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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.