Fiber is a type of carbohydrate present in all foods derived from plants, including whole grains, fruits, vegetables, beans, nuts and seeds. It is not digested or absorbed into the bloodstream and does not add calories to a person’s diet or provide energy for the body. However, fiber provides numerous health benefits to the body.

Fiber is a key component of a healthy diet and contributes to digestive health and appetite control. It also helps reduce blood cholesterol and blood sugar levels in the body and has been linked to preventing or controlling chronic diseases such as heart disease, diabetes, obesity and possibly some forms of cancer.

Fiber is divided into two types, based on its solubility, or ability to dissolve in water:

• Soluble fiber. Forms a gel when mixed in a liquid. Soluble fiber slows digestion and helps the body absorb various nutrients.
  
  
• Insoluble fiber. Provides structure to plant cell walls and does not dissolve in liquids. Insoluble fiber adds bulk to stool, preventing constipation and allowing stool to pass more quickly.

Fiber also can be classified according to its chemical structure. Examples of fiber categorized in this way include cellulose, hemicelluloses, gums and mucilages, lignan, pectin, and restricted starches.

High-fiber foods are a rich source of vitamins and minerals, and are generally low in total fat, saturated fat, cholesterol and salt. Foods high in fiber are known to help promote regular bowel movements and generally have few calories. They also contribute to a feeling of fullness.

Fiber can be found in all plant foods. However, it is not found in any foods that come from animals, including meat, fish, poultry, milk, milk products and eggs. Most foods that contain fiber contain both soluble and insoluble fiber, although the balance leans toward one type or the other. Fiber also is available in supplement form. However, experts generally advise most people to use dietary sources of fiber rather then supplements.

To get adequate amounts of fiber, the U.S. Dietary Guidelines and the Dietary Reference Intakes (DRIs) indicate that people should eat a variety of fruits, vegetables and whole-grain products daily. Helpful tools to increase intake of dietary fiber can be found on food labels.

Fiber is a carbohydrate that the body cannot digest or absorb into the bloodstream. It is the structural part of a plant present in all foods derived from plants, including whole grains, fruits, vegetables, beans, nuts and seeds. Unlike other carbohydrates, fiber does not add calories to a person’s diet and does not provide energy for the body. Because it is not absorbed, fiber is not considered to be a nutrient.

However, fiber contains many health benefits that are still being uncovered. It helps lower so-called “bad” low-density lipoprotein (LDL) blood cholesterol levels. It also promotes digestive regularity, lowers blood sugar and cholesterol levels, and helps control appetite. Studies indicate that a high-fiber diet appears to reduce the risk of developing many chronic diseases and conditions such as heart disease, diabetes, constipation, diverticulitis, obesity and possibly some types of cancer.

Like starch, fibers are made of sugar units and are considered to be polysaccharides (carbohydrates whose molecules contain chains of monosaccharides). However, unlike starch, human digestive enzymes lack the ability to break fiber down into units small enough for digestion. Instead, it passes through the entire digestive system largely intact. Fiber essentially has no calories because it is not absorbed.

Originally referred to as crude fiber or indigestible material, dietary fiber is not one single entity and can be separated into two major groups based on its solubility in water:

• Soluble fiber. Forms a gel when mixed in a liquid. Examples include gums, mucilages and pectins. They are found in beans, oats, barley, and some fruits and vegetables. Soluble fiber slows digestion, can help lower cholesterol and glucose levels, and helps the body absorb various nutrients.
  
  
• Insoluble fiber. Fibers such as cellulose, hemicellulose and lignan that give structure to plant cell walls and that do not dissolve in liquids. They are generally found in most whole wheat and grain products, nuts, and vegetables. Insoluble fiber adds bulk to stool, preventing constipation and allowing stool to pass more quickly from the body.

Resistant starches have recently been more recognized and classified as a third type of dietary fiber. Resistant starches escape digestion in the small intestine of healthy individuals and are fermented in the large intestine, providing some of the benefits of both soluble and insoluble fiber.

