INCREASED EFFICIENCY OF CALCIUM-ABSORPTION DURING SHORT PERIODS OF INADEQUATE CALCIUM INTAKE IN GIRLS

Adequate calcium intake is essential for skeletal integrity, particula rly during the period of peak bone mass acquisition from 9 to 17 y of age. Currently, the calcium intake of many adolescent girls is below t he recommended dietary allowance. The purpose of this study was to eva luate the ability of girls to respond to acute periods of inadequate d ietary calcium intake. Calcium absorption was evaluated in 11 girls ag ed 11.6 +/- 2.4 y after 10 d on both a low-calcium (7.05 +/- 2.03 mmol /d) and a high-calcium (35.30 +/- 2.28 mmol/d) diet. Fractional calciu m absorption was determined by using oral (Ca-46) and intravenous (Ca- 42) Stable isotopes of calcium. During a low calcium intake, fractiona l calcium absorption was significantly greater (0.582 +/- 0.087 compar ed with 0.260 +/- 0.068, P < 0.0001) and urinary calcium excretion was significantly lower (1.30 +/- 0.83 compared with 3.08 +/- 1.98 mmol/d , P < 0.004) than values obtained during a high calcium intake. Concen trations of 1,25-dihydroxyvitamin D (combination of cholecalciferol an d ergocalciferol) were greater during the low calcium intake, although the difference was not significant (108.7 +/- 30.6 compared with 90.0 +/- 25.1 pmol/L, P < 0.1; n = 9). Excretion of N-telopeptide was sign ificantly greater during the low calcium intake (761 +/- 508 compared with 413 +/- 341 nmol bone collagen equivalent (BCM)/mmol creatinine, P < 0.02; n = 9), indicating that bone resorption was increased. These results suggest that during short periods of inadequate calcium intak e, girls are able to significantly increase the efficiency of calcium absorption and decrease urinary calcium losses to conserve calcium req uired for bone mineral acquisition.