THE EFFECT OF SEASON AND LATITUDE ON IN-VITRO VITAMIN-D FORMATION BY SUNLIGHT IN SOUTH-AFRICA

Aims. To assess the effect of season and latitude on the in vitro form ation of previtamin D-3 and vitamin D-3 from 7-dehydrocholesterol (7-D HC) by sunlight in two cities in South Africa, Cape Town and Johannesb urg. Methods. An in vitro study utilising vials containing 7-DHC, whic h were exposed to sunlight for a period of 1 hour between 8:00 and 17: 00 on 1 day a month for a year. Previtamin D-3 and vitamin D-3 were se parated from 7-DHC by high-performance liquid chromatography, and the amounts formed were calculated with the use of external standards. Res ults. A marked seasonal variation in vitamin D-3 production was noted in Cape Town, with very little being formed during the winter months o f April through September. In Johannesburg, in vitro formation changed little throughout the year, and was similar to that found in Cape Tow n during the summer, During sunlit hours, vitamin D-3 production was m aximal at midday and small quantities were still being formed between 8:00 and 9:00, and between 16:00 and 17:00 during the summer, During w inter in Cape Town, peak formation at midday was less than one-third o f that in Johannesburg, and negligible amounts were formed before 10:0 0 and after 15:00. Conclusions. The previously documented seasonal var iation in serum 25-hydroxyvitamin D recorded in patients in Johannesbu rg is probably a consequence of the increased clothing worn and the de creased time spent out of doors during winter, rather than decreased u ltraviolet radiation reaching the earth. The limited in vitro formatio n of vitamin D-3 during winter in Cape Town may have clinical implicat ions insofar as the management of metabolic bone diseases like rickets and osteoporosis is concerned, Breast-fed infants resident in the are a are likely to suffer from vitamin D deficiency rickets unless vitami n D supplements are provided, or the mothers are encouraged to take th eir children out of doors.