Some types of fiber have benefits not usually associated with their class. For example, insoluble rice bran helps lower blood cholesterol, while soluble fiber in the supplement psyllium helps to promote bowel movements.

Some experts have begun to classify fibers based on the source rather than the physical properties. This reclassification accommodates food labels that list total fiber in the product, and includes:

• Dietary fibers. Occur naturally in intact plants.
  
• Functional fibers. Extracted from plants or manufactured.
  
• Total fiber. Includes all dietary fibers and functional fibers.

The U.S. Public Health Service and the U.S. Department of Agriculture (USDA) recently issued daily dietary fiber recommendations as part of the Dietary Guidelines for Americans. They are listed in grams:

However, the average American man consumes only about 17 grams of fiber each day, while the average American woman consumes just 12 grams of dietary fiber per day, according to the USDA. In addition, less than 15 percent of Americans include at least one serving of whole grains in their daily diets. This amount is among the lowest intake level in the world and is about half the intake recommended by health experts.

There are many different types of fiber that can be distinguished by their chemical structure. They include:

• Cellulose. The chief constituent of all plant cells walls. It is found naturally in all vegetables, fruits and legumes. It also can be extracted from wood pulp or cotton. When extracted, this type of fiber is added to foods and serves as an anticaking, thickening and texturizing agent.
  
  
• Gums and mucilages. Gums are the substances secreted by plants when they are cut. Guar gum and gum arabic are used as additives to thicken processed foods. Mucilages resemble gums and types such as psyllium (also a common fiber supplement) and carrageenan are added to foods as stabilizers.
  
  
• Hemicelluloses. The main component of all grain fibers.
  
  
• Lignan. A tough fiber not found in many foods. The woody parts of some vegetables (e.g., carrots) and the small seeds of fruits such as strawberries are made up of lignan.
  
  
• Pectins. Commonly found in vegetables and fruits (e.g., citrus fruits, apples), they easily form gels in water. Pectins sometimes are isolated and used to thicken jelly, prevent separation of salad dressings and provide texture and consistency for various products.
  
  
• Resistant starches. Some starches are classified as fibers because they are not digested or absorbed by the body. This type of starch is found in whole legumes, raw potatoes and unripe bananas and plantains.

Fiber is not absorbed and passes through the entire digestive system largely intact. As a result, it contributes no calories to a person’s diet, which may aid in controlling weight and preventing obesity. Foods rich in fiber, such as fruits, vegetables and whole-grain foods are naturally cholesterol-free and typically low in fat, sodium (salt) and calories (energy). Fiber-rich foods take longer to chew, which may cause people to eat less. Their added bulk also may help a person feel full longer. Insoluble fiber may also help reduce the absorption of dietary fat, which is dense with calories. 

Fiber also contributes toward good digestive health. Insoluble fiber helps move waste through the intestinal tract, which helps prevent harmful substances from lingering in the intestines and coming into contact with the intestinal lining. Recent research has found that fiber causes a release of mucus from the intestinal lining that assists food in passing through the digestive system. In addition, insoluble fiber adds bulk and softness to stool, which promotes regularity and prevents both constipation and hemorrhoids (dilated veins in swollen anal tissue). Fiber can also reduce the risk of irritable bowel syndrome (disorder of the lower intestinal tract) and diverticulosis (condition in which small pockets develop in the colon).

In addition, foods rich in fiber are full of vitamins, minerals, phytochemicals (plant chemicals), carbohydrates and protein, all of which contribute to their significant impact on prevention and treatment of chronic diseases.

Fiber appears to be involved in the prevention or management of chronic diseases. Research conducted by the U.S. Department of Agriculture has found that people who eat three servings or more of whole-grain foods each day have a lower risk of metabolic syndrome, a condition marked by abdominal obesity and poor triglyceride, cholesterol, blood pressure and blood sugar readings. Metabolic syndrome is a risk factor associated with the development of both diabetes and heart disease.

