GPS-derived outdoor time and associated vitamin D status

Background: Vitamin D is essential for health and sunlight (ultraviolet radiation band B (UVB) radiation) is a major source. However, individual sunlight exposure is difficult to measure, limiting our ability to identify people at risk of deficiency and to design effective interventions. Therefore, to address limitations of self-reported data, we developed and assessed a method that uses global positioning system (GPS) data to generate individual outdoor time measures relevant to vitamin D synthesis and compared these with serum vitamin D levels. Method: GPS and accelerometer sensors were used to measure time spent outdoors in San Diego adults (n = 130) for an average of 12.6 days. Three categories of outdoor time measures were generated including total outdoor time, number of outdoor “bouts” (distinct periods of outdoor time), and number of days with outdoor bouts. Within these categories, outdoor time was assessed between 10 am–2 pm (peak UVB) and a broader time UVB-relevant time window 8 am–4 pm. Satellite derived daily Ultraviolet (UV) data were used to generate UV weighted versions of the total outdoor time and number of outdoor bout measures. Participants also completed 24-hour food recalls and provided fasting blood samples. Adjusted generalized linear regression models were estimated for associations of each exposure measure with serum vitamin D level. Results: Across all GPS-derived exposure metrics, point estimates were consistently positive, indicating a uniform directional pattern in which greater outdoor time was associated with higher serum vitamin D concentrations, although most confidence intervals included the null. In sensitivity analyses excluding participants who reported taking vitamin D supplements, this pattern persisted. Two measures of longer outdoor exposure provided stronger evidence of an association: the number of 45-minute bouts between 10 am and 2 pm (β = 4.36, 95% CI 0.32 to 8.40) and the number of days with a 45-minute bout (β = 4.38, 95% CI 0.16 to 8.59). In both cases, the confidence intervals did not include the null. More days with shorter bouts also showed a relatively large point estimate, though with substantial imprecision. UV-weighted exposure metrics did not outperform the simpler GPS-derived time‑based measures. Conclusion: GPS-derived outdoor time measures, including those weighted by satellite-derived daily UV, demonstrated consistent positive associations with vitamin D levels, with the strongest evidence observed for longer exposures during peak UVB periods. These findings indicate that simple, time-based GPS metrics and their UV-weighted variants may provide practical and informative indicators of UVB-relevant exposure patterns in real-world settings.