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

Objective: To investigate the impact of overweight and obesity on the risk of RA.

Methods: 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.

Results: 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.

Conclusion: 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.