Plant residue and bacteria as bases for increased stool weight accompanying consumption of higher dietary fiber diets

Objective: Stool diluting effects of relatively inert material, such as unf ermentable dietary fiber, has been proposed as an effect of fiber beneficia l to the colon. Stool dilution by increasing bacterial mass may be benefici al or deleterious, depending on bacterial metabolic products. The purpose o f this study was to determine the basis for stool weight when two stepwise increases of fiber from all classes of fiber-containing foods were consumed . Methods: Stool from five men consuming three constant diets containing 15, 30 and 42 g/d of dietary fiber were fractionated into plant material and ba cteria and analyzed for neutral and amino sugar content. Fecal nitrogen, fa t and ash were measured. Results: Daily gravimetric yield and sugar content of the plant fraction fr om stool increased with each fiber addition. Compared to the low fiber diet , the medium fiber diet decreased the concentration of the bacterial mass i n wet stool by 11% and the high fiber diet by an additional 32%. The high f iber diet decreased stool fat concentration; the medium and high fiber diet s decreased stool nitrogen concentration to the same extent. Apparent diges tibility of plant-derived neutral sugars decreased with each fiber addition . Conclusions: Inherently less fermentable plant material modulates the colon environment in three beneficial ways: it is a relatively unreactive diluen t of lumenal contents; it adds mass to promote distal movement of waste; it does not promote a large bacterial mass.