Why is gastric bypass so effective?

Gastric bypass surgery is a highly effective treatment for obesity. It involves surgically sealing off or removing most of a person’s stomach and attaching the remaining small stomach pocket to the small intestine. Thus, a person is only able to eat a small amount before feeling full. You may think that this inability to eat large meals is the driving force behind the high success rate of gastric bypass. While that is important, it may not be the main factor. It may be more about the microbes.

Alice Liou of Massachusetts General Hospital and her colleagues used a mouse model of gastric bypass to compare the pre-and post-operative fauna in the digestive tracts of mice with diet-induced obesity. One group (GB) received gastric bypass surgery similar to that used in humans. The second (SHAM) also underwent surgery but that operation did not result in gastric bypass. All the mice were fed a liquid diet for two weeks and then were allowed to eat as much as they liked. 

Post surgery, the GB mice maintained normal weights while the SHAM mice quickly regained their obese statures. So, far this is no surprise. After all the SHAM mice had not had gastric bypass. However, there were significant differences in the composition of the gut microbes between GB mice and the SHAM mice. In other words, having real gastric bypass surgery affected the make-up of the mice’s intestinal flora. This was true regardless of whether the mice were fed a normal or a high fat diet.

Here’s the fascinating part: the scientists inoculated germ-free mice (mice with no gut bacteria of their own) with fecal samples from the GB or SHAM mice. In effect, the researchers were transplanting the intestinal environment of the GB or SHAM mice into the germ-line mice without surgery. The germ-free mice that received gut bacteria from the GB mice lost weight and fat tissue. This was without any diet restrictions. In contrast, mice receiving fecal matter from SHAM mice did not lose weight. This strongly suggests that it is the altered microbial environment of the gut rather than meal size restriction that drives gastric bypass weight loss.

This is actually a hopeful sign. It means that we might be able to achieve the same results as gastric bypass surgery by simply manipulating a person’s microbial content. The bad news is that we’re far from understanding how to do this safely. We don't know what it is about gastric bypass that causes the change in bacterial population and we don't know exactly which of those changes are critical for weight loss. Still, it's a start.

Liou, A., Paziuk, M., Luevano, J., Machineni, S., Turnbaugh, P., & Kaplan, L. (2013). Conserved Shifts in the Gut Microbiota Due to Gastric Bypass Reduce Host Weight and Adiposity Science Translational Medicine, 5 (178), 178-178 DOI: 10.1126/scitranslmed.3005687.