A high body mass index is associated with reduced risk of rheumatoid arthritis in men, but not in women.

To investigate the impact of overweight and obesity on the risk of RA. From two large population-based health surveys (30 447 and 33 346 participants), individuals who developed RA after inclusion were identified by linkage to four different registers and a structured review of the medical records. Matched controls were selected from the corresponding health survey database. The impact of overweight or obesity (BMI > 25 kg/m(2)) compared with normal BMI (18.5-25 kg/m(2)) on the risk of RA was examined in conditional logistic regression models, stratified by sex. A total of 172 (36 men/136 women) and 290 (151 men/139 women) individuals were diagnosed with RA after inclusion in the two health surveys. The median time from inclusion to RA diagnosis was 5 years and 12 years, respectively. In men, being overweight or obese at inclusion in the health survey was associated with a reduced risk of subsequent development of RA in both cohorts [odds ratio (OR) = 0.33; 95% CI: 0.14, 0.76, and 0.60; 95% CI: 0.39, 0.91]. There was no such association in women (OR = 1.01; 95% CI: 0.65, 1.54, and 1.37; 95% CI: 0.86, 2.18). Estimates were similar in analyses adjusted for potential confounders, including smoking. A high BMI was associated with a reduced risk of future RA in men, but not in women. Factors related to adipose tissue may contribute to mechanisms that are protective from RA in men.