Abstract
The plasma 25-hydroxyvitamin D (25-(OH)D) concentration in the Saudi population was studied in relation to seasonal variations in the amount of ultraviolet radiation reaching the earth's surface in Riyadh. During 3 years (1985, 1986, 1987) solar ultraviolet radiation (290-320 nm and 300-400 nm) was monitored hourly, but the seasonal variations observed were non-significant. The plasma concentration of 25-(OH)D in adult males gradually increased from the lowest value recorded in December (6.6 +/- 4.7 ng/ml) reaching a peak value in April (20.1 +/- 7.6 ng/ml). After this the concentration fluctuated during the summer months reaching low value in September (10 +/- 4.2 ng/ml, n = 23).There was no significant difference between the concentration of 25-(OH)D in January and August. The variations in the concentration of 25-(OH)D in adult females showed relatively similar pattern to that of adult males. To examine further the seasonal variations in the plasma concentration of 25-(OH)D, in the Saudi population, the plasma samples collected from housewives only, for 1 year, were analysed for 25-(OH)D and plotted against the months. A similar pattern as that demonstrated for adult males and overall adult females was obtained. It seems that the variations in the circulating concentration of 25-(OH)D in the Saudi population do not follow the seasonal variations in the amount of ultraviolet light.
These results may suggest that as the temperature increases during the summer the exposure of the Saudi population to solar ultraviolet radiations is decreased and hence the concentration of plasma 25-(OH)D decreases.