CLINICAL CASES
231
A nsw ers
[25.1]
D. Pectins and gums are soluble dietary fibers that absorb water and
form mucilaginous gels. In doing so, they delay gastric emptying and
decrease the rate at which monosaccharides such as glucose and fruc-
tose and disaccharides are absorbed by the intestinal tract. By
decreasing the rate of sugar absorption, postprandial spikes in blood
glucose concentration are avoided.
[25.2]
A. Raffinose and stachyose are sucrose molecules that have one and
two galactose residues in an ai—6 glycosidic linkage. These bonds
are not hydrolyzed by intestinal enzymes, but can be broken down by
intestinal bacteria to produce CO2 and H2. Although they contain the
glucose (pi—>2) fructose link, sucrase can hydrolyze that bond. The
galactose (pi——
4) glucose, glucose (a i—4) glucose, and glucose
(pi——
4) glucose linkages are not found in these oligosaccharides.
[25.3]
E. Cellulose is a polymer of glucose in pi,4 glycosidic linkages. This
bond is not hydrolyzed by intestinal enzymes or the flora of the
human intestine. It makes up the bulk of the stool.
B IO C H E M IS T R Y PE A R L S
Dietary fiber comprises those components that are not digestible,
which can be grouped into two main categories, those that are
soluble and those that are insoluble in water.
The soluble fibers include pectins, gums, some hemicelluloses, and
storage polysaccharides (starch and glycogen). The insoluble
fibers include cellulose, most hemicelluloses, and lignins.
Cellulose is a long, linear polymer of glucose (P-
D
-glucopyranose)
units that are joined by b(1—4) glycosidic bonds, which cannot
be broken by human enzymes.
Starch, the plant storage polysaccharide, which is also a polymer of
glucose, differs in its structure in that the glucose monomer units
are joined by a (i—4) glycosidic bonds.
REFERENCES
Chaney SG. Principles of nutrition I: macronutrients. In: Devlin TM, ed. Textbook
of Biochemistry with Clinical Correlations, 5th ed. New York: Wiley-Liss,
2002:1127-8.
Mayes PA. Nutrition. In: Murray RK, Granner KK, Mayes PA, et al., eds. Harper’s
Illustrated Biochemistry, 26th ed. New York: Lange Medical Books/McGraw-Hill,
2003:656-7.
Pettit JL. Fiber. Clinician Reviews 2002;12(9):71-5.
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