Other illnesses that may be influenced by a high-fiber diet include:

• Cancer. There is some controversy about the potential role of fiber in preventing cancer. Numerous studies appear to show a strong link between a diet rich in fiber and a reduced risk of cancer. Experts have speculated that fiber’s role in speeding waste through the digestive tract may prevent harmful substances from lingering in the intestinal tract and contacting the intestinal walls. In addition, fiber’s role in bulking up stools may dilute the concentration of potential carcinogens in food. Finally, fiber may keep the body’s pH levels in a range that reduces the ability of intestinal microbes to produce carcinogens.
  
  However, some recent studies have found less certainty in the previously reported links between a high-fiber diet and colorectal cancer prevention. For now, the role of fiber in cancer prevention remains an open question.
  
  
• Heart disease. The health benefits of dietary fiber can be partially explained by its ability to dissolve in liquids. For example, soluble fiber forms a gel when mixed in a liquid. The viscosity (a measure of a fluid’s resistance to flow) of soluble fibers in foods has been credited with its cholesterol-lowering effect. When people eat, bile acids rich in cholesterol are secreted into the intestine and mix with the food to aid in digestion and absorption. As food is absorbed, the remaining viscous soluble fiber interferes with the re-absorption of these cholesterol-rich bile acids from the intestine. The end result is that the cholesterol is removed from the body, reducing blood cholesterol levels. High levels of low density lipoprotein (LDL) cholesterol can forum into plaques that cause the coronary arteries to harden and narrow (atherosclerosis). Eventually, this process can trigger a heart attack.
  
  There is also some evidence that the change in bile acid content in the body due to dietary intake of fiber may slow the liver’s production of cholesterol, as well as change the LDL cholesterol particles to make them less damaging. Scientific studies, clinical trials and population-based studies have documented the total cholesterol and LDL cholesterol-reducing effects of whole grains, as well as the reduction in heart disease risk due to consumption of a high-fiber diet rich in whole grains, fruits and vegetables.
  
  
• Diabetes. Diabetes is a condition that develops when the body can no longer produce enough insulin or access the insulin that is produced. Insulin is a hormone necessary to control blood sugar levels. Dietary fiber may slow the movement and absorption of foods from the intestine, resulting in favorable effects on blood sugar levels after eating. Studies have also demonstrated that a high-fiber diet may reduce insulin requirements and increase insulin resistance, improving blood sugar levels overall. The secondary benefits of a high-fiber diet for diabetes include its lower calorie (energy), fat and simple carbohydrate content, which may explain its beneficial effects in weight management.

Fiber can be found in all plant foods in varying amounts. However, it is not found in any foods that come from animals, including meat, fish, poultry, milk, milk products and eggs.

To increase intake of dietary fiber, it is important to include whole-grain breads and cereals, fresh fruits, vegetables, and dried beans in one’s daily diet. The Dietary Guidelines and the Food Guide Pyramid recommend eating 2 to 4 servings of fruit, 3 to 5 servings of vegetables and 6 to 11 servings of whole grains and cereals each day.

Most foods that contain fiber contain both soluble fiber and insoluble fiber, although the balance leans toward one type or the other. It is more important to make sure the body receives the proper amount of overall fiber than to worry about striking a balance between soluble and insoluble fiber.

Sources of soluble and insoluble fiber include the following:

The following is a list of food sources that are excellent or good sources of dietary fiber:

• Six or more grams of fiber per serving:
  
  
• • 1/3 cup of all bran, 100 percent bran cereals
  • 1/2 cup of beans, peas, lentils
    
    
• Four or more grams of fiber per serving:
  
  
• • 1/2 cup of blackberries or raspberries, one medium pear with skin
    
    
• One to four grams of fiber per serving:
  
  
• • 1/2 cup of 40 percent bran flakes, puffed wheat, raisin bran cereals or cooked oatmeal
    
