Effect of Dietary Amino Acid Source on the Zinc-deficiency Syndrome in the Chick

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Abstract
The objective of these studies was to compare isolated soybean protein, casein hydrolysate and dried egg white as amino acid sources in chick diets varying in zinc content. Compared with isolated soybean protein in low zinc diets for chicks, casein hydrolysate and dried egg white produced severe zinc deficiency, but the chicks did not develop leg abnormalities, and tibias and femurs of the chicks had higher concentrations of zinc, ash and fat and less water. The length-to-width ratios of femurs from zinc-deficient chicks fed casein hydrolysate or egg white were much greater than for chicks fed zinc-deficient soy protein diets. Dietary amino acid source had no effect on the zinc content of heart, liver or muscle. Supplements of Mn, inorganic P, vitamin D3, cod liver oil, or a mixture of 13 additional trace elements had no effect on the zinc-deficiency syndrome in chicks fed soy protein. Autoclaving the soy protein or adding 80 ppm Cu had slight beneficial effects, whereas doubling of the mineral mix had slight detrimental effects on the zinc-deficient chick fed soy protein. HEDTA in a soy protein diet containing 12 ppm zinc overcame all zinc-deficiency symptoms. Addition of phytic acid in amounts comparable to those in soy protein to egg white and casein hydrolysate diets caused little or no depression in zinc availability and caused no leg abnormalities. It appears that soy protein contains a complicating factor other than phytic acid which affects zinc metabolism, especially in bone.