The relationship between oral vitamin D supplementation and cutaneous photosynthesis is not well understood.

We sought to provide estimates of the equivalency of vitamin D production from natural sun exposure versus oral supplementation.

Using the FastRT simulation tool, we determined sun exposure times needed to achieve serum vitamin D₃ concentrations equivalent to 400 or 1000 IU vitamin D for individuals of various Fitzpatrick skin types living in Miami, FL, and Boston, MA, during the months of January, April, July, and October.

Peak ultraviolet B irradiation for vitamin D synthesis occurs around 12 pm Eastern Standard Time (EST). In Boston, MA, from April to October at 12 pm EST an individual with type III skin, with 25.5% of the body surface area exposed, would need to spend 3 to 8 minutes in the sun to synthesize 400 IU of vitamin D. It is difficult to synthesize vitamin D during the winter in Boston, MA. For all study months in Miami, FL, an individual with type III skin would need to spend 3 to 6 minutes at 12 pm EST to synthesize 400 IU. Vitamin D synthesis occurs faster in individuals with lighter Fitzpatrick skin types. The duration to attain 1000 IU of vitamin D is longer in all scenarios.

Results of the computer model are only approximations. In addition, calculations were made based on the assumption that ¼ of 1 minimal erythema dose directed at ¼ body surface area is equal to 1000 IU of oral vitamin D.

Although it may be tempting to recommend intentional sun exposure based on our findings, it is difficult, if not impossible to titrate one’s exposure. There are well-known detrimental side effects of ultraviolet irradiation. Therefore, oral supplementation remains the safest way for increasing vitamin D status.