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An evaluation of the relative contributions of exposure to sunlight and of diet to the circulating concentrations of 25-hydroxyvitamin D in an elderly nursing home population in Boston. Webb AR, Pilbeam C, Hanafin N, Holick MF. Am J Clin Nutr. 1990 Jun;51(6):1075-81. PMID: 2349922

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.

Ultraviolet exposure scenarios: risks of erythema from recommendations on cutaneous vitamin D synthesis. Webb AR, Engelsen O. Adv Exp Med Biol. 2008;624:72-85. PMID: 18348448 DOI: 10.1007/978-0-387-77574-6_6