Adequate intake values for dietary fibre based on faecal bulking indexes of 66 foods

Objective: To determine an adequate daily intake value for dietary fibre (AI(df)) based on faecal bulking indexes (FBIs) for 66 foods of known total dietary fibre (TDF) content. Design: FBIs of 66 foods were measured and expressed as wheat bran equivalents (WBE(fb)) per 100 g of food. A daily WBE(fb) requirement for humans was calculated from faecal bulk generated per gram of wheat bran TDF in humans, using a critical faecal mass of 200 g/day for protection against large bowel disease. TDF content was regressed against WBE(fb) content for all 66 foods assayed, and an AI(df) value obtained by substituting the calculated human WBE(fb) requirement into the regression equation. Method: FBI was measured using a validated rat assay, with eight large (400+/-50 g) rats per group, preadapted to dietary fibre, and fed adequate restricted diets containing test foods at inclusion rates consistent with human intakes. The critical faecal mass was based on epidemiological studies, and the faecal bulk generated per gram of wheat bran TDF was the mean of 27 published values. Results: WBE(fb) requirements for humans were calculated to be 90 g/day, corresponding to a faecal output of 200 g/day. The regression equation relating WBE(fb) measured in the FBI assay to TDF in all 66 foods was TDF=0.419WBE(fb)+3.19.R2=0.81 Substituting the human WBE(fb) requirement of 90 g into the regression equation gave an AI(df) value of 40.9 g TDF/day. Conclusion: The AI(df) value of 40.9 g TDF/day based on faecal bulking supports the AI(df) of 38 g TDF/day recently set by the Institute of Medicine (USA), for young men, based on protection against heart disease. The AI(df) value is obtained from the relation between the effects of foods and their content of mixed function, plant cell wall dietary fibres within the food matrices, and should not be used to guide intakes of extrinsic functional polysaccharides.