    
  • 1 slice of whole wheat, cracked wheat, pumpernickel or rye bread
    
    
  • 1/2 cup or one serving of an apple, banana, cherries, peaches, cranberries, grapefruit, orange, kiwi fruit, plum, dates
    
    
  • 1/2 cup of bean sprouts, beets, broccoli, cabbage, carrots, cauliflower, corn, eggplant, kale, okra, spinach, potatoes (with skin), tomatoes, turnip greens, yams, zucchini
    
    
  • 1 cup popped popcorn
    
    
  • 2 tablespoons of almonds, peanuts or chunky peanut butter

Fiber also is available in supplement form. Experts generally advise most people to use dietary source of fiber. Excessive fiber supplementation can prevent the absorption of certain nutrients such as calcium, iron, magnesium or zinc. In addition, supplements do not provide the vitamins, minerals and other nutrients found in high-fiber foods.

However, a physician may recommend the use of fiber supplements for people with disorders such as irritable bowel syndrome or chronic constipation. It is important for a patient to consult a physician about any possible interactions. Fiber supplements tend to decrease the absorption of certain medications. As a result, it is usually best to take a fiber supplement two to three hours before or after taking any of the following:

• Aspirin
• Carbamazepine (anti-seizure medication)
• Digoxin (heart medication)
• Lithium (bipolar disorder drug)
• Nitrofurantoin (antibiotic)
• Warfarin (anticoagulant)

Fiber supplements also can reduce blood sugar levels. Therefore, patients with diabetes may want to ask their physician about adjusting their dosage of insulin or other medication.

To get adequate fiber, the U.S. Dietary Guidelines and the Dietary Reference Intakes (DRIs) indicate that people should include a variety of foods rich in fiber in their daily diets. To increase dietary fiber intake, include whole grain breads and cereals in one's diet.

It is important to note that not all wheat products qualify as “whole grain.” The bran and germ is removed from many wheat products, which reduces the fiber content of the food. When purchasing wheat products, look for the term “whole wheat” or “whole grain” high in the list of ingredients to ensure that the product is packed with fiber. Do not be fooled by the label “enriched wheat flour.” This indicates that certain nutrients were added back into the flour during or after processing. However, it does not guarantee that fiber is one of these added nutrients.  

Other good sources of fiber include fresh fruits, vegetables and beans in the diet. Fiber is best obtained from a variety of foods rather than from a supplement.

Helpful tools to increase intake of dietary fiber can be found on food labels. The nutrition facts label and the ingredients list on the label of packaged foods provide information on fiber content.

When increasing dietary fiber content it is important to do so gradually to allow the digestive tract to adjust. Side effects of increasing fiber intake too rapidly include excess gas formation, bloating, abdominal cramps or diarrhea. When increasing dietary fiber intake, it is also important to drink plenty of fluids. Dietary fiber readily absorbs liquid, so people who increase fiber intake without boosting water intake risk depriving their cells of necessary water. Experts recommend that men drink 12 eight-ounce glasses of water or other noncaloric beverages each day, and that women drink at least 9 eight-ounce glasses of water or other noncaloric beverages each day.  

The following are some tips for increasing fiber intake:

• Eat plenty of fruits and vegetables. These foods are naturally high in fiber. Eat fresh fruits and vegetables instead of drinking their juices, which lack fiber-rich skins or membranes. To keep the fiber content of vegetables high, eat them raw or steamed until slightly tender. Leaving the skins on the vegetables also increases fiber. For example, a medium baked potato with the skin provides 5 grams of fiber.
  
  
• Replace refined grains with whole grains. For example, choose brown rice, whole-wheat breads and whole-grain products instead of white rice, bread and pasta.
  
  
• Choose whole grain cereals for breakfast. Instead of sweetened cereals full of added sugars, choose unsweetened whole-grain cereals and add your own dried fruits. Another option is to mix one-quarter cup high-fiber bran cereal with your regular cereal.
  
  
• Add dried fruits, nuts or seeds to plain low fat yogurt, muffins, quick breads, salads, pilafs or other dishes.
  
