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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. J. Nutr. 130: 1850S--1856S, 2000. KEY WORDS: * protein digestibility * apparent digestibility * true digestibility * real digestibility * protein quality evaluation

Background The validation of a dual in vivo-in vitro digestibility assay ('dual digestibility assay') for separately predicting the upper-tract, hindgut and total tract digestibility of nutrients in humans, as estimated using organic matter digestibility ( OMD), is described. Human upper-tract OMD was predicted using an animal (rat) model with digesta from the terminal ileum collected from rats fed one of four complete human diets (wheat bran diet, pectin diet, mixed low-fibre diet, mixed high-fibre diet). Large intestinal OMD was predicted using an in vitro hindgut fermentation assay employing a human faecal inoculum and with the rat ileal digesta as the substrate. Results A comparison of total tract OMD of the four diets from a human balance study ( OMDhuman) with that predicted using the dual digestibility assay ( OMDdual) showed no significant differences ( P > 0.05). OMDhuman and OMDdual were highly correlated ( r = 0.953, P = 0.047). Conclusion The dual digestibility assay accurately predicts the uptake of dietary nutrients (as grams of organic matter) in humans over the total tract. The assay is able to separately quantify the digestibility of nutrients in the upper and lower digestive tracts. The validation of the dual digestibility assay needs to be extended to a wider range of human diets. © 2013 Society of Chemical Industry [ABSTRACT FROM AUTHOR]