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 ((x) over bar +/- SD) birth weight, 29 +/- 4 wk gestational age], an oral dose of either 300 or 1200 mu g Zn-68 . kg(-1) d(-1) was equilibrated with formula or human milk and administered simultaneously with either 50 or 100 mu g Zn-70 . 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 pg kg(-1) . d(-1). EFZ averaged 390 +/- 270 mu g kg(-1) . d(-1) and ranged from 48 to 889 mu g . kg(-1) d(-1) Net absorption was 220 +/- 316 mu g . kg(-1) d(-1) and net retention was 131 +/- 334 mu g . k(-1). d(-1). True absorption was 373 +/- 161 mu g . 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.