of a lignin molecule in the early stage of condensation is shown in Figure 25-2.
These polymers cannot be broken down by the digestive enzymes and, like cel-
lulose and the indigestible portion of hemicelluloses, form the stool bulk.
The soluble fibers such as pectin and true plant gums are mucilaginous and
are digestible. Pectins are predominantly polygalacturonic acids with varying
amounts of other hexose or pentose residues. True plant gums are complex
polysaccharides composed of primarily arabinose, fucose, galactose, mannose,
rhamnose, and xylose. The gums are soluble in water and are digestible by the
enzymes in the intestinal tract. Both pectins and gums are mucilaginous; they
absorb water to form viscous gels in the stomach that decrease the rate of gas-
tric emptying.
Although cellulose and hemicellulose are insoluble, they absorb water to
swell and increase the stool bulk. This results in larger, softer stools. It has
been shown that diets plentiful in insoluble fiber also increase the transit time
of food in the digestive tract and decrease intracolonic pressure. Lignins, in
addition to increasing stool bulk, also bind organic molecules such as choles-
terol and many potential carcinogens. The mucilaginous nature of the soluble
fibers, pectins, and gums tends to decrease the rate at which carbohydrates are
digested and absorbed, thus decreasing both the rise in blood glucose levels
and the ensuing increase in insulin concentration.
Figure 25-2. A lignin molecule in an early stage of condensation. The aro-
matic rings are a result of irreversible dehydration of sugar residues.
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