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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]

Abstract: In vitro assays have been developed to predict dry matter digestibility (DMD) in the human colon, but there is little information on the effect of assay variables. The effect of altering pH, duration of incubation, presence of shaking during incubation and the concentration of faecal inoculum or digestive enzymes on DMD was investigated for three in vitro hindgut digestibility assays. Three mixed human diets varying in the type and ratio of soluble and insoluble dietary fibre were used as substrates. The pH, duration of incubation and the concentration of inoculum relative to substrate significantly (P >0.05) affected predicted DMD for the two in vitro methods employing faecal inocula. The method using synthetic enzymes showed little sensitivity to alteration of assay variables and gave highly variable results and for this reason was not pursued further. Shaking did not affect (P >0.05) digestibility for any method or diet. The different methods led to large differences in predicted hindgut DMD within each of the three diets. In vitro hindgut digestibility assay variables need to be optimised and the predicted DMD data validated against in vivo data. [ABSTRACT FROM AUTHOR]