Environmental determinants of previtamin D synthesis in the United Arab Emirates

Abstract

Despite abundant sunshine throughout the year, vitamin D deficiency is endemic in the UAE. Solar radiation within the UVB range of the spectrum is required for the photosynthesis of previtamin D3 in the skin. Atmospheric transmission of UVB is strongly influenced by atmospheric conditions and solar zenith angle. We investigated the effects of diurnal and seasonal variation on the availability of sufficient UVB radiation for adequate previtamin D3 synthesis using an established in vitro model. Borosilicate ampoules of 7-dehydrocholesterol, the precursor of previtamin D3, in ethanol (50 µg/mL) were exposed to direct sunlight in an urban area of Abu Dhabi, at one hourly intervals between 0800 and 1700, on one day of each month over a period of one year. Conversion to previtamin D3, vitamin D3 and metabolically inactive photoisomers was analyzed using high performance liquid chromatography. The efficiency of 7-dehydrocholesterol conversion to previtamin D3 varied estimated UVB intensity. At the latitude of Abu Dhabi (24.2 N) previtamin D3 synthesis can occur throughout the year. However very little if any previtamin D3 was produced before 0900 hrs.and after 1600 hrs. Local conditions in Abu Dhabi are likely sufficient to maintain vitamin D levels throughout the year given adequate sun exposure.

Keywords: 7-dehydrocholesterol; UVB; atmospheric modeling; lumisterol; previtamin D3; tachysterol; vitamin D3.

Figure 1.

Diurnal variation in 7-dehydrocholesterol conversion in vitro to previtamin D3 or vitamin D3, lumisterol or tachysterol (%) following 1 hour exposure to sunlight, during each month of the year.

Figure 2.

Seasonal variation in in vitro 7-dehydrocholesterol conversion to previtamin D, expressed as area under the curve of percentage conversion throughout the day in each month. Values are normalized to July.

Figure 3.

Observed conversion of 7-dehydrocholesterol to previtamin D3 and vitamin D3 (%) plotted against conversion predicted by the study model, based on estimated irradiance and action spectrum of UVB on conversion (%). Predicted = (0.794 * observed) + 0.123, R² = 0.823, p < 0.0001.

Figure 4.

Seasonal variation in 7-dehydrocholesterol conversion in vitro in ampoules exposed to direct sunlight between 12:00 and 13:00 worldwide. Data supplied by MF Holick.