Role of dietary fiber

A 65-year-old man expresses his concern about developing colon cancer to his physician. He states that his father as well as grandmother died of colon cancer. After performing a physical exam and observing unremarkable colonoscopy, the physician recommends a diet low in fat and high in fiber. The physician explains that studies have shown a decreased risk of colon cancer with insoluble fiber consumption. Which of the following bonds is responsible for the fiber’s inability to be digested?

A. α (1->4)

B. α (1->6)

C. β (1->4)

D. β (1->6)

E. α (1->2)

Answer- The correct answer is – C- β (1->4) Glycosidic linkages

Dietary fiber is indigestible part of plant foods that makes stool soft and thus enables smooth bowel movements, prevents constipation, hemorrhoids and diverticulosis.

It consists of all of the components of the cell walls of plants that are not broken down by the body’s digestive enzymes.

Dietary fiber can be grouped into two main categories, those that are soluble and those that are insoluble in water.

Soluble fiber (viscous fiber) partly dissolves in water and forms gel with it. Foods rich in soluble fiber include beans and other legumes (peas, soy, and lentils), oats, barley, citrus fruits (oranges, grapefruit), psyllium husk and flax seed. Substances found in soluble fiber are gum, pectin, some hemicelluloses, mucilage and storage polysaccharides (starch and glycogen).

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 poly saccharides 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.

Insoluble fiber cannot be dissolved in water. Foods rich in insoluble fiber include whole wheat and other whole grains and most dark green leafy vegetables, like cabbage and cauliflower. Substances found in insoluble fiber include cellulose, hemicellulose and lignin.

The commonest out of theses is cellulose containing β (1->4) glycosidic linkages (figure-1)


Figure-1- Showing the molecular structure of cellulose, indicating the repeating disaccharide unit, cellobiose.

Hemicelluloses are also polysaccharides that are structural components of plant cell walls. However, unlike what their name implies, they are unrelated to cellulose. They are polymers that are made up of a variety of sugar monomers that include glucose, galactose, mannose, arabinose, and Xylose, as well as acidic forms of these monosaccharides. Xylose is the monosaccharide that is most abundant

Lignins are formed by the irreversible dehydration of sugars that result in aromatic structures (figure-2) The remaining alcohol or phenol OH groups can react with each other and with aldehyde and ketone groups to form polymers. These polymers cannot be broken down by the digestive enzymes and, like cellulose and the indigestible portion of hemicelluloses, form the stool bulk.

Structure of Lignin

Figure-2- A lignin molecule in an early stage of condensation. The aromatic rings are a result of irreversible dehydration of sugar residues.

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