Modification of the saturation kinetics model to produce a more versatile protein quality assay

The saturation kinetics (SK) model relates response to nutrient (protein) intake in higher organisms by the equations: r = (bK1 + Rmax In)(K1 + In); where r is response and I is intake. Experimental data are fitted to this equation by interactive computer programs in which the intercept, b; a nutrition constant K1; an asymptotic response value Rmax; and an apparent kinetic order, n, are calculated. This model, unlike linear methods (PER, NPU) closely fits experimental data and predicts protein quality over a wide range of intake values. However, the SK model fails to converge to realistic values of Rmax and K1 unless protein of a sufficient quality or concentration or both, to produce a distinct plateau is fed. Also, the model may fail to distinguish statistically between the qualities of proteins which are easily differentiated by linear methods. These problems are ameliorated by assigning predetermined values to b and Rmax. When b was fixed as the mean response (body nitrogen) of rats fed protein-free diet, and Rmax was assigned a value related to the maximum response of rats fed an excess of high quality protein (casein), reasonable and statistically different values of K1 and n were predicted for casein, peanut protein and wheat gluten.