Background 25-hydroxyvitamin D3 (25(OH)D3) affects immune function, bone health, and reproduction. The precursor of 25(OH)D3, vitamin D3, is synthesized upon ultraviolet (UV) radiation exposure, the levels of which vary. Methods In a one-year observational study, we used multiple statistical approaches to examine 25(OH)D3 metabolism and UV exposure in 217 healthy men (aged 30–50 years) not using vitamin D supplementation, living between Athens and the Arctic Circle. Complementary data were obtained from six consecutive crews of Antarctic expeditioners. Results We show that, while vitamin D3 synthesis tracks UV exposure, resulting 25(OH)D3 concentrations are strongly shaped by the kinetics of 25(OH)D3 synthesis and degradation. Notably, the efficiency of 25(OH)D3 synthesis is high at low vitamin D3, but decreases markedly at higher vitamin D3. This results in comparable summer 25(OH)D3 maxima across European sites. Additionally, the kinetics of 25(OH)D3 degradation induces rapid seasonal concentration shifts, yet also exerts a moderating effect by dampening both seasonal maximum and minimum 25(OH)D3. Comparison of the seasonal patterns of plasma parameters and environmental parameters at European sites shows that vitamin D3 follows the UV exposure pattern, while 25(OH)D3 concentrations align with local temperature patterns. Antarctic data support the alignment between 25(OH)D concentrations and temperature. Conclusions The kinetics of 25(OH)D3 synthesis and degradation modulate the effect of variable UV exposure on 25(OH)D3 concentrations. This results in a regulated plasma signal reflecting local seasonal parameters. While absolute 25(OH)D3 concentrations are commonly investigated, future studies should also examine their temporal dynamics as a biological signal.
Scholze et al. (Thu,) studied this question.