Calcium metabolism in postmenopausal osteoporosis: The influence of dietary calcium and net absorbed calcium

Abstract

A group of 85 females aged 48-77 years with postmenopausal crush fracture osteoporosis were investigated using a 7 day combined calcium balance and calcium⁴⁷ tracer kinetic turnover study to assess the influence of dietary calcium and net absorbed calcium on bone metabolism. During the study, patients were on their habitual diet, as determined by a prestudy registration. Dietary calcium was measured after double serving of all the meals. All urine and feces were collected and analyzed for calcium content. Bone mineralization rate and bone resorption rate were determined by applying the continuously expanding calcium pool model to the tracer kinetic data. Urine calcium excretion and net absorbed calcium were correlated (r = 0.64, p < 0.0001) with the following equation: urinary excreted calcium (mmol/day) = 2.4 + 0.4 x net absorbed calcium (mmol/day). Dermal calcium loss was not correlated with net absorbed calcium or urinary calcium. The net amount of absorbed calcium necessary to balance urinary and dermal losses was calculated to be 4.2 mmol calcium per day. The daily calcium intake necessary for obtaining a net absorbed calcium in excess of the urinary and dermal calcium losses and thereby ensure skeletal integrity was estimated to be 34.2 mmol calcium per day compared to an average intake of 27.9 + 7.6 (mean + SD) mmol/day. Net absorbed calcium correlated negatively to bone resorption rate (r = -0.31, p < 0.005) and positively to bone mineralization rate (r = 0.29, p < 0.01) and to calcium balance (r = 0.66, p < 0.0001). Dietary calcium intake and calcium balance correlated positively (r = 0.38, p < 0.001). The minimum dietary calcium intake to ensure calcium balance was 34.5 mmol calcium per day when using this correlation. We conclude that excess net absorbed calcium in postmenopausal osteoporosis is not quantitatively excreted in the urine but seems to induce a more positive calcium balance by reducing bone resorption rate and increasing bone mineralization rate. The recommended calcium allowance in Western countries does not meet the minimum dietary calcium requirement to ensure calcium balance in patients with postmenopausal osteoporosis when dermal calcium loss is also taken into account