Ulcerative colitis (UC) develops from the interaction of genetic susceptibility, environmental exposures, gut dysbiosis, and dysregulated mucosal immunity; no single risk factor is necessary or sufficient.^[1][2] A family history of inflammatory bowel disease (IBD) is the strongest known risk factor, conferring roughly 4–10 times the background risk in first-degree relatives.^[3][4] UC risk is characteristically increased in former smokers and reduced in current smokers and after appendectomy for appendicitis in early life, the inverse of the pattern seen in Crohn’s disease.^[5][6] Most environmental associations derive from observational, largely retrospective data subject to recall bias and reverse causation, and should be interpreted as risk markers rather than proven causal or modifiable targets.^[7]

• Family history: First-degree relatives of a person with UC have approximately 4–10 times the background risk (adjusted incidence rate ratios 4–10 across population studies; pooled relative risk ~8, 95% CI 5.9–10.7).^[3][4][8]
• Offspring risk: Absolute risk is approximately 6% when a parent has UC (vs ~9% with parental Crohn’s disease). Risk increases with the number of affected relatives and younger age at the relative’s diagnosis, and is higher between siblings than across generations. Maternal and paternal transmission risks are roughly equivalent.^[8]
• Twin concordance: ~15–19% in monozygotic vs ~4% in dizygotic twins, substantially lower than in Crohn’s disease, indicating a dominant contribution of non-genetic factors.^[3][4]
• Susceptibility loci: Genome-wide association studies have identified more than 240 IBD risk loci, most shared between UC and Crohn’s disease, implicating HLA genes, epithelial barrier genes (e.g., HNF4A, CDH1), and IL-23/T-cell pathways (IL23R, STAT3, JAK2). These loci explain only ~7.5–8.2% of UC disease variance.^[9][10][3]
• Clinical implication: Genetic risk scores have no validated role in predicting onset or phenotype, and no preventive intervention has been validated for at-risk relatives.^[9]

• Age: UC can occur at any age; incidence peaks in early-to-mid adulthood (UC peak approximately 30–40 years), and prevalence rises with age before declining after ~80 years. A reproducible bimodal age distribution is not well supported.^[10][11]
• Sex: No consistent sex predominance.^[10]
• Race and ethnicity (United States): IBD prevalence is highest in White Americans (812 per 100,000) and lower in Black (504), Hispanic (458), and Asian (403) Americans; the gap is narrowing. The UC-to-Crohn’s disease ratio is higher in Asian (1.6:1) and Hispanic (1.8:1) than in White or Black (1.2:1) Americans.^[11]
• Jewish ancestry: Historically associated with higher IBD rates.
• Geography and migration: Incidence has historically been highest in northern Europe, Canada, and Australasia but is rising sharply in newly industrialized regions of Asia, the Middle East, and South America in parallel with Westernization. Migrants from low- to high-incidence regions, and particularly their children, acquire the host-country risk, implicating early-life environmental exposures.^[10]

The strength of evidence varies widely; credibility grading is drawn from umbrella reviews and meta-analyses of observational studies.^[7][12]

Several Western risk factors (urban living, antibiotic exposure, the protective effect of appendectomy, and the effect of smoking) do not replicate uniformly in Asian and Middle Eastern populations, and Western data should not be applied uncritically to these patients.^[6][12][10]

• Breastfeeding, high vitamin D levels, tea consumption, physical activity, and Helicobacter pylori colonization are associated with lower IBD risk.^[7]
• Current smoking and early-life appendectomy for appendicitis (see table above).^[5][6]

• UC is associated with dysbiosis characterized by reduced microbial diversity, loss of short-chain fatty acid–producing anaerobes (Firmicutes, Roseburia, Lachnospiraceae), and expansion of facultative anaerobes and Proteobacteria, including adherent Escherichia coli pathobionts, in an oxygen- and nitrate-rich inflamed mucosal environment.^[13][14]
• No single causative organism has been reproducibly identified, and dysbiosis appears to be both a contributor to and a consequence of mucosal inflammation.^[2][15]

• Document a detailed family history of IBD (number and type of affected relatives, age at their diagnosis); it is the most informative single risk descriptor, and siblings carry higher risk than offspring.^[8]
• Recognize recent smoking cessation or a recent episode of bacterial gastroenteritis as a plausible antecedent of new-onset UC.^[5][10]
• Do not recommend starting or resuming smoking to prevent or treat UC despite the epidemiologic protective signal; counsel cessation per usual standards.^[10][6]
• Modest twin concordance does not imply low familial risk; family history still raises risk several-fold.^[3]
• Genetic and microbiome testing have no validated role in predicting UC onset or guiding prevention outside research settings.^[9][2]
• No dietary, antibiotic, or supplement regimen is proven to prevent UC.^[9][7]