Age-related osteoporosis in Chinese: an evaluation of the response of intestinal calcium absorption and calcitropic hormones to dietary calcium deprivation

Background: Age-related osteoporosis may be associated with inefficient int estinal calcium absorption and bone remodeling. Objective: We investigated the pathogenesis of age-related osteoporosis in Chinese women with habitual low calcium intakes. Design: We studied the response of intestinal calcium absorption, calcitrop ic hormones, and biochemical bone markers to graded dietary calcium depriva tion. Results: The osteoporotic subjects (n = 25) had higher urinary calcium excr etion (P < 0.05) and lower plasma 1,25-dihydroxyvitamin D concentrations (P < 0.02) than did age-matched control women (n = 25). Parathyroid hormone w as not significantly different from that in age-matched control women but w as significantly higher than in young women (n = 15, P < 0.05). Fractional Ca-45 absorption was approximate to 61% in all 3 groups when the diet was u nmodified and increased to 71%, 69%, and 68% in the osteoporotic subjects, age-matched control women, and young women, respectively, when dietary calc ium was reduced to 300 mg/d. When the osteoporotic women were calcium depri ved, serum 1,25-dihydroxyvitamin D failed to increase but urinary calcium e xcretion persisted. In contrast, supplementation with 1200 mg Ca resulted i n a lowering of parathyroid hormone (P < 0.005 compared with the unmodified diet) and 1,25-dihydroxyvitamin D (P < 0.01) and decreased fractional Ca-4 5 absorption (P < 0.01), suggesting that the increased calcium intake was a ssociated with a potent compensatory ability of the intestine and calcitrop ic hormones to adapt. Calcium supplementation lowered osteocalcin (P < 0.05 ) but not alkaline phosphatase, which remained elevated in the osteoporotic subjects at all stages. Conclusions: Elderly osteoporotic women had reduced 1,25-dihydroxyvitamin D production, excessive urinary calcium loss, and high bone turnover. The Ch inese women had exceptionally potent intestinal calcium absorption.