Fiber, dietary

What can high-fiber foods do for you?

• Support bowel regularity
• Help maintain normal cholesterol levels
• Help maintain normal blood sugar levels
• Help keep unwanted pounds off

What events can indicate a need for more high fiber foods?

• Constipation
• Hemorrhoids if related to straining from constipation
• High blood sugar levels
• High cholesterol levels

Excellent food sources of fiber include: turnip greens, mustard greens, cauliflower, collard greens, broccoli, Swiss chard, and raspberries.

Description

What is dietary fiber?

Dietary fiber is undoubtedly one of the most talked about nutrients for health promotion and disease prevention. In fact, dietary fiber is the focus of two FDA-approved health claims that appear on foods labels touting the benefits of high fiber foods for the prevention of heart disease and certain types of cancer.

Since the early 1950's, when the term "fiber" first began to be used in scientific journals, there has been considerable controversy among food scientists, nutritionists, and medical experts about the exact definition of dietary fiber.

In fact, even the United States Food and Drug Administration, the federal agency responsible for overseeing food labeling, has no formal, written definition of dietary fiber. For food labeling purposes and the determination of health claims, the FDA has adopted the analytical methods that the Association of Official Analytical Chemists uses for defining dietary fiber.

Although most experts agree that a key defining characteristic of dietary fiber is that it's derived from the edible parts of plants that are not broken down by human digestive enzymes, many people believe that this definition is too ambiguous and that a more clear, internationally-accepted definition is needed to ensure that the total fiber counts on food labels are consistent and accurate.

In recent years there has been a movement among various organizations to include the physiological benefits of dietary fiber in a new definition. For example, the American Association of Cereal Chemists proposed a new definition of dietary fiber that includes the statement “Dietary fibers promote beneficial physiological effects including laxation and/or blood cholesterol attenuation and/or blood glucose attenuation.”

In addition, the Institute of Medicine at the National Academy of Sciences (the organization responsible for issuing Recommended Dietary Allowances) has proposed a new definition that differentiates between dietary fiber and added fiber. According to this definition, dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants.

Added fiber, which refers to fiber that is added to foods during food processing, consists of isolated nondigestible carbohydrates that have proven beneficial physiological effects in humans. For food labeling purposes, the Institute of Medicine defines Total Fiber as the sum of Dietary Fiber and Added Fiber.

Despite the controversy surrounding the exact definition of dietary fiber, experts agree on one important thing – dietary fiber is an important weapon in the fight against heart disease, colon cancer, diabetes, and obesity.

Categories of Dietary Fiber

• Cellulose, found in bran, legumes, peas, root vegetables, cabbage family, outer covering of seeds, and apples
• Hemicellulose, found in bran and whole grains
• Polyfructoses (Inulin and Oligofructans)
• Galactooligosaccharides
• Gums, found oatmeal, barley, and legumes.
• Mucilages
• Pectins, found in apples, strawberries, and citrus fruits
• Lignin, found in root vegetables, wheat, fruits with edible seeds (such as strawberries)
• Resistant Starches, found in ripe bananas, potatoes

How it Functions

What is the function of dietary fiber?

Until very recently, the functions of a specific type of fiber were determined by whether or not the fiber was classified as soluble or insoluble. Soluble fibers, such as the type found in oat bran, are known to reduce blood cholesterol levels and normalize blood sugar levels.

On the other hand, insoluble fiber, such as the type found in wheat bran, are known to promote bowel regularity. Many commonly used plant sources of fiber contain both soluble and insoluble fibers. Psyllium husks, for example, contain a mixture of 70% soluble and 30% insoluble fibers. Despite the widespread use of the terms "soluble" and "insoluble" to describe the health benefits of dietary fiber, many medical and nutrition experts contend that these terms do not adequately describe the physiological effects of all the different types of fiber. These experts are now proposing the use of the terms "viscous" and "fermentability" in place of soluble and insoluble to describe the functions and health benefits of dietary fiber.

Reducing Cholesterol Levels

Like soluble fibers, viscous fibers lower serum cholesterol by reducing the absorption of dietary cholesterol. In addition, viscous fibers complex with bile acids, which are compounds manufactured by the liver from cholesterol that are necessary for the proper digestion of fat. After complexing with bile acids, the compounds are removed from circulation and do not make it back to the liver. As a result, the liver must use additional cholesterol to manufacture new bile acids. Bile acids are necessary for normal digestion of fat. Soluble fiber may also reduce the amount of cholesterol manufactured by the liver.

Normalizing Blood Sugar Levels

Viscous fibers also help normalize blood glucose levels by slowing the rate at which food leaves the stomach and by delaying the absorption of glucose following a meal. Viscous fibers also increase insulin sensitivity. As a result, high intake of viscous fibers play a role in the prevention and treatment of type 2 diabetes. In addition, by slowing the rate at which food leaves the stomach, viscous fibers promote a sense of satiety, or fullness, after a meal, which helps to prevent overeating and weight gain.

Promoting Bowel Regularity

Certain types of fiber are referred to as fermentable fibers because they are fermented by the “friendly” bacteria that live in the large intestine. The fermentation of dietary fiber in the large intestine produces a short-chain fatty acid called butyric acid, which serves as the primary fuel for the cells of the large intestine and helps maintain the health and integrity of the colon.

Two other short-chain fatty acids produced during fermentation, propionic and acetic acid are used as fuel by the cells of the liver and muscles. In addition, propionic acid may be responsible, at least in part, for the cholesterol-lowering properties of fiber.

