Elevated intakes of zinc in infant formulas don not interfere with iron absorption in premature infants.

BACKGROUND: Zinc and iron may share common pathways for absorption and compete for uptake into mucosal cells. We determined whether elevated ratios of zinc to iron would interfere with erythrocyte incorporation of iron in premature infants both during and between feeds. METHODS: In the first experiment, five premature infants (<2500 g birth weight) were enrolled, once receiving full oral feeds by nasogastric tube. They received either high (1200 microg/kg, ratio 4:1) or low (300 microg/kg, ratio 1:1) doses of oral zinc sulfate, together with 300 microg/kg oral 58Fe as chloride in saline with 10 mg/kg vitamin C, between designated feeding periods. Each infant served as its own control and randomly received either high or low doses of zinc or iron and then the alternate dose after 2 weeks. In the second experiment, nine additional premature infants were assigned to the same zinc:iron intake protocol except zinc and iron were given with usual oral feeds (premature formula or human milk) equilibrated before feeding. Iron absorption was measured by the erythrocyte incorporation of 58Fe. RESULTS: High doses of zinc given between feeds significantly inhibited erythrocyte incorporation of iron. 58Fe incorporation (%) with the 1:1 ratio of zinc:iron intake was 7.5 (5.7, 10; geometric mean, -I SD, +1 SD). The percentage of 58Fe incorporation on the 4:1 ratio of zinc:iron intake was 3.6 (2.6, 5.1). Given with feeds, the percentage of 58Fe incorporation on low zinc:iron intake was 7.0 (2.6, 19). Finally, the percentage of 58Fe incorporation on high zinc:iron intake was 6.7 (2.5, 19). CONCLUSION: Elevated intakes of zinc do not interfere with erythrocyte incorporation of iron in premature formulas.

Evaluation of full-term infants fed an evaporated milk formula.

The objective of this prospective, cohort study was to compare the nutritional status of full-term infants who were fed human milk (BF, n = 29), formula (FF, n = 30) or evaporated milk formulae (EM, n = 30) for at least 3 months. Infants were seen at enrollment, 3 and 6 months, at which times a blood sample, diet record and anthropometric data were collected. Infants in the EM group received solids earlier (12 +/- 5 weeks) than did FF infants (15 +/- 4 weeks), and both were earlier than BF infants (19 +/- 4 weeks). Only 26% of the EM fed group received iron supplements as ferrous sulphate drops. Seven BF, 12 FF and 20 EM had abnormal ferritin values (< 10 ng ml-1) at 6 months. Copper intake was lower in the EM infants at 3 and 6 months. However, plasma copper and erythrocyte copper zinc superoxide dismutase (ZnCuSOD) levels did not differ between groups. Selenium intake was lower in the EM group (5 +/- 1 and 10 +/- 5 micrograms d-1; 3 and 6 months) than in the FF infants (13 +/- 4 and 19 +/- 7 micrograms d-1; 3 and 6 months). Erythrocyte SeGHSPx levels in EM infants were lower at 6 months (EM, 33.2 +/- 3.4; FF. 35.2 +/- 3.9: BF, 36.1 +/- 3.8 mU mg Hb-1). Thiamin intake (0.99 +/- 0.08 and 1.24 +/- 0.32; 3 and 6 months, mg 1000 kcal-1) was higher in the FF group than in EM infants (0.38 +/- 0.39 and 0.66 +/- 0.38; 3 and 6 months). There were more (13%) abnormal thiamin assays in the EM group at 6 months than in the BF and FF infants (0%). In conclusion, infants fed evaporated milk formula receive adequate copper but may not receive enough thiamin or selenium. Unless supplemented from birth with medicinal iron, intakes of iron will be inadequate.

Zinc absorption in premature infants: comparison of two isotopic methods.

The fractional absorption of an oral dose of zinc can be measured in adults when given simultaneously with an intravenous dose and subsequently measuring the ratio of the double isotopic enrichment of urine. To test this method in very-low-birth-weight (VLBW) premature infants [n=5 females and 7 males, 1160 +/- 290 g (chi +/- SD) birth weight, 29 +/- 4 wk gestational age], an oral dose of either 300 or 1200 micrograms 68Zn.kg(-1).d(-1) was equilibrated with formula or human milk and administered simultaneously with either 50 or 100 micrograms 70Zn.kg(-1).d(-1) given intravenously 35 +/- 3 wk postconception. Urine and fecal samples were collected for 3-6 d and analyzed by inductively coupled plasma mass spectrometry. Endogenous fecal zinc (EFZ) was determined from isotopic enrichment, whereas net absorption and retention were calculated by traditional methods. The mean fractional absorption calculated from urine was 0.22 +/- 0.09 and from feces it was 0.25 +/- 0.07. Zinc intake averaged 1821 +/- 330, fecal excretion 1637 +/- 419, and urinary excretion 67 +/- 30 micrograms.kg(-1).d(-1). EFZ averaged 390 +/- 270 micrograms.kg(-1).d(-1) and ranged from 48 to 889 micrograms.kg(-1).d(-1). Net absorption was 220 +/- 316 micrograms.kg(-1).d(-1) and net retention was 131 +/- 334 micrograms.kg(-1).d(-1). True absorption was 373 +/- 161 micrograms.kg(-1).d(-1). Fecal collection is difficult, tedious, and often incomplete, and may be replaced by urine collection for the fractional absorption of zinc in groups of premature infants.