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The Protein Requirements of Adult Human Subjects in Terms of the Protein Contained in Individual Foods and Food Combinations¹

One Figure

Division of Animal Nutrition and the Department of Home Economics, University of Illinois, Urbana

Fifty nitrogen balance periods were carried out upon nine women subjects to determine the requirements for protein during adult life for different types of dietary protein. The protein foods tested were milk, soy flour, white flour, a combination of soy flour and white flour containing 13% of the former, and a well balanced mixture of protein foods. All test foods were cooked in some appropriate manner if they are commonly prepared by cooking. The test foods supplied 91% or more of the total protein in the test diets, which were fed in amounts to maintain body weight.

Each food, or food combination, was studied at different levels of nitrogen intake in order to define the relationship between intake and balance. This relationship was shown to be rectilinear and satisfactorily described by the equation of a straight line in which nitrogen balance is the dependent variable (Y). The slope of the line, the b constant in the equation, is the product of the true digestibility of the dietary nitrogen and its biological value in the Thomas sense. The intercept of the line on the ordinate representing zero balance gives the amount of dietary nitrogen required for equilibrium. In these computations, all values for the different subjects are pooled together for each test food by expressing them in milligrams of nitrogen per calorie of basal heat.

For milk, white flour, soy flour, the soy-white flour combination and the mixed foods, the indicated biological values were, respectively, 74, 41, 65, 55 and 65, and the amounts of nitrogen required for equilibrium (as ordinarily measured) were, respectively, 2.76, 4.76, 2.88, 3.38, and 3.12 mg. per basal calorie.

When due allowance is made for probable dermal losses of nitrogen and for the growth of tissues during adult life, the average daily requirements of conventional protein (N x 6.25) calculated to a weight of 70 kg., a surface area of 1.8 square meters and a basal metabolism of 1650 cal. daily, are: 43 gm. for milk, 74 gm. for white flour, 47 gm. for soy flour, 54 gm. for the soy-white flour combination, and 50 gm. for the mixed foods. The limited significance of these numerical values is pointed out.

Some evidence is presented that the human adult, unlike the adult rat, requires lysine for nitrogen equilibrium.

² The data reported in this paper were taken from a thesis presented by Mildred Bricker to the Graduate School of the University of Illinois in partial fulfillment of the requirements for the degree of doctor of philosophy.

Manuscript received 31 May 1945.