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

Abstract

Low dietary calcium is a recognised risk for osteoporotic fractures. We evaluated the pathogenesis of age-related osteoporosis in a group of Chinese women with a mean calcium intake of 561±183 mg/day. When compared to young healthy controls, the osteoporotic subjects had negative calcium balance, raised urinary calcium excretion (0.63±O.47 vs 0.27±O.16 mol/mol, p<O.05) , low calc it rio I level (74±26 vs 108±33 pmol/l, p<0.02) and secondary hyperparathyroidism (3.67±1.54 vs 2.67±1.62 pmol/l, p<O.05). The fractional 45Ca absorption was around 61 % during unmodified diet in both osteoporotic subjects and in young controls which increased to 71 % and 68% respectively during low calcium diet. When subjected to graded dietary calcium deprivation, calcitriollevels failed to increase and urinary calcium excretion perSisted in the osteoporotic subjects. Supplementation of 1200mg calcium in the osteoporotic subjects was able to lower PTH level (2.27±1.09, vs 2.84±1.19 pmol/l, vs unmodified diet, p<0.005) but further suppressed serum calcitriollevel (60±21 vs 79±19 pmol/l, p<0.01) and decreased fractional 45Ca absorption (49±11% vs 61±18%, p<0.01), suggesting thatthis level of calcium intake was excessive and offset the potent compensatory ability of the intestine to absorb calcium. Based on the response of the calcitrophic hormones and fractional 45Ca absorption, a daily intake of 1000 - 1200mg calcium is probably adequate for the Chinese population.