Zinc Homeostasis in Breast-Fed Infants

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Abstract
The needs for dietary zinc are adequately met by most fully breast-fed infants despite relatively low zinc intakes in relation to estimated requirements. The objective of this study was to use stable isotope techniques to evaluate how zinc retention is achieved in normal fully breast-fed infants. Nine male infants, aged 2-5 mo, were fed expressed human milk labeled with70 Zn over a 24-h period. Complete fecal collections were obtained for 8 d. On d 4-7, a metabolic period was initiated which included test weighing and milk sampling, to measure zinc intake, and daily urine collections. Isotopic enrichment of fecal and urine samples was determined by fast atom bombardment mass spectrometry. Results included a mean (±SD) dietary zinc intake of 17.8 ± 6.6 μmol/d; fractional absorption of 0.54 ± 0.075; and total absorbed zinc of 9.5 ± 3.5 μmol/d. Mean endogenous fecal zinc, determined on seven infants by isotope dilution, was 4.7 ± 2.3μmol/d, which resulted in a mean net absorption of 4.8 ± 3.4μmol/d. The results of the study indicated that, for fully breast-fed infants, it is the combination of a relatively high fractional absorption and efficient conservation of intestinal endogenous zinc that results in zinc retention adequate to meet the demands of growth in the face of modest intake.