25-Hydroxyvitamin D₃ metabolism modulates the effect of variable UV exposure on 25-hydroxyvitamin D₃ plasma...

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.

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.

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.

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.