Accurate estimates of endogenous ileal total nitrogen and amino acid flows are necessary to ascertain true dietary amino acid digestibility coefficients and for the factorial estimation of dietary amino acid requirements. Objective: The objective was to ascertain endogenous amino acid losses from the small bowel in human subjects consuming a proteinfree diet or a diet with enzyme-hydrolyzed casein (EHC; MW 5000) as the sole source of nitrogen. Design: The subjects were 8 men and women with

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... nal ileum ileostomies after ulcerative colitis who consumed the protein-free and EHC-based diets in a crossover design. Each subject received each test diet in single meals followed by 2 consecutive 9-h total collections of digesta. Digesta samples for the EHC treatment were centrifuged and ultrafiltered (10 000 MW cutoff), with the precipitate-plus-retentate fraction (10 000 MW) providing a measurement of endogenous ileal amino acids. Results: The mean endogenous flows for most of the amino acids and nitrogen were significantly (P 0.05) higher when determined with the EHC-based diet than with the protein-free diet. Mean (n ҃ 8) endogenous ileal nitrogen flows were 2061 and 4233 g/g dry matter intake for the protein-free and EHC-based diets, respectively. Conclusion: The traditional protein-free method underestimates endogenous ileal amino acid loss in adults. Am J Clin Nutr 2005;81:1359 -65.