The in vitro water-holding properties of 17 dietary fibre preparations, mainly food materials, bulk laxatives, and gel-forming polysaccharides, have been measured. Water uptake was measured by a centrifugation technique and also by a new method using sacs of dialysis tubing containing the material, immersed in simulated gut contents. The centrifugation technique could not be applied to gel-forming polysaccharides but the methods gave broadly similar results for other materials (r=0·85). The gel-forming polysaccharides in general held more water than the food fibres. Studies of matched pairs of materials which differed only slightly in chemical composition suggested that the presence of charged groups on the molecule encouraged water uptake. In the food materials water uptake was related to uronic acid content (r=0·87). Materials ground to a smaller particle size increase their water holding but this effect was small (+28%). The hypothesis that dietary fibre increases faecal bulk by virtue of its ability to hold water was tested by comparing the in vitro water-holding capacity of eight of the fibres with the changes they had produced when fed to human volunteers under controlled conditions. Of these materials pectin had the greatest water-holding capacity (56·2 g water/g material) but produced the smallest change in faecal weight (19%), while bran had the lowest water-holding (4·2 g/g) and the largest faecal weight changes (117%). Overall an inverse relationship (r=0·88) between water-holding and faecal bulking was found, suggesting that dietary fibre does not exert its effect on faecal weight simply by retaining water in the gut.
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