Quantifying the digestibility of dietary protein

The current recommendation, when calculating a protein digestibility-correc ted amino acid score, is to determine the digestibility of a dietary protei n across the entire digestive tract, using the rat as a model animal for hu mans. This fecal digestibility value is subsequently corrected for endogeno us 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 dieta ry 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 discu ss the various methods available for determining ileal digestibility values and compare results obtained for dietary proteins using both fecal and ile al digestibility assays. The relative value of using individual amino acid digestibility values as opposed to nitrogen digestibility values is reviewe d. In addition, we address issues surrounding measurement of endogenous nit rogen flows, and in particular, the relative merits of determining "true" v ersus "real" digestibility values.