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Intrinsic and Extrinsic Stable Isotopic Zinc Absorption by Infants from Formulas^1,2,3,

Robert E. Serfass^*, Ekhard E. Ziegler^**, Barbara B. Edwards^** and Robert S. Houk

^* Department of Food and Nutrition, Iowa State University, Ames, Iowa 50011 ^** Department of Pediatrics, University of Iowa, Iowa City, Iowa 52242 Ames Laboratory—U.S. Department of Energy and Department of Chemistry, Iowa State University, Ames, Iowa 50011

The kinetics of fecal elimination and the extent of fractional absorption of intrinsic and extrinsic zinc from infant formula were studied in 11 human infants by using a direct, simultaneous dual-label method based on fecal monitoring of unabsorbed stable isotope labels. The relative positions of the two stable isotopic tags (⁶⁷Zn, ⁷⁰Zn) as extrinsic and intrinsic labels were reversed in two separate determinations of fractional absorption. Administration of doubly labeled formula for 24 h or less permitted elimination of unabsorbed labels before the end of a 72-h metabolic interval. The extent of enrichment and the kinetics of fecal elimination of unabsorbed extrinsic stable isotopic zinc tag were observed to agree closely with the corresponding values for the intrinsic tag. Fractional absorption of intrinsic and extrinsic tags was highly correlated and did not differ significantly. The ratio of fractional absorption of extrinsic tag to that of intrinsic tag was 1.05 ± 0.19 (mean ± SD). The results constitute strong support for validation of the use of enriched stable isotopes at low levels ( 10%) of extrinsic addition to milk-based diets for studies of zinc absorption by humans.

KEY WORDS: • zinc • stable isotopes • human infant • mineral absorption • intrinsic labeling • mathematical model

¹ Journal Paper No. J-13400 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa; Project No. 2800.

² Supported by U.S. Dept. of Agriculture Competitive Grant 86-CRCR-1-2082, by U.S. Public Health Service grant HD 07578, and by the Director of Energy Research, Office of Basic Energy Sciences, U.S. Department of Energy. The Ames Laboratory is operated by Iowa State University for the U.S. Dept. of Energy under contract number W-7405-Eng-82.

³ Presented in part at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, MO, April 13–18, 1986 [Serfass, R. E., Thompson, J. J. & Ziegler, E. E. (1986) Intrinsic and extrinsic zinc absorption by infants from cow milk formula. Fed. Proc. 45: 588 (abs.)].

Manuscript received 13 February 1989. Revision accepted 13 June 1989.