Influence of Season and Latitude on the Cutaneous Synthesis of Vitamin D₃: Exposure to Winter Sunlight in Boston and Edmonton Will Not Promote Vitamin D₃Synthesis in Human Skin*

TLDR

Sunlight’s UVB radiation converts skin 7‑dehydrocholesterol to previtamin D₃, but the extent of this conversion varies with latitude and season, a relationship that remains poorly understood. The authors built a model to assess how seasonal and latitudinal variations affect sunlight’s capacity to initiate cutaneous vitamin D₃ synthesis. The model shows that winter sunlight in Boston (Nov–Feb) and Edmonton (Oct–Mar) fails to produce previtamin D₃, whereas lower latitudes (34° N and 18° N) can photoconvert it mid‑winter, highlighting a latitudinally extended “vitamin D winter” that may warrant dietary supplementation.

Abstract

Sunlight has long been recognized as a major provider of vitamin D for humans; radiation in the UVB (290-315 nm) portion of the solar spectrum photolyzes 7-dehydrocholesterol in the skin to previtamin D3, which, in turn, is converted by a thermal process to vitamin D3. Latitude and season affect both the quantity and quality of solar radiation reaching the earth's surface, especially in the UVB region of the spectrum, but little is known about how these influence the ability of sunlight to synthesize vitamin D3 in skin. A model has been developed to evaluate the effect of seasonal and latitudinal changes on the potential of sunlight to initiate cutaneous production of vitamin D3. Human skin or [3 alpha-3H]7-dehydrocholesterol exposed to sunlight on cloudless days in Boston (42.2 degrees N) from November through February produced no previtamin D3. In Edmonton (52 degrees N) this ineffective winter period extended from October through March. Further south (34 degrees N and 18 degrees N), sunlight effectively photoconverted 7-dehydrocholesterol to previtamin D3 in the middle of winter. These results quantify the dramatic influence of changes in solar UVB radiation on cutaneous vitamin D3 synthesis and indicate the latitudinal increase in the length of the "vitamin D winter" during which dietary supplementation of the vitamin may be advisable.