Abstract
The concentrations of 25OH-D (n=62) and 1,25OH2D (n=231) were measured in the sera of normal subjects aged 18 months to 35 years. Samples were obtained in all months of the year to assess the effect of the seasons on serum concentration. Serum 25OHD3 was 16.3±7.0 ng/ml (SE) in Jan.-March and rose to 24.4±7.6 in April-June and to 31.2±8.5 in July-Sept., p < .001. In Oct.-Dec. it fell to 20.9±7.2, p < .01. Both total 25OH-D and 25OHD3 are higher during the months April-Sept. No seasonal variation in 250HD2 (x ± SE = 7.3±5.2 ng/ml) or 1,25OH2D (x = 43±2 pg/ml) were found. Age-related differences in 250HD2 and D3 were not apparent. The levels of 1,25OH2D are higher in adolescents and increase from 35±19 pg/ml (SD) at 1.5 to 10 years to 54±21 pg/ml at 10-20 years. In adults, 20-35 years, the levels fall to 28±16 pg/ml. Despite a seasonal variation in the precursor of 1,25OH2D, the levels of this most active metabolite of vitamin D do not vary with the seasons and presumed increased sunlight exposure. The levels of 1,25OH2D are higher during a period of rapid growth. The absence of seasonal variation is further evidence of the tight feedback regulation of the hormonal form of vitamin D.
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