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Vitamin D3 is made in the skin when 7-dehydrocholesterol reacts with UVB ultraviolet light at wavelengths between 270–300 nm, with peak synthesis occurring between 295-297 nm.[12][13] These wavelengths are present in sunlight when the UV index is greater than 3. At this solar elevation, which occurs daily within the tropics, daily during the spring and summer seasons in temperate regions, and almost never within the arctic circles, adequate amounts of vitamin D3 can be made in the skin after only ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen. With longer exposure to UVB rays, an equilibrium is achieved in the skin, and the vitamin simply degrades as fast as it is generated.

Vitamin D3 is the natural human form of vitamin D made in the skin when cholesterol reacts with sunlight. (Do not confuse D3 with D2 which may be found in small amounts in multivitamins.) D3 is being widely studied in larger (than FDA recommended) doses for cancer. In a slightly-altered, patentable form, D3 is currently being developed as a drug.

At an April 2006 cancer conference, evidence was presented that the risk of breast cancer was reduced by 50% when vitamin D in the blood was 52 nanograms per milliliter, which equals to no less than 1000 international units (IU) of vitamin D every day whether in the diet or in supplements. The average person only gets 310 IU a day. (Evidence of Need for Increasing Dietary Vitamin D in Food. Abstract 4008 - AACR Conference.)