Dietary fiber

In addition to being one of the most talked about nutrients for health promotion and disease prevention, dietary fiber is also the focus of two FDA-approved health claims that appear on food labels. These claims reflect the benefits of high fiber foods for the prevention of heart disease and certain types of cancer.

Ever since the early 1950's, when the term "fiber" was first used in scientific journals, there has been considerable controversy among medical experts, food scientists and nutritionists as to the exact definition of dietary fiber. Even the United States Food and Drug Administration (FDA) has no formal, written definition for dietary fiber and has adopted the analytical methods that the Association of Official Analytical Chemists uses for defining dietary fiber for food labeling purposes and the determination of health claims.

Most experts do agree that a key defining characteristic of dietary fiber is that it is derived from the edible parts of plants that are not able to be broken down by human digestive enzymes. Yet, many people still believe that this definition is too ambiguous and that there needs to be a clearer definition that has international acceptance in order to ensure that the total fiber counts on food labels are consistent and accurate.

There has been a growing movement in recent years among various organizations to propose that the physiological benefits of dietary fiber should become part of their new definition. For example, the new definition proposed by the American Association of Cereal Chemists includes the statement Dietary fibers promote beneficial physiological effects including laxation and/or blood cholesterol attenuation and/or blood glucose attenuation. The Institute of Medicine at the National Academy of Sciences (the organization that is responsible for issuing Recommended Dietary Allowances) has proposed a definition that would differentiate between dietary fiber and added fiber. Under this definition, nondigestible carbohydrates and lignins that are intrinsic and intact in plants would fall under the category of dietary fiber. Added fiber would be the category that would include isolated nondigestible carbohydrates that have been proven to have beneficial physiological effects in humans. Total Fiber is defined by the Institute of Medicine for food labeling purposes as the sum of Dietary Fiber and Added Fiber.

While there may still be controversy as to how to exactly define dietary fiber, experts do agree on one important thing that dietary fiber plays a significant role in promoting health and preventing heart disease, colon cancer, diabetes and obesity.

Categories of Dietary Fiber:

• Cellulose: found in bran, legumes, peas, root vegetables, apples, seed coverings and the cabbage family of vegetables
• Hemicellulose: found in bran and whole grains
• Polyfructoses such as inulin and oligofructans
• Galactooligosaccharides
• Gums: found in oatmeal, barley, and legumes
• Mucilages
• Pectins: found in apples, strawberries, and citrus fruits
• Lignin: found in root vegetables, wheat, and fruits, such as strawberries with edible seeds
• Resistant Starches: found in ripe bananas, potatoes and whole grains

 

Physiological functions of dietary fiber

• Supports bowel regularity
• Helps maintain normal cholesterol levels
• Helps maintain normal blood sugar levels

 

Physiological events that can signal a need for greater dietary fiber intake

• Constipation
• Hemorrhoids
• High cholesterol levels
• High blood sugar levels

Functions of dietary fiber

It was commonly accepted, until very recently, that that functions of a specific fiber type was determined based upon whether the fiber was classified as soluble or insoluble. Soluble fibers, like those found in oat bran, are known to be able to normalize blood sugar levels and reduce blood cholesterol while insoluble fiber, such as found in psyllium seed husk, is known to promote bowel regularity.

While these terms may enjoy widespread use in describing the health benefits of dietary fiber, many medical and nutrition experts believe that they are limited in that they do not truly describe the physiological effects of all the different types of fiber. These experts suggest that dietary fiber should be classified as either viscous or fermentable in place of soluble or insoluble to more adequately describe their functions and health benefits.

Viscous fibers: reduction of cholesterol levels and regulation of blood sugar levels

Similar to soluble fibers, viscous fibers lower serum cholesterol by reducing dietary cholesterol absorption. They also complex with bile acids, compounds manufactured from cholesterol by the liver, which play an important role in the proper digestion of fats. Once the viscous fibers complex with the bile acids, these compounds are removed from circulation. Since they are not able to make it back to the liver, the liver, as a result, must use additional cholesterol to manufacture new bile acids.

By slowing the rate at which food exits the stomach and therefore delaying the absorption of glucose following a meal, viscous fibers also help to normalize blood glucose levels. As they have also been found to increase insulin sensitivity, high intake of these fibers may play an instrumental role in the prevention and treatment of non-insulin dependent diabetes mellitus. Additionally, viscous fibers can promote satiety, feeling of fullness, because of their ability to slow the rate at which food leaves the stomach. This can help prevent overeating and weight gain.

Fermentable fibers: promoting bowel regularity and maintaining healthy intestinal cells

Fermentable fibers are given that moniker since they are fermented by the friendly bacteria that reside in the large intestine. This fermentation process produces a short-chain fatty acid called butyric acid, which functions as the primary fuel for the cells of the large intestines, supporting the health and integrity of the colon.

