Is the relationship between deprivation and outcomes in rheumatoid arthritis mediated by body mass index? A longitudinal cohort study

Objectives: To understand the relationships between deprivation and obesity with self-reported disability and disease activity in people with RA; and whether BMI mediates the relationship between area-level deprivation and these outcomes.
Methods: Data came from the Rheumatoid Arthritis Medication Study (RAMS), a one-year multi-centre prospective observational cohort of people with RA recruited from rheumatology centres across England commencing methotrexate for the first time. 1529 and 1626 people were included who had a baseline and at least one follow-up measurement at 6 or 12 months of Health Assessment Questionnaire – Disability Index (HAQ-DI) and Disease Activity Score-28 (DAS28), respectively. Linear mixed models estimated the associations of deprivation and obesity with repeated measures HAQ-DI and DAS28. Causal mediation analyses estimated the mediating effect of BMI on the relationship between deprivation and RA outcomes.
Results: Higher deprivation and obesity were associated with higher disability (adjusted regression coefficients highest vs lowest deprivation fifths 0.32 (95% CI 0.19, 0.45); obesity vs no obesity 0.13 (95% CI 0.06, 0.20)) and higher disease activity (adjusted regression coefficients highest vs lowest deprivation fifths 0.34 (95% CI 0.11, 0.58); obesity vs no obesity 0.17 (95% CI 0.04, 0.31)). BMI mediated part of the association between higher deprivation and self-reported disability (14.24%) and disease activity scores (17.26%).
Conclusions: People with RA living in deprived areas have a higher burden of disease, which is partly mediated through obesity. Weight-loss strategies in RA could be better targeted towards those living in deprived areas.