Association between sunlight exposure and risk of rheumatic diseases: a prospective cohort study in the UK

Objective To investigate the relationships between sunlight exposure and the risk of three major rheumatic diseases (systemic lupus erythematosus [SLE], rheumatoid arthritis [RA], and gout) in a large-scale prospective cohort. Methods Data were derived from the UK Biobank (a nationwide prospective cohort in the United Kingdom), including 363,873 participants followed up from 2006 to 2022. Sunlight exposure was assessed by self-reported average daily outdoor time in summer and winter. Incident SLE, RA, and gout were identified using International Classification of Diseases, Tenth Revision (ICD-10) codes from linked hospital records. Multivariate Cox proportional hazards models, restricted cubic splines, subgroup analyses, and sensitivity analyses were performed to evaluate associations. Results During the 13-year follow-up, 335 SLE, 5,591 RA, and 7,485 gout cases were ascertained. Compared with sunlight exposure of 2.5 h/day, both lower and higher sunlight exposure were associated with increased risks of SLE (< 2.5 h/day: HR = 1.71, 95% CI:1.13, 2.59; >2.5 h/day: HR = 1.75, 95% CI:1.17, 2.63). We also observed increased risk associations of higher sunlight exposure with RA (HR = 1.15, 95% CI:1.06, 1.26) and gout (HR = 1.09, 95% CI:1.02, 1.18). Subgroup analyses suggested an age-related difference in the association between sunlight exposure and gout after Bonferroni correction (adjusted p value = 0.014). Conclusion Sunlight exposure was associated with disease-specific risks of SLE, RA, and gout.