Dorchester County Health Department

3 Cedar Street | Cambridge, MD 21613 | 410-228-3223; Fax 410-228-9319 | Monday - Friday, 8:00 am - 5:00 pm

Fiber...Just for the health of it!

What is it?

Types of Fiber

Sources

Fiber and Your Health

Recommendations

What is it?

Dietary fiber generally refers to parts of fruits, vegetables, grains, nuts and legumes that cannot be broken down by human digestive enzymes or absorbed by the gastrointestinal tract. Meats and dairy products contain no fiber. Until recently, fiber (roughage) was largely ignored by the scientific community. After all, the term fiber represents a variety of carbohydrates that are not digested, absorbed or used by the body either for fuel or for building body tissue.

Actually there are many compounds that belong to the category called dietary fiber. The most important of these are under:

  • Cellulose
  • Hemi-cellulose
  • Lignin
  • Pectin
  • Gums
  • Algal Polysaccharides – from kelp (seaweeds)
  • Indigestible oligosaccharides – from legumes

Because individual types of fiber differ in solubility and in other physical and chemical properties, the effects they have on health may differ. Just knowing the total intake of dietary fiber is of limited value.

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Types of Fiber

Did you know that not all fiber is alike? In fact, there are two kinds of dietary fiber that are important for good health: insoluble fibers and soluble fibers. Most fiber-containing foods feature both, but one or the other type often predominates in specific parts of a food and determines the characteristic texture of that portion of the food.

For example, insoluble fibers produce the tough, chewy feel of wheat kernels, popcorn, apple skin and nuts. Essential to the cellular structure of plants, insoluble fibers include cellulose, hemicelluloses and ligin and do not dissolve in water.

Soluble fibers include pectin, gums, mucilages and algal polysaccharides. Although pectin is part of cell walls, most soluble fibers are found within plant cells. The gummy essence of oat bran and the mushy center of a cooked kidney bean reflect both the soluble fiber content of those foods and the ability of soluble fibers to soak up water.

The fiber content of a food varies according to the species of the plant and stage of maturation, but seeds, berries, fruit skins and the bran layers of cereal grains generally contain larger amounts of a plant's fiber.

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Sources

Because dietary fiber is composed largely of structural components of the cell walls of the plants, the major sources of fiber are whole grains, legumes, vegetables, fruits and nuts. Animal products are not a source of dietary fiber. The distribution of specific types of fiber varies, even within food groups. Good sources of pectin, for example, include apples, cranberries and cherries, but pears and strawberries are low in pectin.

Oatmeal and legumes have a significant gum content. Generally, whole grain is noted for its cellulose content.

The amount and characteristics of dietary fiber may be affected by food processing. Refining of grains, for example, removes nearly all of the fiber. Preparing juice, fruit or vegetable, does likewise. Peeling apples, peaches, potatoes, and the like removes a fiber-rich part of the plant.

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Fiber and Your Health

A high-fiber diet appears to reduce disease risk by increasing fecal bulk, decreasing the transit time of food through the gastrointestinal tract, reducing blood cholesterol levels and helping to control blood sugar levels. With their distinct physical characteristics, insoluble and soluble fibers work differently to produce these results.

Insoluble fibers seem to have their greatest impact on the health of the colon or large intestine. Large amounts of insoluble fibers increase fecal bulk and draw water into the large intestine. The result is a larger, softer stool that exerts less pressure on the colon walls and is eliminated more quickly. Indeed, the most well-established benefit of a high-fiber diet is in the treatment and prevention of constipation. The reduced pressure also may help prevent diverticulosis (small herniations in the colon wall that may become inflamed). In addition, large amounts of insoluble fibers dilute the concentration of potential carcinogens that may be present in the stool, and the decreased transit time reduces the exposure of the intestinal wall to those substances. Furthermore, insoluble fibers alter the pH of the large intestine, interfering with microbial activity that produces carcinogens. The combined effect may be a reduced risk of colon cancer.

Some soluble fibers also add to fecal bulk and increase its water content. But soluble fiber's potential for reducing blood cholesterol levels recently has grabbed the spotlight. Diets rich in soluble fibers such as oat bran may help reduce total cholesterol and low density lipoprotein (LDL) cholesterol in people with both high and normal blood cholesterol levels. Soluble fibers appear to reduce blood cholesterol in two ways:

  • They prevent the reabsorption of vital bile acids from the small intestine. To replace the lost bile acids, cholesterol is drawn from the body, thereby reducing its cholesterol supply.
  • The fermentation of soluble fibers in the intestine produces short-chain fatty acids which block the synthesis of cholesterol.

Soluble fibers also may help control the rise in blood sugar following a meal and reduce insulin requirements in some patients with diabetes mellitus. By increasing the viscosity of gastrointestinal contents, soluble fibers retard gastric emptying, slowing the absorption of glucose in the process.

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Recommendations

While Americans currently consume an average of 11 grams of dietary fiber daily, the National Cancer Institute advises an increase to 25 to 35 grams a day. Although soluble fibers have received much attention, you should increase the level of fiber by increasing foods from all the vegetable, grain and fruit sources and not rely on fiber supplements because they are not recommended as a way to meet dietary guidelines. The Diet and Health report of the National Academy of Sciences has gone one step further by specifying recommended amounts of foods high in fiber. It advises a daily intake of five or more servings of fruits and vegetables and six or more servings of whole grain breads and cereals and legumes.

Increasing fiber consumption too rapidly can result in flatulence[gas], cramping, and intestinal distention. Undesirable side effects may be avoided through the gradual addition of fiber to the diet along with an adequate fluid intake.

Although concerns that fiber may interfere with the absorption of trace minerals have been voiced, people who consume well-balanced and varied meal plans high in fiber are unlikely to experience mineral deficiencies.

While fiber does seem significant to human health, the effects of heredity, the quality of the overall diet and habits such as smoking and physical activity can outweigh any single dietary modification. The best guarantee is an integrated effort that includes a high-fiber diet as part of a healthy lifestyle.

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