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Labs & diagnostics · health · gut · Emerging
Large Population Study Links Gut Microbe Diversity Patterns To Obesity Markers
5,846 people studiedValidated in 275 moreLower diversity, higher BMILean gut: fiber breakers
A study published online in iScience on September 17, 2026 analyzed gut bacteria from 5,846 people and found that lower diversity in the community of microbes living in the gut was linked to obesity markers, with the pattern confirmed in a separate group of 275 people.
Researchers analyzed stool samples from a well-phenotyped random population sample of 5,846 participants, using multivariable-adjusted linear models to test connections between gut bacteria patterns and two obesity measures, body mass index (BMI) and waist-hip ratio (WHR). They validated the BMI finding in a separate group of 275 people. A measure of microbial diversity called Shannon diversity, which captures how many different species are present and how evenly they're distributed, was inversely linked to obesity: lower diversity tracked with higher BMI and WHR. A different diversity measure, simple species counts, showed no such link. BMI was also linked to differences in the overall composition of gut bacteria communities (measured by Bray-Curtis dissimilarity), though people with obesity did not form one distinct cluster. The long-studied Bacteroidota-to-Firmicutes ratio, once a popular obesity marker, showed no link to either BMI or WHR in this sample.
The gut microbes of people with obesity were enriched in species associated with inflammation and involved in lipid metabolism, along with pathways tied to folate-building blocks, membrane lipid proteins and tRNA charging, a process cells use to build proteins. Lean participants' gut microbes were enriched in bacteria tied to breaking down cellulose, the structural fiber found in plant cell walls. In short, obesity-linked gut communities carried more inflammation-associated, lipid-metabolizing species, while lean ones carried more cellulose-degrading bacteria. Fiber feeds the microbes already present in the gut, and the types of fiber a person eats shape which species thrive.
Lean individuals were enriched in bacteria linked to cellulose breakdown
Data Panel
- Who
- 5,846 participants in a well-phenotyped random population sample, with BMI findings validated in a separate 275-person cohort
- Design
- Cross-sectional observational study using multivariable-adjusted linear models to relate gut microbiome features to BMI and waist-hip ratio
- Dose
- Not applicable (observational microbiome analysis, no intervention)
- Primary result
- Lower Shannon diversity (a measure of microbial variety and evenness) was linked to higher BMI and waist-hip ratio; species richness alone showed no link
- Secondary
- Bray-Curtis beta diversity (overall community composition) was linked to BMI with no distinct obesity cluster; Bacteroidota:Firmicutes ratio showed no link to BMI or waist-hip ratio; obesity-linked microbiomes were enriched in inflammation and lipid-metabolism pathways, lean microbiomes enriched in cellulose-degrading bacteria
- Funding / conflicts
- Funders listed in the journal record: Research Council of Finland, Sigrid Juséliuksen Säätiö, European Commission Marie Sklodowska-Curie Actions, Juho Vainion Säätiö, Sydäntutkimussäätiö, Suomen Lääketieteen Säätiö and 2 others. Conflicts declared in the PubMed record: one author is a scientific advisory board member, consultant and equity holder at BiomeSense, a scientific advisory board member and equity holder at GenCirq, and a consultant and equity holder at Cybele; another has received consulting fees from AstraZeneca.
This is a cross-sectional analysis, meaning gut bacteria and body measurements were captured at one point in time, so the study shows a link rather than proof that microbiome patterns and obesity cause one another. Two of the nine authors declared industry ties: one is an adviser to or holds equity in BiomeSense, GenCirq and Cybele, and one has received consulting fees from AstraZeneca.
Dr. Axe's Take
The old Bacteroidota-to-Firmicutes ratio didn't hold up in this sample, which matches what this body of research increasingly shows: diversity matters more than any single bacterial ratio. The functional finding here is the one worth acting on. Lean guts were enriched in cellulose-fermenting bacteria, which is a direct argument for eating a wide variety of plant fibers rather than leaning on one or two sources, since different fibers feed different species. This week, I'd stack fiber diversity across meals, think vegetables, legumes, berries, and ground flaxseed, instead of relying on a single fiber supplement, and pair that with fermented foods to support a broader microbial community.
— Dr. Axe