Digestion and fermentation pattern of various dietary fiber sources in the rat

Abstract

Wheat bran, beet pulp, soya bean hulls and copra meal are potential dietary fiber sources which have very different composition. Their total apparent digestibilities in rats have been shown to vary greatly: non-starch glucose digestibility was 20% for soya bean hulls and 63% for beet pulp. Arabinose from lateral chains of the beet pulp pectins was highly digestible whereas arabinans of wheat bran were poorly digestible. Mannans of copra meal were also highly digestible. The present study indicates that endogenous secretions or bacteria induce an underevaluation of the digestibility of minor neutral sugars in the diets. Parts of the neutral sugars from beet pulp (arabinose and galactose) and from copra meal (mannose) disappear from the stomach faster than others. That could indicate digestion of the polysaccharide containing the sugar and/or a quicker transit of this polysaccharide. In the small intestine, digestion or disappearance of the sugars, initiated in the stomach, was accentuated, but xylose and glucose from wheat bran disappeared faster than arabinose. Digestion in the cecum and colon is known to be due to bacteria which proliferate at this level. They produce, from dietary fibers, variable amounts of fermentation products (volatile fatty acids and gas). This production is not correlated to total digestibility of the dietary fiber. Wheat bran is poorly fermentable and is characterized by its fermentation profile, with a high proportion of butyric acid. Digestion of dietary fibers in the stomach and small intestine has been reported by several authors. Its real mechanism — chemical action by secretion or bacterial fermentation of the gut content — has yet to be identified.