Your Genes Shape Your Gut Bacteria More Than Previously Known
The trillions of bacteria living in your gut are not entirely a product of what you eat, where you live, or how old you are. Your DNA plays a role too - and a pair of coordinated international studies suggest that role is considerably larger than scientists had confirmed before now.
Across 28,669 participants drawn from Swedish and Norwegian population biobanks, researchers identified 11 regions of the human genome with clear, statistically robust links to the composition and activity of gut bacteria. Before this work, only two such genomic regions had been established with confidence. Nine are newly confirmed. Some of the variants also connect to elevated risk of gluten intolerance, hemorrhoids, and cardiovascular disease - suggesting that changes in gut bacterial communities may be one pathway through which genetic predispositions translate into illness.
Why the Genetics of the Microbiome Has Been So Hard to Pin Down
The gut microbiome is enormously complex. A typical adult hosts hundreds of bacterial species, and their relative abundances shift constantly in response to diet, antibiotics, illness, stress, and aging. That variability makes it difficult to detect the comparatively subtle signal of genetic influence. Earlier genome-wide association studies - the standard tool for finding DNA variants linked to a trait - repeatedly found hints of genetic effects on gut bacteria but struggled to replicate results across different study populations.
The new analysis addressed that limitation through scale. The participant pool combined individuals from four Swedish cohorts - SCAPIS-Malmo, SCAPIS-Uppsala, MOS, and SIMPLER, coordinated through Lund and Uppsala Universities - with 12,652 participants from the Trondelag Health Study (HUNT) in Norway. Gut bacterial communities were characterized using detailed sequencing of microbial DNA, which identifies species present and infers their metabolic activity. Genetic data from the same individuals were then analyzed alongside those microbiome profiles to find consistent associations.
What the 11 Genomic Regions Do
The 11 confirmed regions are not random scatterings across the genome. Several cluster around genes that govern fundamental gut biology. Some relate to molecules displayed on the surface of intestinal epithelial cells - the lining cells of the gut - that bacteria can bind to or use as nutrients. Others involve how the gut immune system recognizes and responds to bacterial products. These biological roles make intuitive sense: the gut's physical and immunological environment determines which bacterial strains can survive and thrive there, so genes that shape that environment will inevitably shape the microbiome.
"Several of the genetic connections that we found have to do with very specific biological mechanisms," said Tove Fall, Professor of Molecular Epidemiology at Uppsala University, who led one of the two studies. "These concern, for instance, which molecules are present on the surface of gut cells and are thereby available as food for bacteria. They also relate to the way in which the gut reacts to molecules produced by bacteria."
The second study was led by Claes Ohlsson, Professor at the University of Gothenburg, and Kristian Hveem, Professor at the Norwegian University of Science and Technology. Ohlsson noted that some of the newly identified variants overlapped with known genetic risk factors for disease: "We saw that some of these genetic variants were linked to the risk of gluten intolerance, haemorrhoids and cardiovascular diseases. This suggests that changes in the composition of intestinal bacteria could provide a way to better understand how genetic risks affect health."
A Biobank Built for This Kind of Work
The studies drew on what the researchers describe as one of the world's largest gut-microbiome biobanks. Building that resource required years of participant recruitment, sample collection, bacterial DNA sequencing, and data harmonization across multiple institutions and two countries. The scale is what makes the genetic signals detectable: rare variants and modest effect sizes that would disappear in a study of a few thousand participants become statistically clear at nearly 29,000.
The study design also benefits from the relative homogeneity of Scandinavian population cohorts - a methodological strength that also carries a limitation. Genetic associations found in predominantly northern European populations do not necessarily generalize to people of other ancestries. Microbiome composition itself varies across populations with different diets, environments, and ancestry. Whether the same 11 genomic regions show equivalent effects in African, East Asian, or South Asian populations remains to be tested.
From Association to Mechanism
Finding a genomic region associated with microbiome composition is not the same as understanding how it works. Association studies identify statistical correlations between DNA variants and traits; they do not by themselves establish whether the genetic variant causes a change in the microbiome, or whether that change in the microbiome in turn affects disease risk. The path from association to mechanism requires follow-up work - cell culture experiments, animal models, and eventually clinical studies in humans.
That work is ongoing. Fall framed the long-term goal clearly: "Given that many aspects of our health are linked to the gut microbiome, we naturally want our research to contribute to better ways of preventing and treating diseases by paying attention to the interaction between genes, gut biology and the microbiome."
One concrete possibility is that understanding which genetic variants alter bacterial communities could help explain why the same diet or probiotic supplement works well for some people and poorly for others. If a person's genetics shift their gut toward a particular bacterial profile, dietary interventions might need to be calibrated to that baseline rather than applied uniformly. That kind of precision approach to gut health is still years away from clinical practice, but studies of this scale and design are the necessary foundation for building toward it.