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The Determination of Metabolic Fecal Nitrogen and Protein Digestibility

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Department of Biochemistry, Medical Research Division, Sharp and Dohme, Inc., Glenolden, Pennsylvania

"True digestibilities" of the proteins of whole egg, wheat gluten, and casein and the fecal nitrogen of metabolic origin have been determined under conditions of protein feeding with growing mice. The metabolic fecal nitrogen was determined by plotting the nitrogen intake per unit of food consumed at different dietary protein levels as a function of the fecal nitrogen excretion per unit of food consumed. The calculated straight line was extropolated to the point of zero nitrogen intake. The Y intercept was then used as the value for the metabolic fecal nitrogen per unit of food consumed.

Under conditions of ad libitum feeding the average "true digestibilities" determined by this method were: casein 98.3 ± 0.1%, wheat gluten 98.3 ± 0.3%, and extracted whole egg 91.5 ± 0.6%. When the food intake of mature mice was restricted to approximately 30% of the normal caloric requirement the average "true digestibility" of extracted whole egg was found to be 95.4 ± 0.2%.

The data indicate that metabolic fecal nitrogen values determined with protein-free or low protein diets are not safe indices of the metabolic fecal nitrogen under conditions of protein feeding.