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Supplement

Quantifying the Digestibility of Dietary Protein¹

Milk and Health Research Centre, Institute of Food Nutrition and Human Health, Massey University, Palmerston North, New Zealand

²To whom correspondence should be addressed.

The current recommendation, when calculating a protein digestibility–corrected amino acid score, is to determine the digestibility of a dietary protein across the entire digestive tract, using the rat as a model animal for humans. This fecal digestibility value is subsequently corrected for endogenous contributions of protein using a metabolic nitrogen value determined by feeding rats a protein-free diet. The limitations inherent with this method are well recognized, however, and determining the digestibility of a dietary protein to the end of the small intestine is the preferred alternative. Unlike the fecal digestibility assay, which has only one basic methodology, ileal digestibility values can be determined in a number of ways. We discuss the various methods available for determining ileal digestibility values and compare results obtained for dietary proteins using both fecal and ileal digestibility assays. The relative value of using individual amino acid digestibility values as opposed to nitrogen digestibility values is reviewed. In addition, we address issues surrounding measurement of endogenous nitrogen flows, and in particular, the relative merits of determining "true" versus "real" digestibility values.

KEY WORDS: • protein digestibility • apparent digestibility • true digestibility • real digestibility • protein quality evaluation

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