Does Bariatric Surgery Induce Long-Term Changes in Gut Bugs That Promote Weight Loss?

sharma-obesity-gastric_bypass_roux-en-y3With all the talk about how gut bugs may play a key role in susceptibility to weight gain (and resistance to weight loss?), we are learning much about this issue from animal studies.

However, now a human study by Valentina Tremaroli and colleagues published in Cell Metabolism, not only shows that bariatric surgery can induce distinct changes in gut bacteria but also that transferring stool from these individuals to germ-free mice changes their metabolism towards greater utilization of carbohydrates for fuel.

As a first step, the researchers examined the gut microbiota of 14 weight-stable women 9 years after randomization to either gastric bypass (n=7) or vertical banded gastroplasty (n=7) and matched for weight and fat mass loss.

As a control, they also analyzed the gut microbiota of two groups of non-operated women with a BMI matched to the patients’ pre-surgical BMI (OBS) and post-surgical BMI (Ob).

The gut bacteriome was significantly altered in both surgical groups with some difference between the two procedures: changes in the gastric-bypass patients suggest an  increased energy flux into sugar metabolism and glycolysis, whereas the VBG patients showed an enrichment of pathways for amino acid uptake and metabolism and for glyoxylate metabolism pointed to the utilization of amino acids and acetate for energy production.

There were also important difference in bile acid metabolism in the surgery groups.

The authors are comfortable that these changes result from bariatric surgery and are not related to BMI per se, since the OBS and Ob microbiomes were similar. Also, similar changes in gut microbiota have not been seen during dietary interventions for weight loss.

Thus, the authors note that,

“…our results suggest that bariatric surgery produces a specific shift in the microbiota that persists for up to a decade after surgery and is different from shifts related to dietary interventions for weight loss.”

In a second set of experiments, the researchers transplanted the microbiome from the various groups to germ free mice.

Mice colonized with gastric-bypass and banding microbiota for 2 weeks accumulated 43% and 26% less body fat, respectively, than mice colonized with OBS microbiota despite the fact that body weight gain and food intake did not differ between the groups during the 2-week colonization period.

In addition, metabolic studies showed decreased utilization of carbohydrates and increased utilization of lipids as fuel in recipients of gastric-bypass microbiota.


“…this study clearly shows that bariatric surgery has long-term effects on the composition and functional capacity of the gut microbiota and that these changes have the potential to modulate host metabolic regulation, thus adding evidence for the transmissibility of the human adiposity phenotype through the gut microbiota.”

“Our results also show that two different bariatric surgery procedures, namely RYGB and VBG, have similar long-term effects on the gut microbiome in women matched for BMI and fat mass loss. However, the two bariatric surgery procedures might result in different functionality due to different intestinal environmental conditions…. Importantly, the changes in the microbiome were not dependent on BMI or degree of weight and fat mass loss, thus revealing shifts in the gut microbiota that were specific to bariatric surgery.”

These findings should certainly do away with the simplistic notions of how and why bariatric surgery works – perhaps it is high time we discarded the notions of “restrictive” or “malabsorptive” surgery.

Edmonton, AB