INTERRELATIONSHIPS BETWEEN ZINC AND PROTEIN LEVEL AND SOURCE IN WEANLING RATS

Feeding trials were conducted to compare body weight, food consumption, and serum zinc responses of weanling rats to marginal or more generous levels of zinc chloride (ZnCl2) added to diets supplying either casein (CS) or isolated soybean protein (SI) at two levels. Proteins were washed with ethylenediaminetetraacetate (EDTA) to remove inherent zinc. Diets containing 10 or 20% protein (+ DL-methionine) were supplemented with ZnCl2 to approximate concentrations of 6, 12, or 24 ppm. After 6 weeks, serum zinc values were similar in most cases for CS-6, SI-6, and SI-12 groups fed marginal levels of zinc and significantly lower than values for CS-12, CS-24, and SI-24 groups. Serum zinc correlated with dietary zinc (r2 = 0.73) but was not affected by dietary protein at the levels used. Body weights of SI-fed rats were significantly lower when either protein or zinc was limiting. To compare the response of rats to inherent or added zinc with CS or SI, proteins were washed without EDTA. After 4 weeks, rats fed SI with inherent zinc (8.2 ppm) weighed significantly less than rats fed EDTA-washed, ZnCl2-supplemented SI (7.0 ppm). The results indicate that added zinc salt was better utilized by rats than zinc inherent to SI. Also, added zinc salt, as well as inherent zinc, was less well utilized when SI rather than CS was the protein.