Comorbidity and lifestyle, reproductive factors, and environmental exposures associated with rheumatoid arthritis

Objective-To evaluate the influence of lifestyle, reproduction, and some ex ternal factors on the development of rheumatoid arthritis (RA) and to descr ibe its comorbidity. Methods-Cases were identified retrospectively from 1980 to 1995 at the Univ ersity Hospital in Linkoping, Sweden. The study comprised 422 cases and 859 randomly selected population referents. Data on possible aetiological fact ors and comorbidity were collected by postal questionnaire. Results-The response rates were 67% among cases and 59% among referents. A decrease in the occurrence of atopic allergy was seen in the cases (odds ra tio (OR) 0.6, 95% confidence interval (CI) 0.4 to 1.0). There was a positiv e association between RA and insulin treatment (OR 10.2, 95% CI 1.7 to 60.8 ) in women, and women with a short fertile period had an increased risk of RA (OR 2.5, 95% CI 1.1 to 5.4). Current and previous smoking were associate d with increased risks for RA in both sexes, and in men a dose-response rel ationship was found with number of tobacco pack years (p for trend <0.005). The risk for RA decreased with increasing level of education in both men a nd women. Increased risks were seen in men born into households with privat e wells (OR 2.8, 95% CI 1.5 to 5.2), residentially exposed to mould (OR 4.6 , 95% CI 1.1 to 20.2), or exposed to farm animals (OR 3.3, 95% CI 0.7 to 16 .6). In women there were positive associations between RA and reporting a p revious joint injury (OR 2.5, 95% CI 1.0 to 6.6) and prolonged exposure to hair dyes (OR 1.9, 95% CI 0.8 to 4.5). Conclusions-RA, a disease with features of T helper 1 (Th1) dominated immun e response, was inversely associated with atopic allergy, a Th2 dominated c ondition, while there were indications of a strong positive association wit h Th1 related diabetes mellitus. The results support a causal relationship between smoking and RA. The level of education was inversely associated wit h RA, while there was a positive association between RA and certain residen tial factors in men and a short fertile period in women.