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Seasonal Differences in Serum Vitamin D Binding Protein in Exclusively Breast‐Fed Infants: Negative Relationship to Sunshine Exposure and 25‐Hydroxyvitamin D

Summary: Vitamin D binding protein (DBP) is the major carrier for vitamin D and its metabolites in serum. DBP increases in pregnancy and decreases in cirrhosis; no seasonal variation has been reported in adults. We observed significant seasonal differences in 41 exclusively breast‐fed infants who were less than 6 months of age. Winter DBP concentrations exceeded summer DBP concentrations: 398 ± 22 versus 297 ± 20 &mgr;g/ml (mean ± SEM). The mean concentration for spring and fall was 329 ± 25 &mgr;g/ml. Maternal DBP concentrations did not differ by season. A sunshine exposure score, previously verified, was used to document time and body surface exposed to the sun. DBP was inversely related to sun exposure (r = −0.46, p = 0.005). Infant DBP was significantly and negatively correlated with 25‐hydroxyvitamin D concentrations (r = − 0.38, p = 0.02). We speculate that serum DBP fluctuations are a response to varying vitamin D needs: increased serum DBP occurs in low vitamin D status to maximize uptake of vitamin D from skin.