A COMPARATIVE-STUDY IN RATS OF MEASURES OF THE AVAILABILITY OF DIETARY ZINC AND IRON

The purpose of this study was to compare three measures of the availability of dietary Zn and Fe in order to test their validity. Thirty-six 5-wk-old rats were fed deionized water and wheat crispbread made from endosperm flour, whole-grain flour, or endosperm flour supplemented with Zn and Fe to the whole-grain levels ad libitum for 14 d. The retention of Zn-65 and Fe-59 from test meals of the same breads after 1 wk and the sum of the excretion of endogenous Zn and Fe (injected Zn-65 and Fe-59) with the Zn and Fe balances, respectively, were used as independent measures of Zn and Fe absorption. Measurements of Zn absorption, Zn balance, and serum Zn concentration gave quite different results with regard to the availability of Zn in the three breads, presumably because of the homeostatic regulation of the absorption and excretion of Zn when the Zn in the diet is in excess of the body's needs. Measurements of Fe absorption, Fe balance, and Fe concentrations in liver and serum were consistent in demonstrating overloading of Fe in the group given wheat-endosperm crispbread supplemented with Zn and Fe, but there was evidence that the isotope retention method overestimated iron absorption.