Nitrogen Balance Experiments : Some Theoretical Considerations

Abstract

THE method¹ for the determination of nitrogen balance indices (K) by measuring the slope of the line relating either absorbed or intake N to N balance has been well tried. The interpretation of the results depends on treating the experimental points as falling on a straight line in the region below nitrogen equilibrium, and values for K may be shown to be theoretically equal to values for either biological or net protein utilization (NPU) depending on whether data for absorbed or intake nitrogen are used. For example: N balance = endogenous excretion + K × N intake (1) However, it has recently been shown² that NPU varies with the concentration of protein in the diet (P) according to the equation: NPU = b(1 − kP) (3) where b and k are constants. In view of the work of Allison and Anderson¹, we^2,3 took the view that below maintenance NPU was constant, but re-examination of the literature suggests that the efficiency of nitrogen utilization falls over the whole range of protein concentration and that the relationship plotted by the American workers should be slightly curvilinear. This does not invalidate nitrogen balance index as a method of assaying protein value since the slopes of the lines would be similar, but raises some points of theoretical interest. For example, it is possible to explain why some values for K and indeed some biological values would indicate efficiencies of retention that are greater than 100 per cent. Several such values appear in the literature, and one might suspect that others have been found but rejected as apparently impossible.