Propionic and acetic acid are two other short-chain fatty acids produced during this fermentation process. These compounds are used as fuel by liver and muscle cells. Propionic acid is also thought to be partially responsible for the cholesterol-lowering properties of fiber as it has been shown to inhibit HMG-CoA reductase, an enzyme that is involved in cholesterol manufacture in the liver. Blood cholesterol levels may be lowered as the activity of this enzyme is reduced.

Fermentable fibers can also help maintain healthy populations of the friendly bacteria that play an important role in supporting immune system function by preventing the pathogenic (disease-causing) bacteria from surviving in the intestinal tract. As is the case with insoluble fiber, these fibers can also maintain bowel regularity by increasing the bulk of the feces and decreasing the transit time of fecal matter through the intestines. A decrease risk for colon cancer and hemorrhoids is associated with bowel regularity.

Causes and symptoms of dietary fiber deficiency

The inadequate fiber intake associated with the standard American diet is a primary reason contributing to a dietary fiber deficiency. Additionally, a healthy gastrointestinal system and proper digestive function are important for deriving full benefits from dietary fiber intake. Another factor that can inhibit the full realization of fibers benefit is inadequate chewing since in order for the benefits of some types of fiber (lignins, celluloses and some hemicelluloses) to be fully realized as participants in biochemical processes, they need to be mechanically broken down by chewing.

Lack of fiber in the diet does not result in a clearly identifiable isolated deficiency disease. Yet, there is a clear association between low dietary fiber intake (less than 20 grams per day) over the course of a lifetime and the development of various health conditions such as obesity, elevated cholesterol levels, hemorrhoids, constipation and colon cancer.

Causes and symptoms of dietary fiber toxicity

In most people, intake of substantial levels of dietary fiber, even in excess of 50 grams per day, will improve bowel health. Yet, in certain susceptible individuals it may cause intestinal obstructions.

Excessive intake of nonfermentable fiber, especially in supplemental form, can reduce the absorption or increase the excretion of minerals, leading to mineral deficiencies. This may be of special concern during times when mineral intake is especially low or when mineral needs are increased as in adolescence, pregnancy and lactation. Since excessive intake of fiber can cause fluid imbalance that can lead to dehydration, person who want to suddenly double or triple their fiber intake are advised to proportionally increase their water intake as well.

Effects of cooking, storage and processing on dietary fiber

Much of the fiber content of whole foods is lost when these foods undergo processing. Wheat flour is a stellar example of how processing reduces fiber content. Most breads in the United States use extraction flour in which 60-70% of the original wheat grain remains in the flour while 30-40% is discarded. The parts that are discarded are the bran and the germ. As these are the components that contain most of the wheats fiber, this extraction wheat flour contains virtually no fiber while whole, unprocessed wheat grain contains a significant amount.

Another example of how processing affects fiber content is juice. Fruits and vegetables are high fiber foods, yet when they are processed into juice they oftentimes end up with minimal, if any, fiber.

Interactions between medications and dietary fiber

Dietary fiber interacts with the following medications:

• HMG-CoA reductase inhibitors (e.g., lovastatin)
  • Fiber, especially that which is found in fruit, beans and oat bran, binds to these drugs in the gastrointestinal tract and reduces their absorption.
• Hydralazine
  • Fiber decreases its absorption.
• Digoxin
  • Fiber decreases its absorption.
• Lithium
  • Fiber decreases its absorption.
• Insulin or oral glucose lowering medications
  • Since diets high in dietary fiber may improve glucose control in individuals with non-insulin dependent diabetes mellitus, these diets may reduce the dose of these medications needed to control blood sugar.
• Pain medications (e.g. codeine)
  • These medications may cause constipation which increased dietary fiber intake may relieve.
• Calcium channel blockers (e.g. verapamil)
  • These medications may cause constipation which increased dietary fiber intake may relieve.

   

Interactions that occur between dietary fiber and other nutrients

Nonfermentable fiber, the type that passes unchanged through the gastrointestinal tract, may reduce the absorption and/or increase the excretion of minerals such as calcium and iron.

Health conditions that require special emphasis on dietary fiber

Individuals who have the following health conditions should pay special attention to their dietary fiber status:

• Breast cancer
• Cardiovascular disease
• Colon cancer
• Constipation
• Diabetes
• Diverticulitis
• Gallstones
• High cholesterol
• Irritable bowel syndrome
• Obesity

Forms in which dietary fiber is found in dietary supplements.

Fiber is available in powder form as either a dietary supplement or an over-the-counter medication These powders oftentimes contain psyllium, pectin or guar gum and can be mixed with water or juice. Oat bran, a fiber-concentrated food ingredient, is also available.

Foods that are concentrated sources of dietary fiber

Excellent sources of dietary fiber include cabbage, cauliflower, celery, collard greens, eggplant, lentils, mustard greens, raspberries and Swiss chard. Very good sources include barley, black beans, Brussel sprouts, bulgur wheat, carrots, flax seeds, pinto beans, summer squash and winter squash.