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The Protein Requirements of the Adult Rat in Terms of the Protein Contained in Egg, Milk and Soy Flour¹

One Figure

Division of Animal Nutrition, University of Illinois, Urbana

Ten adult male rats were fed 2 dietary levels each of milk and soy flour proteins and 3 dietary levels of egg protein; all of the former and 2 of the latter dietary levels were associated with negative nitrogen balances. The linear relationship between truly absorbed nitrogen and nitrogen balance was described by regression equations fitted to the data by the method of least squares.

The amount of truly absorbed nitrogen (corrected for the metabolic nitrogen in the feces) required for nitrogen equilibrium was computed from these regression equations to be 2.78 mg of egg nitrogen, 3.18 mg of milk nitrogen and 5.48 mg of soy flour nitrogen per cal. of basal heat. The biological values indicated by these regression equations are 99 for whole egg nitrogen, 86 for milk nitrogen and 49 for soy flour nitrogen.

The nitrogen of the defatted dehydrated whole egg preparation used in these studies was completely digestible by the adult rat and, at the low levels of feeding used (2.38 to 4.62% conventional protein) the absorbed nitrogen was practically completely utilized in adult rodent metabolism. Neither in this experiment, nor in previous ones on adult rats carried out in this laboratory, has egg protein been observed to depress appreciably the minimum endogenous metabolism of nitrogen.

The total requirements of milk nitrogen per basal cal. for nitrogen equilibrium in the adult rat and the adult human are not greatly different, being 3.34 mg and 2.76 mg, respectively. The requirements of soy flour nitrogen on the same basis are much greater for the rat than for the human, 6.07 mg and 2.88 mg, respectively, testifying to the greater relative requirement of the rat than of the human for the amino acid severely limiting the nutritive value of soybean protein, namely, methionine (or cystine).

Manuscript received 7 July 1947.