Dual tracer stable isotopic assessment of calcium absorption and endogenous fecal excretion in low birth weight infants

Using a dual tracer (⁴⁴Ca orally and⁴⁶Ca i.v.) stable isotope technique, true dietary Ca absorption, endogenous fecal Ca excretion, and net Ca retention were measured in 12 low birth weight (1426 ± 260 g) infants fed a high Ca-containing formula. Endogenous fecal Ca excretion averaged 7.2 ± 4.1% of intake, and exceeded 10% of intake in three infants. Net Ca retention, 103 ± 38 mg/kg/d, was consistent with previous studies of Ca retention obtained using mass balance techniques and correlated closely (r = 0.98, p < 0.001) with true Ca absorption but not with endogenous fecal excretion (r = — 0.40, p = 0.19). Although endogenous fecal excretion may represent a significant source of Ca loss for some low birth weight infants, these data suggest that net Ca retention in low birth weight infants fed a high Ca-containing formula is primarily determined by the total dietary Ca absorbed.