Jm. Pettifor et al., THE EFFECT OF SEASON AND LATITUDE ON IN-VITRO VITAMIN-D FORMATION BY SUNLIGHT IN SOUTH-AFRICA, South African medical journal, 86(10), 1996, pp. 1270-1272
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.