Calcium absorption, kinetics, bone density, and bone structure in patients with hereditary vitamin D-resistant rickets.
Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) is caused by mutations in the vitamin D receptor gene. Children with HVDRR suffer from severe hypocalcemia and rickets that are treatable with extremely high-dose calcium supplements. Surprisingly, spontaneous recovery of calcium metabolism occurs after the end of puberty without the need for further calcium supplementation. To evaluate the role of vitamin D receptor in intestinal calcium absorption and bone, we investigated intestinal fractional calcium absorption (FCA), bone calcium accretion (Vo+), bone mineral density (BMD), and bone structure parameters in HVDRR patients from infancy into adulthood. Seventeen HVDRR patients aged 1.5-37 yr were investigated. FCA and Vo+ were determined by stable-calcium isotopes. BMD was determined by dual-energy x-ray absorptiometry and bone structure by high-resolution magnetic resonance imaging. FCA in patients aged 1.5-17 yr was 34.9 ± 11.2% compared with 57.3 ± 2.0% in age-matched controls (P < 0.00004), whereas in patients aged 18-26 yr, it was 82.0 ± 7.8 and 53.6 ± 1.2% in controls (P < 0.001). FCA of patients older than 29 yr was comparable to controls. Patients aged 18-26 yr had higher Vo+ than controls (P < 0.02). Patients under 18 and over 29 yr of age had Vo+ comparable to controls. Femoral-neck BMD Z-score was -2.38 ± 0.3 in patients under 18 yr and 0.28 ± 0.87 in postpubertal patients (P < 0.0001). Bone structure by high-resolution magnetic resonance imaging and bone parameters of HVDRR patients and controls were similar. Evidence from HVDRR patients reveals that calcium absorption is highly vitamin D dependent during infancy until the end of puberty, after which there is a period of about 10 yr in which mechanisms other than vitamin D-dependent ones are substantially involved in calcium absorption.