Effects of dietary protein level and ascorbic acid supplementation on the contents of tyrosine metabolites in droppings and plasma of chicks fed a diet containing excess tyrosine.

A study on chickens was conducted to investigate whether or not: a) excess dietary tyrosine increases the content of tyrosine metabolites in plasma and excreta, b) these elevations of tyrosine metabolites are presented by increasing dietary protein level or supplementing with ascorbic acid (AA), and c) urine is a major excretory route of tyrosine metabolites. Chicks fed a 10% protein diet with excess tyrosine developed external foot lesions accompanied by retarded growth and depressed feed intake. These adverse effects were alleviated by elevating dietary protein level or supplementing with AA. Excreta and plasma of chicks fed the 10% protein diet contained small or undetectable amounts of free tyrosine, 4-hydroxyphenylpyruvate (4-HPP), 4-hydroxyphenylacetate (4-HPL), and 4-hydroxyphenylacetate (4-HPA), while these metabolites were markedly increased by the addition of excess tyrosine to the 10% protein diet. From the results with colostomized cocks, the major source of 4-HPP, 4-HPL, and 4-HPA excreted by chicks fed a tyrosine excess diet was considered more likely to be of urinary than fecal origin. Elevated contents of tyrosine and its metabolites in plasma were partially counteracted by increasing dietary protein level or AA supplementation. In excreta, elevated contents of tyrosine and its metabolites caused by excess tyrosine were reduced by increasing dietary protein level and supplementing with AA when expressed in the proportion of tyrosine intake. These results suggest that the beneficial effects of increased dietary protein level and supplementation with AA are related to enhanced ability of chicks to degrade excessively ingested tyrosine.