The digestible amino acid composition of several milk proteins: application of a new bioassay.

The aim of the present study was to determine the composition of amino acids that were truly digestible in the ileum. Several bovine milk products and two soy protein products were tested using the newly developed enzymatically hydrolyzed casein-ultrafiltration (UF) method. This method provides a novel approach for determining endogenous flows of amino acids at the terminal ileum, which are required for correcting apparent ileal digestibility values to true digestibility values. Digestibility was determined by sampling digesta of Sprague-Dawley male rats at the end of the small intestine (ileum). Chromic oxide was used as an indigestible marker. The traditional protein-free method for determining endogenous losses of amino acids was also used for comparison with the enzymatically hydrolyzed casein-UF method. Flows of endogenous amino acids at the terminal ileum of the rat following peptide alimentation were generally higher (1.8-fold) than those determined after a protein-free diet was fed. Compared with values for true amino acid digestibility, apparent values underestimated digestibility by 2 to 30%. True amino acid digestibility was high (79 to 102%) for all of the protein sources. The digestible amounts of methionine and lysine were 2 and 1.3 times higher, respectively, in dairy proteins than in soy proteins. The enzymatically hydrolyzed casein-UF method provides a physiological estimate of endogenous amino acid flow and appeared to be an appropriate method for correcting apparent digestibility values to true digestibility values. The data for true ileal digestibility of amino acids obtained using this technique demonstrated the high quality of bovine milk proteins.