Objective: To evaluate the effect of seasonal variations in UV B-exposure on calcium absorption and bone turnover in young women with the overall goal to assess the potential benefit of a vitamin D supplementation during wintertime.
Design: Cross-sectional study.
Setting: Area of Bonn, Germany (51 degrees N).
Subjects: Thirty-eight women (24.5+/-0.5 y) studied in winter and 38 females of the same age (24.7+/-0.4 y) studied in summer.
Results: As estimated by a 4 d food record, both groups had similar dietary calcium and phosphorus intakes (> 1200 mg/d, respectively) covering actual recommendations. Significant reductions in serum concentrations of 25-hydroxyvitamin D (25OHD) and calcitriol, fractional calcium absorption (Fc220, measured by means of a stable strontium test), 24h urinary calcium and 24h urinary phosphorus excretion were observed during wintertime. 25OHD but not calcitriol was correlated with Fc220 values and with 24h urinary phosphorus excretion. Moreover, Fc220 was related to 24 h urinary calcium excretion. Fasting 2 h-urinary deoxypyridinoline concentrations (biomarker of bone resorption) and serum levels of carboxyterminal propeptide of type I procollagen (biomarker of bone formation) showed no differences between summer and winter.
Conclusions: Our data indicate a decrease in intestinal calcium and phosphorus absorption during wintertime, most likely because of a reduction in serum 25OHD levels. Since bone turnover was not affected by the seasonal differences in mineral metabolism, there is no objective for young women with high calcium intake to supplement vitamin D during wintertime.