Objective Obesity is an underrecognized risk factor for rheumatoid arthritis (RA). In recent years, both the prevalence of obesity and the incidence of RA have been rising. Our objective was to determine whether the "obesity epidemic" could explain the recent rise in the incidence of RA. Methods An inception cohort of Olmsted County, Minnesota residents who fulfilled the 1987 American College of Rheumatology criteria for RA in 1980-2007 was compared to population-based controls (matched on age, sex, and calendar year). Heights, weights, and smoking statuses were collected from medical records. Obesity was defined as a body mass index ≥30 kg/m2. Conditional logistic regression was used to assess the influence of obesity on developing RA. Population attributable risk was used to estimate the incidence of RA in the absence of obesity. Results The study included 813 patients with RA and 813 controls. Both groups had extensive medical history available prior to the incidence/index date (mean 32.2 years), and ∼30% of each group was obese at the incidence/index date. The history of obesity was significantly associated with developing RA (odds ratio 1.24, 95% confidence interval 1.01-1.53; adjusted for smoking status). In 1985-2007, the incidence of RA rose by an increment of 9.2 per 100,000 among women. Obesity accounted for 4.8 per 100,000 (52%) of this increase. Conclusion Obesity is associated with a modest risk for developing RA. Given the rapidly increasing prevalence of obesity, this has had a significant impact on RA incidence and may account for much of the recent increase in the incidence of RA.
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