Faecal bulking index: A physiological basis for dietary management of bulk in the distal colon

Bulk in the distal colon provides protection against a range of large bowel disorders, but a simple standardized measure of the relative bulking efficacy of foods, for use in dietary management of distal colonic bulk, has not been available. This paper describes a faecal bulking index (FBI) for standardized measurement of the relative colonic bulking efficacy of foods relative to a reference material. Faecal bulking index is defined as the mass of fully rehydrated faecal matter accumulated by the distal colon per gram of a food consumed, as a percentage of the matter accumulated from the same weight of a reference food. The FBI of foods was measured after feeding adult rats at moderate levels by partially or completely replacing sucrose in a baseline diet already containing mixed dietary fiber. Faeces were collected, dried, weighed, allowed to imbibe water until fully rehydrated, reweighed and their mass and water holding capacity measured. The FBI was calculated as the increase over baseline in rehydrated faecal mass induced by a test food as a percentage of the increase due to wheat bran (reference). The FBI values were measured for 69 diets including breakfast cereals, breads and other bakery products, fruits, vegetables, food ingredients and polysaccharides. Values for most foods ranged between almost zero for some starch-based foods to about 50 for wheat bran-enriched breakfast cereals, but laxatives based on fermentation-resistant hydrated polysaccharide had FBI values well in excess of 100 (FBI for psyllium = 500). The FBI values allow foods to be ranked according to their faecal bulking efficacy on an equal edible weight basis. They can also be used to calculate the bulking action of any amount of food in terms of equivalents to a reference material such as wheat bran. Wheat bran equivalents allow the cumulative intake of potential distal colonic bulk to be monitored for single foods or mixed meals, and shortfalls to be quantified for dietary modification or supplementation. Measures such as FBI or wheat bran equivalents would prove more useful than dietary fiber in controlling 'functional foods' promoted as effective bulking agents.