Sunlight regulates the cutaneous production of vitamin D3 by causing its photodegradation.
Webb AR, DeCosta BR, Holick MF.
J Clin Endocrinol Metab. 1989 May;68(5):882-7.
PMID: 2541158
doi:10.1210/jcem-68-5-882
Vitamin D3 proved to be exquisitely sensitive to sunlight, and once formed in the skin, exposure to sunlight resulted in its rapid photodegradation to a variety of photoproducts, including 5,6-transvitamin D3, suprasterol I, and suprasterol II.suprasterol I, and suprasterol II.
nfluence 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.
Webb AR, Kline L, Holick MF.
J Clin Endocrinol Metab. 1988 Aug;67(2):373-8.
PMID: 2839537
doi:10.1210/jcem-67-2-373
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.