Apparent Digestibility of Dietary Protein as a Function of Protein Level

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Abstract
Coefficients of apparent digestibility of protein for rats were obtained on a wide range of combinations of wheat, methocel, egg and cheese. Partial regression and multiple correlation analysis of the data revealed that the greatest portion of the variation in the protein digestibility coefficient is accounted for by the percentage of protein in the diet. Therefore, although different types of foods frequently have characteristic protein levels, any relation between type of food and the apparent digestibility of its protein is coincidental. Adjustment of the total fecal protein to average dry matter intake revealed that there was a small increase in fecal protein with increasing percentage of protein in the diet. It was postulated that this increase was due to bacterial protein, and that the true digestibility of the protein in all of these diets was 100%. Thus, differences in apparent digestibility of protein of different types of foods are not primarily due to inherent differences in the true digestibility of their protein, but are mainly a consequence of the effect of a relatively constant metabolic fecal nitrogen on the calculation of the proportion of the protein intake which “disappears” between its ingestion and subsequent fecal output.