Significance of the Endogenous Nitrogen Excretion in Protein Evaluation Studies with Rats

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Abstract
Four experiments with young male rats were carried out to determine whether any particular urinary N constituents varied closely with the quantity and quality of dietary protein and to compare the “endogenous” N loss determined from the excretion of rats fed an N-free diet with that calculated from regression equations relating urinary N excretion to dietary protein intake. The dietary protein sources, which included casein and isolated soy with and without methionine supplementation, were fed at zero, 2, 4, 6, and 10% levels. The total urinary nitrogen excretion with a protein-free diet was much higher than the calculated value from the regression relating protein intake to total urinary nitrogen excretion extrapolated to zero N intake. The extrapolated values for total nitrogen excretion at zero N intake for the various experiments and different levels of dietary protein varied considerably, as did the observed nitrogen excretion values of the rats fed the protein-free diet. In contrast with these variable results, the non-urea nitrogen values, calculated from the regression equations relating total urinary N and urinary urea nitrogen to protein intake, were remarkably similar at about 50 mg/m2. Biological values, calculated by the Thomas-Mitchell method, differed greatly from values determined by the direct carcass nitrogen retention method. When the Thomas-Mitchell equation was modified by substituting the calculated non-urea nitrogen value for the “endogenous” nitrogen there was good agreement of the biological values thus calculated with those obtained by the carcass retention method. The significance of the relation of these findings to the concept of “endogenous” nitrogen metabolism is discussed.