Dietary Fiber – Soluble vs. Insoluble Fiber

What Is Dietary Fiber?

Dietary fiber (fibre) is the indigestible part of plant foods that makes stool soft and thus enables smooth bowel movements, prevents constipation and reduces severity of hemorrhoids and diverticulosis. Other effects of fiber depend on the type – soluble or insoluble fiber (see below).

Dietary Fiber and Total Fiber

“Dietary fiber”, as noted on the “Nutrition Facts Label” of commercial foods, is the original fiber present in the food. “Total fiber” consists of dietary fiber and added fiber – substances that are added to the original state of food to increase the fiber content or to change its physical properties. Pectin, for example, is added to jam to give it a gel form. In fiber content charts “total fiber” means a sum of soluble and insoluble fiber, though.

Soluble Fiber

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

BENEFICIAL Effects of Soluble Fiber

  • Soluble fiber may prevent both diarrhea or constipation. It absorbs water from the gut and thus makes stool soft, but not bulky. People with irritable bowel syndrome can try foods rich in soluble fiber on an empty stomach.
  • Soluble fiber may help in weight loss. It slows down 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 spikes of glucose in the blood after a meal.
  • Soluble fiber may lower total and LDL cholesterol and thus helps to prevent hypercholesterolemia, which leads to ischemic heart disease and stroke. It binds the bile acids in the bowel and removes them from the body thereby reducing their absorption into the blood. Lost bile acids are replaced by synthesis from blood cholesterol resulting in drop of its blood level. 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 excessive gas
  • Dehydration, if taken without water
  • Pectin may reduce absorption of cholesterol-lowering drugs, like lovastatin

Soluble Fiber Supplements

Examples of soluble fiber supplements:

  • Metamucil (psyllium – isphagula)
  • Citrucel (methycellulose)
  • Benefiber (wheat dextrin)
  • FiberChoise (inulin)
  • Pectin

Insoluble Fiber

Insoluble fiber can not dissolve 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 diverticula. It absorbs 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 intake of supplements containing insoluble fiber, especially in small children, may reduce absorption of calcium, magnesium, iron, copper and zinc.

Does Food Processing Affect Fiber?

The capacity of fiber to hold water can be decreased by food processing, therefore procesing may affect the quality of fiber. Freezing and canning does not affect capacity of fiber to hold water, but cooking, crushing and drying can do so. Almost all fiber is usually removed from clear fruit juices.

End Notes

  • Investigations have not reliably proven, if dietary fiber reduces the risk of colorectal cancer or other malignancies.
  • Experiment with different fiber-rich foods to find out which helps you maintain regular bowel movements.

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About Jan Modric (209 Articles)
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