The relationship between ultraviolet radiation exposure and vitamin D status

Abstract

This paper reviews the main factors influencing the synthesis of vitamin D, with particular focus on ultraviolet radiation exposure. On the global level, the main source of vitamin D is the sun. The effect of solar radiation on vitamin D synthesis depends to some extent on the initial vitamin D levels. At moderate to high latitudes, diet becomes an increasingly important source of vitamin D due to decreased solar intensity and cold temperatures, which discourage skin exposure. During the mid-winter season, these factors result in decreased solar radiation exposure, hindering extensively the synthesis of vitamin D in these populations.

Keywords: UV; sun; ultraviolet; vitamin D.

Figure 1

Outline of vitamin D synthesis and metabolism. From [7].

Figure 2

CIE erythema action spectrum [60] (bold line) and the action spectrum for the formation of previtamin D in human skin [37 ](dots) with solar spectra (thin solid lines) measured at solar zenith angles of 25 and 75 degrees. The units of the irradiances in the solar spectra are Wm^-2nm^-1. From [15], and the relative effectiveness of the action spectra are unitless.

Figure 3

Required UV exposure times around noon for a cloudless sky, and typical conditions with respect to latitude and day of year to obtain approximately the equivalent of 400 IU when the face, neck and hands with type I skin are exposed. The red areas illustrate when this vitamin amount is not achievable from the sun. The required dose can be obtained in minutes in the black area. From [36,53].

Figure 4

Daily period (in hours) of vitamin D production in terms of time and latitude for a typical clear atmosphere and no surface reflection. From [83].