Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin.
Level 5 - mechanism / opinion, no new human data
Ex vivo and in vitro photochemical bench model
PubMed 2839537 · doi:10.1210/jcem-67-2-373
What was done
Human skin samples and radiolabeled [3 alpha-3H]7-dehydrocholesterol were exposed to sunlight on cloudless days across multiple latitudes—Boston (42.2° N), Edmonton (52° N), and southern locations (34° N and 18° N)—to quantify the photochemical conversion of 7-dehydrocholesterol to previtamin D3 by solar UVB radiation across different seasons.
What was found
No previtamin D3 was produced in human skin or substrate solutions exposed to sunlight in Boston from November through February. In Edmonton, absent previtamin D3 synthesis extended from October through March. At 34° N and 18° N, sunlight effectively photoconverted 7-dehydrocholesterol to previtamin D3 in the middle of winter. Specific quantitative conversion yields were not reported in the abstract.
Why it matters
This study defines the geographic and seasonal boundaries of the "vitamin D winter," showing that cutaneous vitamin D3 synthesis completely ceases for several months at northern latitudes regardless of time spent outdoors.
Limits
The study utilized ex vivo skin and in vitro models rather than measuring in vivo serum vitamin D levels in live human cohorts. The number of skin samples tested was not reported in the abstract. Factors such as cloud cover, air pollution, skin pigmentation variability, and clothing coverage were not evaluated in real-world human trials.
Cited by
- supports Living above the 35th parallel prevents sufficient atmospheric penetration of UVB radiation to synthesize adequate vitamin D in the skin during winter months.