SECTION 9.1
Classification
149
F I G U R E 9 - 2 4
Structure of cellulose, (a) /1(1 -> 4) linkage of glucose residues in cellulose, (b) Conformational formula
showing hydrogen bonds ( ---------- ) between the ring oxygens and the C
3
hydroxyl groups.
hydrogen, carbon dioxide, and methane. Undigested cel-
lulose forms a part of the indigestible component of the
diet, known as
dietary fiber.
Ruminants and termites are
able to digest cellulose, which is a primary energy source
for them, because they harbor microorganisms in their in-
testinal tract that elaborate cellulase, which catalyzes hy-
drolysis of the /1(1 -> 4) glucosidic linkages.
Dietary fiber consists of cellulose and other polysaccha-
rides (hemicelluloses, pectins, gums, and alginates) and
a nonpolysaccharide component, lignin. These different
types of dietary fiber are derived from the cell wall and
the sap components of plant cells, and their properties are
listed in Table 9-2. The exact role of dietary fiber in hu-
man nutrition is not clearly understood. Each component
has a different chemical composition and exhibits differ-
ent actions in the gastrointestinal tract. Dietary fiber has
effects on the rate of absorption of nutrients and on the
fecal mass. Depending on the amount and type of the in-
digestible component, gastric emptying is decreased, thus
delaying of entry of digestible components into the small
intestine, where the nutrients are absorbed. Thus, the ab-
sorption of nutrients can be modulated by the presence
of fiber, particularly pectins or guar gums. Pectins, guar
gums, and lignins may reduce plasma cholesterol levels.
Thus, fiber may ameliorate
diabetes mellitus
(Chapter 22)
and
atherosclerosis
(Chapter 20). Since the degree of sati-
ety is related to the bulk of food that is consumed, the
amount of fiber in the diet regulates the total energy con-
sumed. Fiber also affects nutrient density, so that diets high
in fiber are normally low in energy content. These aspects
are important in the development and management of obe-
sity. The relationship of dietary fiber to colonic cancer has
been the subject of epidemiological studies showing an in-
verse relationship between fiber consumption and risk of
colonic cancer. The postulated mechanism is that the high-
fiber diet results in rapid intestinal transit, so that potential
carcinogens or cocarcinogens have less time to interact
with the mucosa. Various components of fiber may also
bind carcinogens or dilute their concentrations by increas-
ing the intraluminal bulk.
Although dietary fiber is resistant to human diges-
tive enzymes, some fiber is digested by microorganisms
that reside in the colon, yielding organic anions such as
acetate, propionate, and butyrate. Fiber and the osmoti-
cally active anions increase the size and wetness of the
stools because they attract water and form gels. The over-
all effect of a high-fiber diet is a larger fecal mass, decrease
in intraluminal pressure, reduction in transit time through
the colon, and greater ease and frequency of defecation.
These effects are useful in the treatment of some gastroin-
testinal disorders. For example, a high-fiber diet has been
recommended in the treatment of
diverticulosis,
in which
outpouchings of colonic mucosa occur through the muscu-
laris layer at points of weakness in the muscle wall. The di-
verticuli can become infected (
diverticulitis
) and produce
bleeding, altered bowel function, and pain. Cross-cultural
epidemiological studies link low-fiber diets with diver-
ticulosis, cancer of the colon and rectum, coronary heart
disease, diabetes mellitus, appendicitis, varicose veins (di-
lated tortuous veins), and hemorrhoids (enlarged veins of
the lower rectum and anus). Despite gaps in our knowledge
about the role of dietary fiber in human nutrition, adequate
amounts probably are beneficial in the prevention of some
chronic diseases. Consumption of carbohydrates in their
