Role of Change in Vitamin D Metabolism with Age in Calcium and Phosphorus Metabolism in Normal Human Subjects*

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Abstract
This study describes the age-related changes of vitamin D metabolism and its related hormones, immunoreactive PTH (iPTH) and calcitonin (CT) in normal human subjects. The objective was to assess their roles in the changes in metabolism of calcium and phosphorus with age. Serum calcium and phosphorus levels declined linearly with age from newborn infants to older adults (r = −0.385, P < 0.01; r = −0.568, P < 0.01). The serum calcium and phosphorus levels in adults of 51 yr of age or more were significantly lower than those in children and younger adults of 50 yr of age or less (P < 0.025, P < 0.01), whereas the calcium and phosphorus levels in cord blood were significantly higher than those in children and younger adults (P < 0.025, P < 0.01). The serum concentration of lα,25-dihydroxyvitamin D (lα,25-(OH)2-Vit D) did not change in children and younger adults, being 42.0 ± 1.4 (SE) pg/ml, but it significantly decreased to 31.4 ± 1.9 pg/ml in older adults (P < 0.01). There were no significant age-related changes in the serum concentrations of 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, or vitamin D-binding protein (DBP) among children, younger adults and older adults. The concentrations of all vitamin D metabolites and DBP in cord serum were significantly lower than those in children and younger adults (P < 0.01). Serum iPTH levels were higher in older adults (P < 0.05) and lower in cord blood (P < 0.1), compared with those in children and younger adults, whereas the serum CT level was higher in cord serum (P < 0.01). No sex differences were found in the serum concentrations of calcium, phosphorus, vitamin D metabolites, DBP, iPTH, and CT. The serum concentration of calcium or phosphorus did not correlate significantly with that of lα,25-(OH)2-Vit D by simple correlation analysis. Multivariate analysis, however, showed that the change in the serum concentration of la,25-(OH)2-Vit D, as well as iPTH and CT, contributed to their correlation with the change in the serum concentrations of calcium and phosphorus. These data indicate that change in vitamin D metabolism might play some role in the age-related change of serum calcium and phosphorus levels in children and adults, but that calcium and phosphorus metabolism in the fetus might be regulated by some mechanisms other than vitamin D metabolism.