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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.

25-Hydroxylation of vitamin D3: relation to circulating vitamin D3 under various input conditions. Heaney RP, Armas LA, Shary JR, Bell NH, Binkley N, Hollis BW. Am J Clin Nutr. 2008 Jun;87(6):1738-42. PMID: 18541563 Conclusions: At physiologic inputs, there is rapid conversion of precursor to product at low vitamin D3 concentrations and a much slower rate of conversion at higher concentrations. These data suggest that, at typical vitamin D3 inputs and serum concentrations, there is very little native cholecalciferol in the body, and 25(OH)D constitutes the bulk of vitamin D reserves. However, at supraphysiologic inputs, large quantities of vitamin D3 are stored as the native compound, presumably in body fat, and are slowly released to be converted to 25(OH)D.

Benefits of Vitamin D Supplementation Joel M. Kauffman, Ph.D. Journal of American Physicians and Surgeons Volume 14 Number 2 - Summer 2009 Clinical trials show that vitamin D supplementation at higher levels than previously recommended is beneficial for many conditions. It decreases the frequency of falls and fractures, helps prevent cardiovascular disease, and reduces symptoms of colds or influenza. Benefits are also seen in diabetes mellitus, multiple sclerosis, Crohn disease, pain, depression, and possibly autism. Sunlight does not cause an overdose of vitamin D production, and toxicity from supplementation is rare. Dose recommendations are increasing, but appear to be lagging the favorable trial results. A number of common drugs deplete vitamin D levels, and others may limit its biosynthesis from sunlight. People with adequate levels from sun exposure will not benefit from supplementation. While dietary intake is helpful, supplementation is better able to raise serum 25-hydroxyvitamin D , the major circulating metabolite, to the level now thought adequate, 30-50 ng/mL. Where there is inadequate daily sun exposure, oral doses of 1,000-2,000 IU/d are now considered routine, with much higher doses (up to 50,000 IU) for rapid repletion now considered safe.

Dusso AS, Brown AJ, Slatopolsky E. Vitamin D. Am J Physiol Renal Physiol. 2005 Jul;289(1):F8-28. Review. PMID: 15951480 [PubMed - indexed for MEDLINE]