COMPARTMENTAL ANALYSIS OF CALCIUM-METABOLISM IN VERY-LOW-BIRTH-WEIGHTINFANTS

The calcium metabolism of 13 very-low-birth-weight infants fed a high- calcium diet was evaluated by means of stable isotope kinetic and bala nce studies. The studies used orally and i.v. administered stable isot opes, and the kinetic data were evaluated with the aid of a sequential , three-compartment model. The infants (postmenstrual age 33 +/- 1 wk, weight 1.34 +/- 0.03 kg) had higher bone calcium deposition rates (16 0 +/- 7 mg.kg(-1)d(-1) or 4.00 +/- 0.18 mmol.kg(-1)d(-1)) than those p reviously reported for either older children or adults. Furthermore, w hen analyzed as a function of net calcium absorption, bone calcium dep osition rates increased markedly and significantly as net calcium abso rption increased (r = 0.70, p < 0.01), whereas in older individuals, b one calcium deposition is a relatively invariant function of absorptio n. A relatively smaller response of bone calcium removal to calcium ab sorption was found for the very-low-birth-weight infants in this study (r = -0.39, p = 0.18), whereas in adults, bone calcium removal consti tutes the major regulatory response. It is suggested that the calcium kinetic results in the very-low-birth-weight infants reflect the high rate of bone growth typical of the third trimester of gestation.