ROLE OF CALCIUM INTAKE IN MODULATING AGE-RELATED INCREASES IN PARATHYROID FUNCTION AND BONE-RESORPTION

Serum parathyroid hormone (PTH) and bone resorption increase in elderl y women and contribute to age-related bone loss. Whether these abnorma lities are caused by calcium deficiency resulting from age-related dec reases in absorption and renal conservation is unclear. We studied 28 normal elderly women (mean +/- SD, age 69.3 +/- 2.7 yr) who were maint ained for 3 yr on usual calcium intake levels (20.4 +/- 7.2 mmol/day [ 815 +/- 289 mg/day]; n = 15) (known as the usual calcium group) or hig h calcium intake levels (60.4 +/- 6.5 mmol/day [2414 +/- 260 mg/day]; n = 13) (known as the high calcium group) and a reference group of 12 normal young adult women (age 30.1 +/- 4.4 yr), whose calcium intake w as 23.0 +/- 4.8 mmol/day (918 +/- 193 mg/day) (known as the young grou p). Serum PTH was measured every 2 h, and urinary excretion of deoxypy ridinoline (Dpd), a new marker for bone resorption, was measured in 4 h collections. Parathyroid gland secretory capacity was assessed durin g induced hypocalcemia. The mean 24 h serum PTH was 40% lower (P < 0.0 01), and the mean 24 h urinary Dpd was 35% lower (P < 0.005) in the hi gh than in the usual calcium group. Mean parathyroid gland secretory c apacity also was 47% lower (P < 0.005) in the high calcium group than in the usual calcium group. However, the usual calcium group had a mea n 24 h serum PTH level that was 70% higher (P < 0.001) and a mean 24 h urinary Dpd level that was 30% higher (P < 0.005) than the young grou p, whereas the high calcium group was indistinguishable from the young group. Thus, failure of elderly women to increase their calcium intak e to offset age-related increases in calcium requirement contributes s ubstantially to their development of increased parathyroid activity an d increased bone resorption, whereas a high calcium intake can reverse both abnormalities.