In animal studies, propionic acid has been shown to inhibit HMG-CoA reductase, an enzyme involved in the production of cholesterol by the liver. By lowering the activity of this enzyme, blood cholesterol levels may be lowered.

In addition, fermentable fibers help maintain healthy populations of friendly bacteria. In addition to producing necessary short-chain fatty acids, these bacteria play an important role in the immune system by preventing pathogenic (disease-causing) bacteria from surviving in the intestinal tract.

As is the case with insoluble fiber, fibers that are not fermentable in the large intestine help maintain bowel regularity by increasing the bulk of the feces and decreasing the transit time of fecal matter through the intestines. Bowel regularity is associated with a decreased risk for colon cancer and hemorrhoids (when the hemorrhoids are related to straining and constipation).

Deficiency Symptoms

What are deficiency symptoms for dietary fiber?

There is no identifiable, isolated deficiency disease caused by lack of fiber in the diet. However, research clearly indicates that low intake of dietary fiber (less than 20 grams per day) over the course of a lifetime is associated with development of numerous health problems including constipation, hemorrhoids, colon cancer, obesity and elevated cholesterol levels.

Toxicity Symptoms

What are toxicity symptoms for dietary fiber?

Intake of dietary fiber in excess of 50 grams per day may cause an intestinal obstruction in susceptible individuals. In most individuals, however, this amount of fiber will improve (rather than compromise) bowel health.

Excessive intake of fiber can also cause a fluid imbalance, leading to dehydration. Individuals who decide to suddenly double or triple their fiber intake are often advised to double or triple their water intake for this reason.

In addition, excessive intake of nonfermentable fiber, typically in supplemental form, may lead to mineral deficiencies by reducing the absorption or increasing the excretion of minerals, especially when mineral intake is too low or when mineral needs are increased such as during pregnancy, lactation, or adolescence.

Impact of Cooking, Storage and Processing

How do cooking, storage, or processing affect dietary fiber?

Many whole foods contain 5 or more grams of fiber, and in their whole, unprocessed form, would be highly supportive of health. When foods are processed, however, most or all of this fiber is often lost.

For example, most breads sold nationally in the United States use a 60% extraction process in which 60% of the original wheat grain is kept in the flour, but 40% is discarded. The discarded part of the wheat includes the bran and the germ; these two components of the grain contain virtually all of its fiber.

As a result, 60% extraction wheat flour contains almost no fiber, even though the whole, unprocessed wheat grain contains an ample amount. Fruit juices and vegetable juices are also good examples of products which started out high-fiber in their whole, unprocessed state but ended up with virtually no fiber as a result of processing.

Factors that Affect Function

What factors might contribute to a deficiency of dietary fiber?

Even though fiber is often defined as the "undigestable" part of food, a certain amount of healthy digestive function is important for realizing the health benefits of this nutrient.

Inadequate chewing can prevent the health benefits of fiber from being realized, since fibers that cannot be solubilized (like lignins, celluloses, and some hemicelluloses) require extra chewing in order to participate in biochemical processes.

Drug-Nutrient Interactions

What medications affect dietary fiber?

• Dietary fiber, especially the fiber found in fruit, beans, and oat bran, reduces the absorption of a class of cholesterol-lowering medications called HMG-CoA reductase inhibitors (for example, lovastatin) by binding to the drug in the gastrointestinal tract.
• Dietary fiber decreases the absorption of hydralazine, digoxin, and lithium.
• Diets high in dietary fiber may improve glucose control in people with type 2 diabetes, thereby reducing the dose of insulin or oral glucose lowering medications needed to control blood sugar.
• Certain medications, including pain medications (for example, codeine) and calcium channel blockers (for example, verapamil) can cause constipation.
• Increased intake of dietary fiber can reduce the constipation caused by these medications.

Nutrient Interactions

How do other nutrients interact with dietary fiber?

Foods high in nonfermentable fiber, or the fiber that passes all the way through the intestines unchanged, may reduce the absorption and/or increase the excretion of several minerals, including calcium and iron.

Health Conditions

What health conditions require special emphasis on dietary fiber?

A diet high in fiber may play a role in the prevention and/or treatment of the following health conditions:

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

Form in Dietary Supplements

What forms of dietary fiber are found in dietary supplements?

As a dietary supplement and over-the-counter medication, fiber is available in powders that can be mixed with water or juice. These products often contain psyllium as the source of fiber, but may also contain pectin or guar gum. In addition, oat bran is available as a fiber-rich food ingredient that can be added to baked goods or hot cereal.

The Bottom Line Recommendations for Fiber Intake

Fiber is an important part of a healthy diet, and you should get a least the minimum recommended amount of 20-35 grams of dietary fiber per day for adults. For children over age 2, the recommended intake is the child's age + 5 grams.

The best sources are fresh fruits and vegetables, nuts and legumes, and whole-grain foods.
Some tips for increasing fiber intake:

• Eat whole fruits instead of drinking fruit juices.
• Replace white rice, bread, and pasta with brown rice and whole-grain products.
• Choose whole-grain cereals for breakfast.
• Snack on raw vegetables instead of chips, crackers, or chocolate bars.
• Substitute legumes for meat two to three times per week in chili and soups.
• Experiment with international dishes (such as Indian or Middle Eastern) that use whole grains and legumes as part of the main meal (as in Indian dahls) or in salads (for example, tabbouleh).

By The George Mateljan Foundation for The World's Healthiest Foods, Harvard School of Public Health