A comparison of the bodywater and nitrogen balance-sheet methods for determining the nutritive value of proteins.

Biological methods available for determining the nutritive value of proteins have recently been reviewed by Allison (1955), Bender (1958) and Adrian & Rerat (1958), and several reports of comparisons between methods for the evaluation of selected proteins have appeared (Rutger’s University : Bureau of Biological Research, 1950; Forbes & Yohe, 1955 ; Chapman, Castillo & Campbell, 1959; Rippon, 1959; Tomarelli, Minnick, d’Amato & Bernhart, 1959). It is generally agreed (Forbes & Yohe, 1955; Bender, 1958 ; Rippon, 1959) that the balance-sheet method of Mitchell (Mitchell, 1923-4; Mitchell & Carman, 1926), measuring the biologicalvalue and true digestibility of proteins, gives more fundamental information and is more precise than other available methods. This method has been in use in this laboratory for many years and has been preferred to the growth method of Osborne, Mendel & Ferry (1919) because of its greater precision, particularly for relatively small differences (cf. Henry, Kon, Lea & White, 1947-8). It is, however, time-consuming, and the method, based on body-water determinations, described by Miller & Bender (1955) seemed to offer a more rapid way of assessing Mitchell’s (Mitchell & Villegas, 1923) net protein value (NPV), that is biological value x true digestibility -+ 100. We have applied the method of Miller & Bender (1955) to a series of thirty-five proteins or protein foods of different quality and compared the results with the NPV determined by the balance-sheet method. Before doing so it was necessary to establish, for our colony, the relationship between the age of the rat and the water and nitrogen content of its body. For this purpose two series of experiments were carried out, in the second of which the livers of the rats were analysed separately from the rest of the body, with a view to the possible application of the method to larger animals with which carcass analysis would be impracticable. In the second series some preliminary tests were also done to establish whether the ratio between body content of water and N was affected by dietary changes.