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Supplementation of Bread Protein with Lysine and Threonine

Stine Laboratory, E. I. du Pont de Nemours and Company, Inc., Newark, Delaware

With a bread diet of low protein content (1.5% of nitrogen) the responses of growing rats to various levels of lysine and threonine supplementation were studied. An incomplete factorial design based on ratio and product of the two essential amino acids was used for the two experiments reported. From the results, optimal performance was calculated to occur at a ratio of lysine to threonine of 1.67 to 1. This figure is in agreement with the requirement data of Rose but is higher than the 1.4:1 ratio determined recently for rice.

In these studies on the protein of bread as well as in the earlier studies on the proteins of rice and of white corn meal there were observed at suboptimal levels of amino acid supplementation two different sets of maximal responses, one for each amino acid at fixed levels of the other amino acid. "Proper balance" between the two amino acids, at any level of supplementation, occurred when maximal response was obtained from a given product of the two amino acids.

The present study confirmed that threonine is the next limiting amino acid in bread protein after lysine. However, it was necessary to reduce the level of bread protein in the diet below 12.5% (dry weight) in order to permit threonine to become a limiting nutrient. At a 9.3% protein level, the maximal weight gain obtained by supplementation with lysine alone was about 75% of that reached with the optimal combination of lysine and threonine.