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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 and Total Fiber

“Dietary fiber” as noted on the “Nutrition Facts Label” of commercial foods, is fiber originally present in the food. “Total fiber” consists of dietary fiber and added fiber – substances that are adding to original food to increase their fiber content or to change their physical properties. Pectin, for example, is added to jam to give it a gel form.

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

Soluble Fiber

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 seedSubstances found in soluble fiber are gum, pectin, some hemicelluloses, mucilage and storage polysaccharides (starch and glycogen).

Beneficial Effects of Soluble Fiber

  • Soluble fiber may prevent both diarrhea and constipation. It absorbs water from the gut and thus makes stool soft, but not bulky.
  • Soluble fiber may help in weight loss. It slows down the speed of the passage of food through the gut thus giving a feeling of fullness.
  • Soluble fiber may help to prevent and treat diabetes type 2. It slows down absorption of glucose from the intestine into the blood thus preventing high spikes of glucose in the blood after a meal.
  • Soluble fiber may lower total and LDL cholesterol and thus helps to prevent ischemic heart disease and stroke. It binds bile acids in the bowel and removes them from the body and thus reduces their absorption into the blood. Lost bile acids are replaced by synthesis from blood cholesterol. This is one theory about how soluble fiber lowers blood cholesterol levels.
  • Soluble fiber may prevent bile salt diarrhea after a gallbladder removal.   

Unwanted Effects of Soluble Fiber

Soluble fiber, if ingested in excess may cause:

  • Abdominal bloating and flatulence
  • Dehydration, if ingested without water
  • Pectin may reduce absorption of cholesterol-lowering drugs, like Lovastatin

Soluble Fiber Supplements

Examples of soluble fiber supplements:

  • Metamucil (psyllium – isphagula)
  • Citrucel (methylcellulose)
  • Benefiber (wheat dextrin)
  • FiberChoise (Inulin)
  • Pectin

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. Both pectins and gums are mucilaginous; they absorb water to form viscous gels in the stomach that decrease the rate of gastric emptying. 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.

Insoluble Fiber

Insoluble fiber can not 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.

Beneficial Effects of Insoluble Fiber

  • Insoluble fiber may help to prevent constipation, hemorrhoids and diverticulosis. It binds water and thus makes stool soft and bulky; it also speeds up the passage of food through the intestine.

Unwanted Effects of Insoluble Fiber

  • Ingesting foods with insoluble fiber containing sulphur (garlic, onions) may result in excessive gas.
  • Insoluble fiber eaten on an empty stomach may aggravate symptoms of irritable bowel syndrome.
  • Insoluble fiber ingested without water may result in severe constipation or even intestinal obstruction.
  • Certain types of insoluble fiber may trigger diarrhea in sensitive people.
  • Excessive ingestion of supplements containing insoluble fiber, especially in small children, may reduce absorption of calcium, magnesium, iron, copper and zinc.

1) Cellulose is a major structural component of plant cell walls. Cellulose is a long, linear polymer of glucose (β-D-glucopyranose) units that are joined by β(1→4) glycosidic bonds . Cellulose molecules have an extended, rigid structure that is stabilized by interchain hydrogen bonds.

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 α(1→4) glycosidic bonds .Starch is composed of two types of polymers, amylose, which has a nonbranched helical structure, and amylopectin, which is branched with α(1→6) glycosidic bonds joining the branches to the main polymer chain. Although starch is easily digested by salivary and pancreatic amylase and the disaccharidases present on the brush border of intestinal mucosal cells, cellulose cannot be hydrolyzed. The β(1→4) glycosidic bonds of the cellulose chain cannot be cleaved by the amylases present in the digestive tract.


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



Figure- 2-The molecular structure of starch, indicating the repeating disaccharide unit, maltose, as well as the α-1,6-glycosidic bond present in the branch points of amylopectin.

2) 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. Hemicelluloses have a random, amorphous structure that is suitable for their location in the plant cell wall matrix. Depending on their molecular structure, hemicelluloses are partially digestible.

 3) Lignins are formed by the irreversible dehydration of sugars that result in aromatic structures. 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.


Figure-3-  A lignin molecule in an early stage of condensation. The aromatic rings are a result of irreversible dehydration of sugar residues.

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 cholesterol and many potential carcinogens. 

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