CALCIUM RETENTION IN RELATION TO CALCIUM INTAKE AND POSTMENARCHEAL AGE IN ADOLESCENT FEMALES

Achievement of maximal calcium retention during adolescence may influe nce the magnitude of peak bone mass and subsequently lower the risk of osteoporosis. Calcium retention is generally considered to reach a pl ateau at a certain calcium intake. To test this hypothesis, calcium ba lance was measured in 35 females with a mean (+/- SD) age of 12.7 +/- 1.2 y (range: 12-15 y) who consumed from 841 +/- 153 to 2173 +/- 149 m g Ca/d. Subjects ate a basal diet that included a fortified beverage c ontaining different amounts of calcium citrate malate. Twenty-one subj ects were studied at two dietary calcium intakes with use of a crossov er design. Results from a previous study in 14 subjects who were studi ed at only one calcium intake were included in the data analysis. Calc ium retention was modeled as a nonlinear function of calcium intake th at included a parameter representing mean maximal retention. Mean maxi mal calcium retention was 473 mg/d (95% CI: 245, 701 mg Ca/d). At high er postmenarcheal ages, maximal calcium retention was lower but the in take required to achieve this was nor affected. Calcium intake explain ed 79% and 6%, respectively, of the variation in fecal and urinary cal cium excretion. Intake of 1200 mg Ca/d, the recommended dietary allowa nce for calcium published in 1989, resulted in a mean calcium retentio n that was 57% of the maximal value (95%; CI: 25%, 89%). Intake of 130 0 mg Ca/d was the smallest intake that allowed some adolescent females to achieve 100% of maximal calcium retention (95% CI: 26%, 100%). The se data support the idea that calcium retention plateaus at a certain calcium intake although it continues to increase at intakes > 2 g/d.