Zinc absorption, distribution, excretion, and retention by healthy preterm infants

Zinc (Zn) is an essential nutrient for growth, but little is known about Zn absorption, distribution, excretion, and retention in preterm infants. Nine infants with gestational age 32 +/- 1 wk (mean +/- SE), birth weight 1.44 +/- 0.08 kg, postnatal age 14 +/- 3 d, on Zn intake of 23 +/- 3 mu mol/kg per d via enteral feeding of preterm formula were studied. A stable Zn isotope (Zn-70) was administered orally or i.v., and plasma, red blood cells, urine, and feces were sampled for up to 30 d. Samples were analyzed for Zn by inductively coupled plasma atomic emission spectrometry and for isotope enrichment by inductively coupled plasma mass spectrometry. Data were analyzed by compartmental analysis using the Simulation Analysis and Modeling program, and absorption, distribution, excretion, and retention were calculated. Absorption was 36 +/- 5% or 7 +/- 1 mu mol/kg per d; distribution in plasma was 15 +/- 1 mu mol Zn/L and in RBC was 41 +/- 4 mu mol Zn/L; excretion in urine was 0.55 +/- 0.03 mu mol Zn/kg per d and in feces was 17 +/- 3 mu mol Zn/kg per d and retention was 5 +/- 1 mu mol/kg per d. Results show that healthy preterm infants with Zn intake of 23 mu mol/kg per d and expected growth rates (>15 g/kg per d) absorb and retain Zn at rates comparable to in utero accretion. The values for absorption, distribution, and excretion by this population of healthy preterm infants provide a normal range for future studies, although further studies are required to determine endogenous excretion rates in healthy preterm infants. We speculate that these values can be used to determine whether Zn kinetics are abnormal in sick infants or in infants with slow growth.