Researchers have studied gut microbiota changes following bariatric surgery (BS), but not gut diversity and function in patients who fail to reduce weight. Stool samples were collected from three groups of women: 15 women who did not lose weight after BS (“Yes” group), 9 overweight women without surgery, and 8 slim women (“No” group). 16S ribosomal RNA gene sequencing and PICRUSt2 were used for the analysis. The surgery and control groups had equal alpha and beta diversity, perhaps due to the high proportion of overweight participants (n = 24). All groupings were dominated by Bacteroidota and Bacillota. Barnesiellaceae decreased with BS, although Streptococcaceae remained frequent in overweight people. The iron supplementation group had High abundance of Atopobiaceae and Prevotellaceae. Barnesiellaceae abundance was considerably lower in both surgical groups (with and without iron supplementation) than in the no-iron and no-surgery groups. The ornithine degradation and haem biosynthesis routes use different metabolites than the glycine super system. Finally, the “Yes” group significantly upregulated PWY0–1241, PWY-5177, and PWY-5855 signaling pathways. In conclusion, gut bacteria and metabolic functions may predict weight loss after surgery better than diversity markers. The requirement for orthogonal validation assays is suggested by pathway analysis outperforming diversity metrics.

Oral administration of omega-3 polyunsaturated fatty acids (PUFAs) to rodents and humans is associated with an increase in gut bacteria that are predicted to synthesise short-chain fatty acids (SCFAs). We tested the hypothesis that physiological levels of omega-3 PUFAs in the distal intestinal lumen (1–50 μg/mL) are associated with increased SCFA synthesis in an in vitro fermentation model using faecal slurry from 10 healthy participants (mean age 30 years), with and without exogenous dietary fibres. SCFAs were measured by gas chromatography-flame ionisation detection (n = 10), and changes in bacterial composition were analysed by shotgun metagenomic sequencing (n = 6). In the presence of omega-3 PUFAs, there was a mean 9.3% (no inulin; P = 0.03) and 19.3% (+ 0.01 mg/mL inulin; P = 0.01) increase in total SCFA concentration at 24 h compared with paired control fermentations. Omega-3 PUFAs had a limited effect on the fermentation model microbiome in the absence of inulin. However, omega-3 PUFAs (50 μg/mL) were associated with increased abundance of Bifidobacteriaceae compared with paired control fermentations, if inulin (0.01 mg/mL) was present. Prebiotic activity of omega-3 PUFAs drives SCFA synthesis in an in vitro colonic fermentation model and is augmented by the soluble fibre inulin.