Higher Serum 25-Hydroxyvitamin D Levels in School-Age Children Are Inconsistently Associated with Increased Calcium Absorption

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Abstract
Context: Increasing serum 25-hydroxyvitamin D (25-OHD) in adults may enhance calcium absorption (Ca-abs). There are few similar pediatric data leading to uncertainty about the optimal target for 25-OHD to maximize Ca-abs. Objective: Our objective was to evaluate the relationship between 25-OHD and Ca-abs in a large cohort of school-age children and adolescents. Design: We evaluated data from 439 Ca-abs measurements performed using dual-tracer stable isotope techniques conducted at our center over a 15-yr period in 251 healthy children, 4.9–16.7 yr of age. Results: Serum 25-OHD ranged from 28 to 197 nmol/liter (mean 85 ± 2 nmol/liter) (sem). Total Ca-abs (intake times fractional absorption) were significantly correlated to 25-OHD in the whole population (r = 0.16, P = 0.001). This relationship was closer in the 197 studies in early puberty (Tanner 2 or 3, r = 0.35, P < 0.001) and not significant in pre- or late pubertal subjects. For the whole population, fractional Ca-abs adjusted for calcium intake were slightly but significantly higher at 25-OHD of 28–50 nmol/liter (0.344 ± 0.019) compared with 25-OHD of 50–80 nmol/liter (0.280 ± 0.014) or 25-OHD greater than 80 nmol/liter (0.297 ± 0.015, P < 0.01 for each), suggesting adaptation to moderately low 25-OHD values. Conclusion: There is no consistent pattern of relationship between 25-OHD and either fractional or total calcium absorption in school-age children. However, there appears to be a modest calcium absorptive response to higher 25-OHD during early puberty.