Evidence suggests that vitamin D status in adults, as assessed by serum 25-hydroxyvitamin D (25-OHD), is positively associated with calcium absorption fraction and inversely associated with serum PTH. Few comparable pediatric data exist.
The objective of this study was to evaluate the relationships among vitamin D status, PTH, and calcium absorption in mid-pubertal boys and girls.
Calcium absorption was measured as part of an evaluation of the effects of prebiotics (inulin-type fructans) using a stable isotope method in 93 young adolescents, 12.7 ± 1.0 yr of age, receiving diets averaging approximately 900 mg/d calcium.
A significant positive relation to calcium absorption was found for serum 1,25-dihydroxyvitamin D(P = 0.048) and PTH(P = 0.007), but not for 25-OHD (P = 0.77). PTH was significantly inversely related to 25-OHD and was positively related to serum 1,25-dihydroxyvitamin D and osteocalcin. PTH was marginally significantly inversely related to lumbar spinal, but not whole body, bone mineral density.
These data suggest that in adolescents, especially in the presence of vitamin D insufficiency, PTH secretion increases to adapt to higher rates of bone formation associated with growth. This results in higher serum 1,25(OH)2D concentrations and increased calcium absorption results. Vitamin D status, as reflected by the serum 25-OHD level, is not closely related to calcium absorption. Whether adaptation to low serum 25-OHD is adequate under physiologically stressful situations, including those leading to very low serum 25-OHD levels, is unknown.