Relationships among vitamin D levels, parathyroid hormone, and calcium absorption in young adolescents

Abstract

Background: 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.

Objectives: The objective of this study was to evaluate the relationships among vitamin D status, PTH, and calcium absorption in midpubertal boys and girls.

Methods: 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.

Results: 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.

Conclusions: 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.

Figures

Fig. 1

Distribution of calcium intake and absorption (percentage) in 93 study subjects (15 distribution groups).

Fig. 2

Distributions of serum 25-OHD in 93 study subjects (15 distribution groups), of serum intact PTH in 93 study subjects (15 distribution groups), and of serum 1,25(OH)2D in 92 study subjects (15 distribution groups) are shown. For 5-OHD, multiply by 2.496 for nanomoles per liter; for 1,25(OH)2D, multiply by 2.6 for picomoles per liter; and for PTH, multiply by 0.105 for picomoles per liter.

Fig. 3

A, Linear regression of serum 25-OHD and PTH: n = 93; y = −1.01 × x + 64.7; se of slope, 0.24; r = −0.41; P < 0.001. For 5-OHD, multiply by 2.496 for nanomoles per liter; for PTH, multiply by 0.105 for picomoles per liter. B, Linear regression of serum osteocalcin and PTH; n = 85, y = 0.28*x +28.3, se of slope = 0.10, r = 0.29, P = 0.008. For PTH, multiply by 0.105 for picomoles per liter. C, Linear regression of serum 25-OHD and calcium absorption (percentage): n = 93; r = 0.073; P = 0.49. For 25-OHD levels, less than 25 ng/ml (n = 35); y = −1.69 × x + 72.1; se of slope = 0.60; r = −0.45; P = 0.009. For 25-OHD, multiply by 2.496 for nanomoles per liter. D, Linear regression of 1,25(OH)2D and calcium absorption (percentage): y = 0.30 × x + 18.0; se of slope, 0.084; r = 0.35; P = 0.001. For 1,25(OH)2D, multiply by 2.6 for picomoles per liter.