Sunlight exposure is associated with lower blood glucose levels and type 2 diabetes

Abstract Sunlight exposure is a known risk factor for skin cancer, but a growing body of evidence indicates that it also provides significant health benefits through mechanisms potentially protective against type 2 diabetes. We examined associations between acute sunlight exposure and glucose levels and chronic sunlight exposure and type 2 diabetes using data from participants of the UK Biobank cohort study. Daily ultraviolet (UV)-A estimates from the Japan Aerospace Exploration Agency were used to calculate 2-day and 7-day average ambient sunlight exposures before blood glucose tests for each participant, based on their clinic assessment date and residential location. Linear fixed-effects regression models with robust standard errors were employed to estimate associations between sunlight exposure and blood glucose levels, adjusting for maximum temperature. Restricted cubic spline models were used to assess potential non-linearity in these associations. We also derived a solarium user variable (non-user, occasional and frequent) and estimated adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for associations between solarium use and type 2 diabetes. In winter, a one W/m 2 increase in 2-day and 7-day average sunlight exposure (as measured by UVA) was associated with 0.015 mmol/L and 0.027 mmol/L lower blood glucose levels. In spring, a one W/m 2 increase in 7-day average sunlight exposure was associated with 0.007 mmol/L lower blood glucose levels. Nonlinear analyses indicated that the inverse association between sunlight and glucose was non-linear, with the steepest reductions occurring at low sunlight levels and a plateau at higher exposures. Occasional and frequent solarium users were at a lower hazard of type 2 diabetes over follow-up compared to non-users (HR = 0.86; 95% CI 0.77 to 0.96 and HR = 0.77; 95% CI 0.63 to 0.95). Our findings suggest that acute sunlight exposure may lower blood glucose levels, while chronic exposure may reduce the likelihood of type 2 diabetes diagnosis.