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 to determine whether BMI mediates the relationship between area-level deprivation and these outcomes. Methods: Data came from the Rheumatoid Arthritis Medication Study (RAMS), a 1-year multicentre prospective observational cohort of people with RA recruited from rheumatology centres across England commencing MTX for the first time. A total of 1529 and 1626 people were included who had a baseline and at least one follow-up measurement at 6 or 12 months of HAQ-Disability Index (HAQ-DI) and DAS in 28 joints (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 DAS (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.