. These findings provide the first empirical support for the claim that changes in the gut microbiota contribute to reduced host weight and adiposity after RYGB surgery. http://stm.sciencemag.org/content/5/178/178ra41 http://stm.sciencemag.org/content/5/178/178ra41