Changes in Calcium Kinetics in Adolescent Girls Induced by High Calcium Intake

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Abstract
To identify the mechanism/s whereby calcium retention is in- creased by calcium intake in adolescent girls, kinetic studies were performed using stable calcium isotope tracers. Girls (n 5 10; 12 6 1 yr old, mean 6 SD) were studied while on a controlled diet con- taining a low (21.2 mmol/day) and a high (47.4 mmol/day) calcium intake, in randomized order, using a cross-over design. Studies were separated by 1 month. Calcium tracers were administered after 1 week on the study diet, orally and iv; and serum, urine, and feces were collected for the following 14 days. Tracers were mea- sured using fast atom bombardment mass spectrometry, and ki- netic data were analyzed by compartmental modeling. Biochemical markers of bone turnover were measured in serum and urine sam- ples. On high (compared with low) calcium intake, fractional ab- sorption did not differ, absorbed calcium increased (19.6 6 7.5 vs. 8.0 6 2.5 mmol/day, mean 6 SD, P , 0.001), calcium excreted in urine increased (2.8 6 1.7 vs. 2.1 6 1.1 mmol/day, P , 0.01), calcium retained in bone increased (14.5 6 8.9 vs. 3.2 6 3.6 mmol/ day, P , 0.001), bone formation did not change, and bone resorption decreased by 32%. These changes, measured by kinetics, were corroborated by changes in markers of bone turnover. We conclude that increased bone retention of calcium, with high calcium intake in adolescent girls, is attributable to an increase in absorption and a decrease in bone resorption. (J Clin Endocrinol Metab 85: 4470 - 4475, 2000)

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