Genetic dissection of stool frequency implicates vitamin B1 metabolism and other actionable pathways in the modulation of gut motility

Background Genetic studies of stool frequency (SF), an indirect proxy for gastrointestinal transit, may reveal therapeutically tractable pathways relevant to IBS and other dysmotility disorders. Objective To identify genes and mechanisms involved in gut motility, providing a foundation for clinical translation. Design We performed a multiancestry genome-wide association study (GWAS) meta-analysis of SF in 268 606 European and East Asian individuals. Heritability and genetic correlations with other traits were estimated, and Mendelian randomisation was used to test causal relationships. GWAS signals were fine-mapped and functionally annotated to prioritise candidate genes and pathways. Findings implicating thiamine metabolism were followed-up with dietary interaction analyses in UK Biobank (UKB). Results SF heritability was comparable in Europeans (7.0%) and East Asians (5.6%). We observed strong genetic correlations with gastrointestinal and psychiatric disorders (r g =0.18–0.47), and causal effects on IBS. Novel correlations with cardiovascular traits (r g =0.12–0.14) were supported by drug signature enrichment analyses. We identified 21 independent loci, including 10 novel signals implicating bile acid synthesis ( KLB ) and cholinergic signalling ( COLQ ). Fine-mapping converged on vitamin B1 metabolism, highlighting single-variant causal effects at SLC35F3 (a thiamine transporter) and XPR1 (phosphate exporter essential for thiamine activation). In 98 449 UKB participants, thiamine intake was positively associated with SF (p