Abnormalities in zinc homeostasis in young infants with cystic fibrosis

Low plasma zinc concentrations have been reported in approximately 30% of y oung infants with cystic fibrosis identified by newborn screening. The obje ctive of this study was to examine zinc homeostasis in this population by a pplication of stable isotope methodology. Fifteen infants with cystic fibro sis (9 male, 6 female; 7 breast-fed, 8 formula-fed) were studied at a mean (+/-SD) age of 1.8 +/- 0.7 mo. On d 1, Zn-70 was administered intravenously , and Zn-67 was quantitatively administered with all human milk/formula fee ds during the day. Three days later, a 3-d metabolic period was initiated, during which time intake was measured and complete urine and fecal collecti ons were obtained. Fractional zinc absorption, total absorbed zinc, endogen ous fecal zinc, and net absorbed zinc were measured; fecal fat excretion wa s also determined. Fractional absorption was significantly higher for the b reast-fed infants (0.40 +/- 0.21) compared with the formula-fed group (0.13 +/- 0.06) (p = 0.01), but with the significantly higher dietary zinc intak e of the formula-fed group, total absorbed zinc was higher for those receiv ing formula (p = 0.01). In 11 infants with complete zinc metabolic data, ex cretion of endogenous zinc was twofold greater for the formula-fed infants (p < 0.05); net absorption (mg zinc/d) was negative for both feeding groups : -0.04 +/- 0.52 for breast-fed; -0.28 +/- 0.57 for formula-fed. Endogenous fecal zinc losses correlated with fecal fat excretion (r = 0.89, n = 9, p = 0.001), suggesting interference with normal conservation of endogenously secreted zinc. These findings indicate impaired zinc homeostasis in this po pulation and suggest an explanation for the observations of suboptimal zinc status in many young infants with cystic fibrosis prior to diagnosis and t reatment.