A Method for Determining “True” Availability of Magnesium from Foodstuffs using Chickens

Abstract

Three experiments were conducted with mature male chickens to establish a valid and reliable technique to determine magnesium (Mg) availability from foodstuffs. Chromic oxide (Cr₂O₃) marked semipurified diets were fed to facilitate collection of comparative-balance data. Magnesium-28 (²⁸Mg) was administered intramuscularly to label the endogenous Mg pool. Relative specific activities of Mg showed that when dietary Mg did not exceed 400 ppm, ²⁸Mg equilibrated among plasma, urine, bile, pancreas, heart, liver, and intestinal mucosa within 50 hours. On this basis, it was concluded that the specific activity (SA) of plasma Mg could be used to estimate SA of the Mg present in endogenous secretions of the digestive tract, and that the SAs of the plasma and digesta Mg could be used to partition dietary and endogenous Mg of the excreta. A formula was derived into which comparative-balance and isotope-dilution data could be applied to calculate Mg availability. Average “true” availability of Mg from MgSO₄ was 57.4% and assigned a relative availability index of 100. The relative availabilities in descending order for the foodstuffs tested were as follows: oats, 144; walnuts, 119; skim milk, 108; soybean meal (48.5%), 105; whole egg, 99; wheat, 99; corn, 97; barley, 95; great northern beans, 89; peas, 84; cream of wheat, 76; polished rice, 74; rib steak (separable lean), 73; oatmeal cereal (baby food), 67; rice cereal (baby food), 64; and minute breakfast oats, 58.