Relationship Between Body Mass Index, Disease Activity, and Exercise in Ankylosing Spondylitis

Ankylosing spondylitis (AS) is associated with elevated cardiovascular risk, and obesity is a common, modifiable risk factor. Our aims were to assess the relationship of body mass index (BMI) with disease activity in AS patients and to assess the extent to which the effect is mediated through exercise.

We used data from a prospective AS cohort with a median follow-up of 7 years. To determine the association of BMI (kg/m² ) with disease activity as measured by the Ankylosing Spondylitis Disease Activity Score (ASDAS), we used generalized estimating equations with inverse probability weighting to account for repeated measures per subject and time-varying confounding. To estimate the direct effect of overweight/obese BMI on disease activity and the indirect effect through exercise, we performed a mediation analysis.

There were 183 subjects with available BMI and disease activity data (77% male, 70% White, mean ± SD age 40.8 ± 13.3 years). Higher BMI was significantly associated with higher disease activity over time; on average, for a 1 kg/m² higher BMI, the ASDAS was 0.06 units higher (95% confidence interval 0.04-0.08) after adjustment for important confounders. The direct effect of an overweight/obese BMI accounted for most of the total effect on disease activity, with a smaller indirect effect mediated by exercise (7%).

Higher BMI was associated with higher disease activity in a prospective AS cohort. We found that being overweight/obese largely influenced disease activity directly rather than indirectly through exercise. Other mechanisms, such as increased inflammation, may better explain the obesity-disease activity association.