1265 SEASONAL CHANGES IN SERUM VITAMIN D BINDING PROTEIN IN INFANCY: RELATION TO SUN EXPOSURE

Abstract

Vitamin D binding protein (DBP) is the major carrier for vitamin D and its metabolites in serum. Its physiologic regulation is unclear; DBP increases in pregnancy and decreases in cirrhosis; no seasonal variation has been reported in adults. We hypothesized that serum DBP in infants would not vary by season. 41 exclusively breast-fed, non-D supplemented infants <6 mos of age were studied. DBP was measured by radial immunodiffusion: adult range, 276-505 ng/ml, cv 2.9% intra- and 7.6% inter-assay. 25-Hydroxyvitamin D (25-OHD) as an indicator of vitamin D status was measured by protein binding assay. Winter DBP exceeded summer: 377±12 vs 302±7 ug/ml (x+sem, p=.003). Serum 25-OHD exhibited an opposite pattern: 14±2 vs 26±2 ng/ml (p<.001). Maternal DBP did not differ by season: 374 and 373 ug/ml for winter and summer. An ultraviolet exposure score, previously verified, was used to document time and body surface exposed to sun. DBP was inversely related to sun exposure (r= -.41, p=.01). Infant DBP was significantly & negatively correlated with 25-OHD (r=-0.38, p=0.02). Two subsequent independent studies, 1 cross-sectional & 1 longitudinal, yielded similar results: infant DBP being significantly higher in winter vs summer. Thus vitamin D binding protein in exclusively breast fed infants is elevated in winter (vs summer), in low sun exposure, and in low vitamin D status as reflected by low serum 25-OHD; we speculate that serum vitamin D binding protein fluctuations are a response to varying vitamin D needs: increased DBP occurs in low vitamin D status to maximize uptake of vitamin D from skin.

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