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Roux-en-Y gastric bypass (RYGB) surgery produced marked changes in gut microbes that may facilitate weight loss, scientists said.

More than 9 months after surgery, the gut microbiota of patients who underwent RYGB were significantly more diverse than in patients who underwent laparoscopic adjustable gastric banding (LAGB) or in obese patients who had not had bariatric surgery (P<0.05 for both comparisons), reported Zehra Ilhan, PhD, of Arizona State University in Tempe, and colleagues.

In addition, the composition of the gut microbiome in post-RYGB patients was distinct from that of post-LABG and obese nonsurgical patients, with significant changes in the numbers of five bacterial classes: Clostridia, Bacilli, Erysipelotrichi, Gammaproteobacteria, and Bacteroida, Ilhan and colleagues wrote online in .

On the level of phylotypes, there was a greater abundance of Escherichia, Streptococcus, Veillonella, Trabulsiella, and Coprococcus in post-RYBG patients. Furthermore, the abundance of these bacteria correlated positively with the percentage of excess weight these patients lost.

"The high diversity in RYGB participants may be explained by the altered environmental conditions after RYGB," Ilhan and colleagues wrote. "For instance, a slight increase in the gut pH after RYGB can enhance the survival of acid-sensitive microorganisms, and increased oxygen in the gut can promote the growth of facultative anaerobes. Additionally, an altered bile flow after RYGB can stimulate the growth of bile-acid-transforming bacteria. Since anatomical rearrangement is not part of LAGB, similar changes would not be anticipated. Therefore, only RYGB creates the large environmental changes in the gut critical for colonization and succession of novel microbial species in the colon."

Ilhan noted in a statement that this was one of the first studies to show that anatomically different surgeries with different success rates have different microbiome and microbiome-related outcomes.

Also commenting, the study's senior investigator, Rosa Krajmalnik-Brown, PhD, also of Arizona State University, said: "One of the key findings of the paper confirms what we had already observed in earlier research. RYGB had a huge effect on the microbial community structure. This fact may have profound implications for both the understanding and management of obesity."

The study included 63 patients who were recruited from 2011 to 2014. Of these, 24 had undergone RYGB, 14 had undergone LAGB, 10 were morbidly obese controls scheduled to undergo bariatric surgery, and 10 were normal-weight individuals for comparison. The investigators wanted to study long-term changes in the gut microbiota after surgery, so all surgical patents had undergone surgery more than 9 months previously.

The investigators collected and analyzed fecal samples from each participant, who were not allowed to take antibiotics for 2 months or probiotic agents for 2 weeks prior to sample collection. In addition, the participants were asked not to take medications such as proton pump inhibitors that could affect gut flora for 2 weeks prior to sample collection.

Ilhan and colleagues also found important differences in the metabolomes of study participants, which mirrored the difference in their microbiomes. The team used principal component analysis to assess the distribution of 71 metabolites in the four patient groups. The largest difference in metabolomes was between the post-RYGB patients and the pre-surgical obese patients.

"These findings suggest that obesity and RYGB surgery not only changed the microbial community structure, but also changed the microbial functions, reflected by the metabolome," the investigators said.

Previous studies in mice reported that fecal transplants from post-RYBG subjects induced weight loss, independent of diet, Ilhan and colleagues noted. "Although the microbiota-associated mechanism of weight loss remains unclear, several microbial metabolites, such as short chain fatty acids, have been suggested to play roles in host energy regulation." For example, propionate and butyrate, which increased most in the post-RYGB patients in the current study, have been implicated as signaling molecules of free fatty acid receptors and seem to protect against diet-induced obesity, they explained.

The mouse studies have yet to be replicated in humans, but do suggest that the properties of the gut microbiome might one day be used to induce weight loss. "A probiotic that would replace surgery would be great," Ilhan said. "Another positive outcome would be if we can find a microbial biomarker that will identify the best candidates for surgery and sustained weight loss."

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