Differences in calcium kinetics between adolescent girls and young women

Rapid bone accretion occurs throughout childhood but peaks during adolescence. The achievement of optimal peak bone mass, which can protect against osteoporosis later in life, is greatly dependent on rates of bone accretion. To identify differences in calcium metabolism during rapid vs. slower bone accretion, calcium kinetics were compared in 14 healthy girls aged 11-14 yr and 11 women aged 19-31 yr. Calcium kinetics were measured while subjects were undergoing a calcium balance study in a camp simulating a free-living environment. After 7 days on a diet containing 1,330 mg Ca/day, two stable isotopes were administered (44Ca orally and 42Ca intravenously), and blood samples and all urine and feces were collected for 14 days. Samples were analyzed for total calcium by atomic absorption spectrophotometry and for enrichment of 42Ca and 44Ca by fast atom bombardment mass spectrometry. Data from serum, urine, and feces were analyzed using the Simulation, Analysis and Modeling (SAAM) software. Data were fitted by a three-compartment model; the first pool was the same size in girls and women (1.6 g) but the second and third pools were larger in girls (2.85 vs. 1.66 g and 12 vs. 5 g). Compared with the women, girls absorbed more calcium from the diet (38 vs. 22% or 494 vs. 283 mg/day), excreted less calcium in urine (100 vs. 203 mg/day), deposited more calcium in bone (1,459 vs. 501 mg/day), and resorbed more calcium from the skeleton (1,177 vs. 542 mg/day), whereas endogenous calcium excretion did not differ between girls and women (112 vs. 121 mg/day). Girls retained more calcium than women (282 vs. -41 mg/day) through increased absorption, lower urine excretion, and higher bone turnover.