Harnessing natural sunlight indoors: sensor-regulated therapeutic approach to enhance vitamin D status in humans

Vitamin D deficiency is a widespread global health issue, even in sun-rich regions like the United Arab Emirates (UAE), where environmental, behavioral, and cultural factors limit skin exposure to ultraviolet B (UVB) radiation. To address this, we developed and assessed a sensor-guided indoor sunlight reflection system that redirects natural sunlight indoors for controlled vitamin D synthesis. Sixteen healthy adults participated in a pre-post interventional study using this system, which features automated solar tracking and UV-index-based exposure control. Participants underwent four sessions weekly for eight weeks, each lasting 10–20 min depending on skin tone and UV intensity. Serum samples were taken at the start, middle, and end of the study. We measured eight vitamin D metabolites and epimers with a high-specificity LC-MS/MS workflow that differentiates 25(OH)D2/D3 and C-3 epimers. The intervention was associated with significant, time-dependent increases in serum 25(OH)D3, total 25-hydroxyvitamin D, and 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] (p < 0.001 for all), with large effect sizes (partial η² = 0.35–0.44). Mean 25(OH)D3 increased from 1.99 to 4.11 ng/mL (+ 106%), total 25(OH)D from 5.0 to 7.7 ng/mL (+ 54%), and 1α 25(OH)2D3 from 0.33 to 0.74 ng/mL (+ 124%). Meanwhile, 25(OH)D2 and its epimer decreased slightly (-28% and − 30%), supporting endogenous D3 production from sunlight. No changes in 7α-hydroxycholesten-3-one (7αC4) suggested stable hepatic cholesterol breakdown. Throughout the study, no adverse events or safety issues occurred. Controlled indoor exposure to full-spectrum sunlight was associated with increased dermal vitamin D3 synthesis, providing physiological effects comparable to outdoor sunlight exposure without heat or overexposure risks. This study suggests that sensor-guided indoor full-spectrum sunlight exposure is a feasible and well-tolerated approach to improve vitamin D status in populations with limited sun exposure. By integrating optical engineering, biosafety algorithms, and precise analytics, this approach offers a potentially sustainable strategy for addressing vitamin D deficiency in indoor-living populations.