SOLAR ULTRAVIOLET-B RADIATION AND PHOTOPRODUCTION OF VITAMIN-D-3 IN CENTRAL AND SOUTHERN AREAS OF ARGENTINA

The incidence of nutritional rickets in the southern part of Argentina is 8-12 times higher than in the rest of the country. Winter 25(OH)D serum levels in normal population of southern areas are lower than in central and northern areas. To elucidate these differences, we compare d the photoconversion of provitamin D-3 (7-DHC) to previtamin D-3 in t wo cities: Ushuaia (latitude 55 degrees S) and Buenos Aires (34 degree s S). Ampules containing 7-DHC were exposed to sunlight one day in the middle of each month either from 10:30 a.m. to 2:30 p.m. or from 8:00 a.m. to 5:00 p.m. The percentages of photoproducts formed were determ ined by high performance liquid chromatography (HPLC). Previous studie s have proved that this is a valid model to assess ''in vitro'' the ph otoproduction of vitamin D-3 in human skin. Previtamin D-3 + vitamin D -3 formed in Ushuaia were less (p < 0.02) than those found in Buenos A ires during all seasons: summer, (X +/- SEM) 6.4 +/- 0.8% vs. 13.2 +/- 1.8%; autumn, 1.2 +/- 0.7% vs. 6.3 +/- 1.3%; winter, 0.8 +/- 0.7% vs. 3.6 +/- 0.7%; spring, 3.4 +/- 0.5% vs. 9.1 +/- 1.1% The photoproducts produced from 10:30 a.m. to 2:30 p.m. were similar for each month and latitude to those formed when the ampules were exposed from 8:00 a.m. to 5:00 p.m. We conclude that in Ushuaia there is a prolonged ''vitam in D winter'' during which cutaneous synthesis of vitamin D is absent, leading to lower serum values of 25(OH)D and contributing to the high er incidence of rickets.