Background We have previously reported a negative association between body mass index (BMI) and the risk of RA in men, whereas BMI did not affect RA development in women. The role of physical activity (PA) has not been studied in this context.
Objectives To investigate the impact of self-reported PA on the risk of RA in a nested case-control study, based on a population based survey.
Methods A total of 30447 subjects (12121 men; 18326 women) were included in a population based health survey. Data on PA were based on a modified questionnaire adapted from the Minnesota Leisure Time Physical Activity Questionnaire. Participants were asked to estimate the number of minutes per week, for each of the four seasons, they spent performing 17 different physical activities. The answer was multiplied by an intensity factor depending on the activity, creating a physical activity score (PAS). This method has been validated against accelerometer-monitoring, an objective measure of PA, in a subset of the present health survey population. From this population, we identified individuals who developed RA after inclusion by linkage to 4 different RA registers. In a structured review of the medical records, patients were classified according to the 1987 American College of Rheumatology criteria for RA. Four controls for each validated case, matched for sex, year of birth and year of screening, who were alive and free of RA when the index person was diagnosed with RA, were selected from the health survey database. The impact of the PAS on the risk of RA was examined in conditional logistic regression models, stratified by sex.
Results One-hundred and seventy-two patients (36 men/136 women, mean age at RA diagnosis 63 years) had a validated diagnosis of RA after inclusion in the health survey. The median time from inclusion to RA diagnosis was 5 years (range 1–13). Data on PA was available from 161 cases (31 men) and 644 controls (132 men). The PAS had no significant impact on future development of RA in women [odds ratio (OR) 1.07 per standard deviation (SD); 95% confidence interval (CI) 0.86-1.34]. In contrast, men with higher PAS had an increased risk of RA (OR 1.65 per SD; 95% CI 1.05-2.58). The association between higher PAS and subsequent RA development in men remained significant in models adjusted for smoking (p=0.033) or level of formal education (p=0.048). Men with a high self-reported PA (highest quartile of PAS) had a higher estimated risk of RA compared to those with low to moderate PA (quartiles 1-3 of PAS), although the association did not reach statistical significance (OR 2.62; 95% CI 0.90-7.65; adjusted for smoking). The PAS was negatively correlated with BMI in men (r: -0.23; p=0.003), and there was a similar, but weaker, correlation in women (r: -0.09; p=0.017).
Conclusions In this nested case-control study, men reporting high levels of PA were found to be at greater risk of developing RA. There was no such association in women. Higher level of PAS was found to correlate with lower BMI, especially in men. Factors related to PA and metabolic status may influence the development of RA in men.
Disclosure of Interest None declared