Vitamin D status and sun exposure in India

Abstract

Background: Little if any cutaneous production of vitamin D3 occurs at latitudes above and below 35° N and 35° S during the winter months. It was postulated that those residing in tropics synthesize enough vitamin D3 year round. Several studies have documented the effect of latitude, season and time of the day on the cutaneous production of vitamin D3 in an ampoule model. Studies from India have shown high prevalence of vitamin D deficiency despite abundant sunshine.

Methods: We studied the influence of season and time of the day on synthesis of previtamin D3 in an ampoule model in Tirupati, (latitude 13.40° N and longitude 77.2° E) south India, between May 2007 to August 2008. Sealed borosilicate glass ampoules containing 50 μg of 7-DHC in 1 ml of methanol were exposed to sunlight hourly from 8 a.m. until 4 p.m. The percent conversion of 7-DHC to previtamin D3 and its photoproducts and the percent of previtamin D3 and vitamin D3 formed was estimated and related to solar zenith angle.

Results: The percent conversion of 7-DHC to previtamin D3 and its photoproducts and formation of previtamin D3 and vitamin D3 was maximal between 11 a.m. to 2 p.m. of the day during the entire year (median 11.5% and 10.2% respectively at 12.30 p.m.).

Conclusions: Therefore at this latitude exposure to sunlight between the hours of 11 a.m. and 2 p.m. will promote vitamin D production in the skin year round.

Keywords: 7-dehydrocholesterol (7-DHC); India; previtamin D3; sun exposure; vitamin D3; zenith angle.

Figure 1. Influence of time of day and season on synthesis of previtamin D₃ at Tirupati located at 13.4° N and 79.2° E. Figure shows the relation between zenith angles (ZA), percent conversion of 7-Dehydrocholesterol (7-DHC) to previtamin D₃ and photoproducts, and the percentage of previtamin D₃ and vitamin D₃ (%Pre D₃+D₃) formed from May 2007 to August 2008. The correlations between them are shown individually for each day. The maximum (max) and minimum (min) temperature (Temp) on the day of the study is given above each day studied. August 2007 is not depicted. r, correlation coefficient; P, significance.

Figure 2. Showing the mean ± SD of the zenith angles, percent conversion of 7-Dehydrocholesterol (7-DHC) to previtamin D₃ and photoproducts, and the percentage of previtamin D₃ and vitamin D₃ against time (for the study duration). The table below gives the individual values, minimum and maximum of the variables.

Figure 3. Figure depicting the percent conversion of 7-Dehydrocholesterol (7-DHC) to previtamin D₃ and photoproducts, and the percentage of previtamin D₃ and vitamin D₃ formed from May 2007 to August 2008 along with zenith angles of various time of the day during the whole study period.

Figure 4. Three-dimensional depictions of the percent conversion of 7-DHC to previtamin D₃ and photoproducts and percentage of previtaminD₃ and vitamin D₃ along with zenith angles of various time of the day during the whole study period. Maximum temperature on the day of the study is also depicted in the same graph.

Figure 5. Satellite picture of the country on the day of study at 11.30 h. First picture on the left upper panel (row 1) shows the location of study site (TIRUPATI—latitude 13.40° N and longitude 77.2° E). The date and maximum and minimum temperature on the day of study is shown in each picture. The satellite picture is downloaded from www.hinduonnnet.com under section miscellaneous—weather chart.

Figure 6. Percent conversion of 7-DHC to previtamin D₃ and photoproducts and formation of previtamin D₃ and vitamin D₃ for three similar months in consecutive years—May, June and July in 2007 and 2008.

Figure 7. (A) Graph showing the inverse correlation between the 25 (OH) D levels and latitude (r = −0.48; p < 0.0001) from various studies conducted in the country (Table 1). (B) The 25 (OH) D levels of various studies from India along with latitude and location from various studies conducted in the country (Table 1).