Symposium: Physiological and Metabolic Effects of Dietary Fiber

Abstract

The generally accepted definition of dietary fiber is that it is comprised of plant material which cannot be degraded by the endogenous secretions of the human digestive tract (1). There are a number of terms which have been used to describe this material, among them unavailable carbohydrates (2), nonnutritive fiber (3), edible fiber (4), and plantix (5). However, the term “dietary fiber” coined by Hipsley (6) and popularized by Trowel1 (1) has become common usage in the lay and scientific communities and will not be dislodged easily.

The major constituents of fiber are:

Cellulose: an unbranched 1-4 β-glucose polymer containing between 3000 and 100,000 glucose units. It is degraded to some extent by colonic bacteria.

Hemicelluloses: also called noncellulosic polysaccharides. They are complex polymers which may be branched extensively. Generally, hemicelluloses are comprised of a spine containing xylose, mannose, galactose, and glucose with arabinose, galactose, and galacturonic acid distributed randomly throughout the polymer. The usual hemicellulose unit contains 1 50-200 sugar units. Hemicelluloses, too, are degraded by colonic bacteria.

Pectins (or pectic substances): these substances represent 4% or less of cell wall polysaccharides. Pectins are polymers of 1-4 β-D-galacturonic acids which contain 10-25% of neutral sugars-arabinose, galactose, xylose, rhamnose, and fucose. The acidic components of pectins may be methylated to varying degrees and the extent of methylation can influence their physiological functions. Pectins are completely degraded by the intestinal flora.

Gums and mucilages: these are not strictly cell wall components but they are secreted by plants and because their physical and physiological properties resemble those of cell wall constituents, they are accepted under the designation of dietary fiber. The gums are highly branched polymers of uronic acids, principally glucuronic and galacturonic acids. They also contain xylose, fucose, rhamnose, and galactose.