Calcium and zinc absorption from lactose-containing and lactose-free infant formulas

Background: Calcium absorption is enhanced by the presence of lactose, but the quantitative significance of this effect in infant formulas is uncertain. It is also not known whether lactose affects zinc absorption. Objective: We measured the absorption of calcium and zinc from infant formulas by using a multitracer, stable-isotope technique. Design: Eighteen full-term infants (aged 8-12 wk at enrollment) were fed 2 partially hydrolyzed whey-protein-based formulas ad libitum for 2 wk per formula. The carbohydrate source was lactose in one formula and glucose polymers in the other (lactose-free). Infants were studied in a blinded crossover fashion after 2 wk of adaptation to each formula. Isotope absorption studies were conducted with a 4-tracer method in which Zn-70 and Ca-44 were provided orally and Zn-67 and Ca-46 intravenously. Zinc and calcium absorption was measured from the fractional excretion of the oral and intravenous isotopes in urine. Results: Fractional and total calcium absorption was significantly greater from the lactose-containing formula than from the lactose-free formula. For total calcium absorption, the mean difference between formulas was 10.3% (P = 0.002) and 60 mg/d (P = 0.006). For zinc, fractional absorption (32 +/- 11%), total absorption, and intake did not differ significantly between the 2 formulas. Conclusions: The presence of lactose in a formula based on cow-milk protein increases absorption of calcium but not of zinc. Absorption of calcium from a lactose-free infant formula is, however, adequate to meet the calcium needs of full-term infants when the formula's calcium content is similar to that of lactose-containing, cow-milk-based infant formulas.