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.