Association between time spent outdoors during the day and biological age acceleration among UK Biobank participants

We aimed to determine whether time spent outdoors during the day (TOD) is associated with biological age (BA) acceleration, and to identify the optimal TOD. BA acceleration was defined as the residual from regressing Klemera-Doubal method BA (KDM-BA) or phenotypic age (PhenoAge) on chronological age. Generalized additive models were applied to evaluate the associations of TOD with BA acceleration in summer and winter, separately. The optimal TOD associated with the lowest KDM-BA acceleration was two hours/day in summer and one hour/day in winter. For PhenoAge acceleration, the optimal TOD was one hour longer. Both KDM-BA and PhenoAge acceleration increased with TOD when it was longer than the optimal TOD. However, PhenoAge acceleration decreased with TOD when it was shorter than the values of optimal TOD in both summer (change in BA acceleration associated with each hour increase in TOD [$\hat{\beta }$] = -0.077 [95% CI, -0.099, -0.055]) and winter ($\hat{\beta }$ = -0.106 [95% CI, -0.130, -0.082]); a negative association was only observed between TOD and KDM-BA acceleration in winter ($\hat{\beta }$ = -0.048 [95% CI, -0.089, -0.007]). Our results highlight the role of appropriate daytime outdoor activity time in promoting healthy aging.