BIOAVAILABILITY TO RATS OF ZINC, MAGNESIUM AND CALCIUM IN CASEIN-CONTAINING EGG-CONTAINING AND SOY PROTEIN-CONTAINING DIETS

The bioavailability of zinc (Zn) and magnesium (Mg) from several soybean products (SP) as well as the effect of the presence of dietary SP upon the bioavailability of added Zn or calcium (Ca) was investigated. Male, weanling rats were fed all or part of their dietary protein as full fat soy flour (SF), freeze-dried soy beverage (SB) or soy concentrate (SC). In experiments testing the bioavailability of minerals from SP, incremental levels of a SP were substituted on an isonitrogenous basis for egg white (Zn studies) or casein (Mg studies). Graded levels of ZnCO3 or MgCO3 were added to egg white or casein basal diets. In experiments testing the effect of the presence of SP upon the bioavailability of supplemental minerals, graded levels of CaCO3 or ZnCO3 were added to SP-containing diets or casein (Ca studies) or egg white (Zn study) basal diets. Linear regression analyses related total tibia (or femur) Zn, Mg, or Ca to increased dietary mineral. Growth (Zn studies) and serum Mg were also related to the dietary mineral concentration. The results showed that Zn was poorly available from soy products, especially SC. Mg was highly available from SF and SB; Mg utilization from SC was good but less than from the other SP. Ca added to all SP was highly available, while Zn added to SC was not fully available. Conditions involved in processing SC or its elevated phytate to zinc molar ratio may have resulted in reduced bioavailability of endogenous Zn and Mg and of added Zn.