  
• Beans, lentils and peas are low in fat and an excellent source of fiber, protein, and vitamins and minerals. Keep canned or frozen beans or peas handy to add to soups, stews, salads and rice dishes.
  
  
• Nuts are a good source of fiber and protein, but they are also high in fat. However,  these fats are mostly unsaturated fats that reduce the “bad” low-density lipoprotein (LDL) cholesterol levels.
  
  
• Snack on high-fiber foods. These include fresh and dried fruit, raw vegetables, low-fat popcorn and whole-grain crackers.
  
  
• Sprinkle crushed bran cereal or unprocessed wheat bran on breakfast cereals and baked products such as bread, cakes, casseroles, cookies, meat loaf and muffins.
  
  
• Use whole-grain flour instead of white flour. Because whole-grain flour is heavier than white flour, it may help to add more yeast or baking powder when baking.

Parents are urged to use caution when adding extra fiber to their child’s diet. Excessive amounts of high-fiber foods may cause a child to fill up quickly, reducing appetite and potentially depriving the child of needed nutrients from a well-rounded diet.

Elderly people and those who have had gastrointestinal surgery also are urged to exercise caution when boosting fiber intake. Such populations are more likely to feel the added effects of additional fiber consumption.

Although it is very rare in the typical American diet, people are urged not to eat too much fiber. Excessive amounts of fiber may move food through the digestive tract so quickly that some nutrients will not be absorbed. More than 50 to 60 grams of fiber daily can lower the absorption of vitamins and minerals such as calcium, iron, magnesium and zinc.

Because fiber delays emptying of the stomach, patients with gastroparesis are advised to consult their physician about whether they need to restrict or avoid consumption. Gastroparesis is a condition that causes delayed or incomplete digestion of food.

The U.S. Food and Drug Administration (FDA) may approve a health claim or nutrient content claim on food labels. These claims are provided to guide consumers to make informed decisions when purchasing foods and planning diets. If many studies agree that certain nutrients contribute to good health, the FDA may allow manufacturers to label foods containing these nutrients as helpful in reducing the risk of developing heart disease, cancer, bone disease, high blood pressure and other serious conditions.

The following are some dietary fiber claims that can be listed on food labels:

• Good Source of Fiber, Contains Fiber or Provides Fiber: Any food product that contains 2.5 to less than 5 grams of fiber per serving (less than 20 percent of the daily value of dietary fiber) and is low in total fat per serving (3 grams or less fat per serving) can carry these claims on the label.
  
  
• High Fiber, Rich in Fiber, Excellent Source of Fiber: Any food product that contains at least 5 grams or more of dietary fiber per serving (20 percent or more of the fiber daily value) and is low in total fat per serving (3 grams or less fat per serving) can have these claims on their label.

The FDA has also approved specific claims about fiber and its role in reducing illness. Food producers may state that diets rich in fruits, vegetables and grains that contain fiber (especially soluble fiber) may help reduce the risk of coronary heart disease. In addition, manufacturers can state that diets low in fat and rich in fiber-containing grain products, fruits and vegetables may reduce the risk of certain cancers.

Preparing questions in advance can help patients have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to fiber:

1. How can I tell if I am getting enough fiber in my diet?
   
   
2. Where can I find information and help about planning a high-fiber diet?
   
   
3. What are some simple steps for boosting the fiber intake of my meals?
   
   
4. How can I minimize any symptoms that may occur when switching to a high-fiber diet?
   
   
5. What are my alternatives if my body continues to react to the fiber?
   
   
6. Should I take a fiber supplement?
   
   
7. What precautions do I need to take while I am on a fiber supplement?
   
   
8. How will I know if I am getting too much fiber in my diet?
   
   
9. What are some tips for encouraging fiber in my child’s diet?
   
   
10. Should I eat one type of fiber more than another if I’m trying to accomplish a specific health goal (e.g., lower my cholesterol or combat constipation)?