Gut Microbiota Contribution to Weight-Independent Glycemic Improvements after Gastric Bypass Surgery

The global rise in obesity and related comorbidities like type 2 diabetes necessitates effective treatments. Metabolic surgeries, particularly Roux-en-Y gastric bypass (RYGB), are highly effective, but their mechanisms extend beyond simple food restriction and malabsorption. Emerging evidence suggests a crucial role for the gut microbiota in mediating the beneficial effects of RYGB. Previous studies have shown that antibiotic treatment attenuates the weight loss and glycemic improvements observed after RYGB, and fecal microbiota transplants from RYGB-treated animals can improve metabolic parameters in recipients. However, the interplay between gut microbiota, weight loss, and glycemic control remains unclear. Weight changes themselves can influence the microbiota and glycemia, confounding the assessment of the microbiota's independent contribution. This study aimed to disentangle these factors by comparing the fecal microbiota of RYGB-treated Zucker fatty rats with sham-operated and body weight-matched controls, followed by fecal microbiota transplants to germfree mice to assess the causal role of the gut microbiota in weight-independent glycemic improvement after RYGB.

Existing literature establishes the strong link between obesity and altered gut microbiota, with the latter contributing to increased energy extraction from food. Metabolic surgeries like RYGB and vertical sleeve gastrectomy (VSG) are the most effective treatments for severe obesity, initially believed to act solely through mechanical means. However, recent studies reveal significant molecular, cellular, and systemic changes post-surgery, including profound shifts in gut microbiota composition. RYGB, in particular, is associated with decreased Firmicutes and increased Proteobacteria. Studies using antibiotic depletion and fecal microbiota transplantation have shown a causal role for the gut microbiota in some of the benefits of metabolic surgery; however, these studies didn't fully separate the effects of weight loss and the gut microbiota on glycemic control. This study builds on this knowledge to specifically assess the independent contribution of the gut microbiota to glycemic improvement after RYGB.

The study utilized male Zucker fatty rats, randomly assigned to undergo sham surgery (Sham), RYGB, or sham surgery with caloric restriction to match RYGB-treated rat body weight (BWM). Body weight and food intake were monitored for 28 days post-surgery, followed by an oral glucose tolerance test (OGTT) on day 27. Fecal samples were collected on day 28 and subjected to 16S rRNA sequencing for microbiota analysis. Beta-diversity was assessed using principal coordinate analysis (PCoA), and differential abundance analysis was performed to identify bacteria altered by RYGB at the phylum and species levels. Correlation analysis explored the association between specific bacterial species and glycemic control indices (OGTT AUC, HOMA-IR, ISI-M). To investigate causality, fecal microbiota transplants from RYGB and BWM rats were performed in germfree mice, followed by OGTT 14 days post-transplant. Further experiments focused on an unidentified Erysipelotrichaceae species strongly associated with glycemic control. This species was identified through sequence alignment and named Longibaculum muris. Quantitative PCR was used to confirm L. muris abundance, and additional experiments were performed to assess the effect of L. muris supplementation in RYGB-recipient mice and in conventionally raised mice fed either chow or a Western-style diet. Statistical analysis included PERMANOVA, Kruskal-Wallis test, Mann-Whitney U test, ANOVA, and Spearman's correlation.

RYGB-treated rats exhibited improved glycemic control compared to both Sham and BWM rats, despite similar body weight reduction. 16S rRNA sequencing revealed distinct microbiota profiles in RYGB rats compared to Sham and BWM rats, with increased Proteobacteria and decreased Firmicutes and Verrucomicrobia at the phylum level. At the species level, an unidentified Erysipelotrichaceae species showed significantly lower abundance in RYGB rats and positively correlated with OGTT AUC and HOMA-IR, and negatively with ISI-M, specifically in RYGB rats. Fecal microbiota transplantation from RYGB rats to germfree mice resulted in improved oral glucose tolerance (lower peak glucose) in recipients, independently of body weight. Interestingly, supplementing RYGB recipient mice with L. muris, the identified Erysipelotrichaceae species, further improved oral glucose tolerance, while administering L. muris alone to chow-fed or Western-style diet-fed mice had minimal metabolic effects. These findings demonstrate that a decrease in the abundance of L. muris contributes to the improved glycemic control seen in RYGB-treated rats but that this species does not exert this effect in mice on regular diets.

This study provides strong evidence that the gut microbiota contributes significantly to weight-loss independent improvements in glycemic control after RYGB. The observed transfer of improved glucose tolerance to germ-free mice through fecal microbiota transplantation establishes a causal link. The specific role of the Erysipelotrichaceae family, particularly L. muris, in glucose metabolism is highlighted, although its effect seems context-dependent, appearing to be associated with enhanced glucose tolerance in the context of RYGB. The unexpected finding that supplementing L. muris further improved glucose tolerance in RYGB recipients underscores the complexity of microbiota-host interactions and the potential for multiple microbiota members to contribute to the overall metabolic benefits of RYGB. This research suggests that manipulating the gut microbiota could provide novel therapeutic strategies for type 2 diabetes, potentially complementing existing treatments.

This study demonstrates a significant contribution of the gut microbiota to weight-independent glycemic improvements after RYGB. The causal link between RYGB-associated microbiota shifts, specifically a reduction in L. muris, and improved glucose tolerance is shown. Further research should explore the mechanisms by which specific bacterial species interact with the host and modulate glucose metabolism. Identifying other key bacterial players and their metabolic pathways is essential for developing gut microbiota-based therapies to improve glycemic control in type 2 diabetes.

The study uses a preclinical model, and findings may not fully translate to humans. The focus on a single bacterial species (L. muris) overlooks the potential contributions of other gut microbiota members. The study did not use a larger sample size for the FMT part of the study. Further investigation is needed to determine the long-term effects of microbiota manipulation and the mechanisms by which specific bacteria exert their effects on glucose metabolism.