Roux-en-Y gastric bypass surgery and vertical banded gastroplasty are two types of bariatric weight loss surgeries that are highly effective in promoting weight loss. The mechanisms for their efficacy are complex and not completely known, but both surgeries are shown to reduce caloric intake, suppress hunger and increase gastric emptying. Little is known about how the microbiome changes during these surgeries, and how this change may effect subsequent weight loss. A team of Swedish scientists investigated this topic and showed the gut microbiota undergo important changes. They published their results in the journal Cell Metabolism.

The researchers compared the microbiomes of women that were obese and hadn’t had surgery with those who were of similar BMI presurgery, but had undergone surgery at least nine years earlier. They observed some major differences in the women’s microbiomes, with the post-operative women had much higher levels of Gammaproteobacteria and lower levels of Firmicutes. When the scientists looked at actual genetic variations they found many differences. Some notable differences were a decrease in short chain fatty acid (SCFA) and in increase in trimethylamine N-oxide (TMAO) creation in women who had surgery. As we have written about in this blog before, SCFAs are often associated with health, while TMAO is a risk factor for some cardiovascular diseases. Interestingly, when they took the microbiomes from both groups of women and transferred them into germ-free mice, the mice receiving microbiomes of women that had undergone surgery gained less weight than the mice that received microbiomes of obese women.

Gastric bypass surgery is often a last resort for folks that have severe obesity. While not normally considered, the microbiome is drastically affected by this procedure. The microbiome is certainly altered by the procedure, and it appears that it may even be helping keep the weight off. However, there may be some negative microbiome-mediated consequences as well, derived from alterations to micrbiome metabolism, such as an increased level of TMAO. Like all surgeries, folks undergoing this one need to balance the risks and rewards of the procedure, and hopefully after this study, the microbiome will be considered.