What is dietary fiber?

Dietary fiber includes all of the complex plant carbohydrates that are not digested or absorbed in the small intestine. This includes whole-grain breads and cereals, beans, peas and other fruits and vegetables. Several different types of fiber with different chemical structures and differing abilities to dissolve in water have been identified, but it is most useful to think of fiber as either soluble or insoluble. Both types of fiber are important for healthy digestive function:

• Soluble fiber disperses well in water and liquid and forms a soft gel in the digestive tract. It is not broken down until it reaches the colon, where its digestion and fermentation causes the production of gas. Examples of soluble fiber are oats, barley, beans, peas and many types of fruit.
• Insoluble fiber does not disperse in water and liquid and undergoes only minimal change as it passes through the digestive tract. Examples of insoluble fiber include wheat bran, whole-grain breads and many vegetables.

Dietary fiber is plant material that is normally left undigested after passing through the body’s digestive system. Food sources of dietary fiber are often classified according to whether they contain predominantly soluble or insoluble fiber. Plant foods usually contain a combination of both types of fiber in varying degrees, according to the plant's characteristics. 

• Insoluble fiber, found in certain plant foods, particularly whole grains, absorbs water throughout the digestive system, helping to promote regular, healthy bowel movements by binding with water and forming a gel, which allows the body’s waste to form into soft and bulky stools which helps to efficiently rid the body of toxins. Wheat bran is a good source of insoluble fiber. While we cannot digest the insoluble fiber in wheat bran, it is partially digested by beneficial bacteria in the gut, helping to ferment them into short-chain fatty acids that nourish our intestines.
• Soluble fiber, found in all plant foods, dissolves in water to form a gel-like substance. It is readily fermented in the colon and includes other plant components such as lignans, oligo- and polysaccharides, resistant starches, and inulin. Soluble fiber also promotes soft and bulky stools, delays gastric emptying, and binds with bile acids, helping to lower cholesterol levels. Foods that are considered good sources of soluble fiber include: oat bran, guar gum, dried beans and peas, apples with skin, seaweeds, and flaxseeds.

Lignans, a soluble fiber found in flax seeds, wheat, and legumes, are known to possess anticancer, antibacterial, and antiviral properties. They bind to estrogen receptors in the body, interfering with cancer promotion and helping to regulate estrogen levels. Soluble fiber also promotes a delay in the absorption of glucose and increases insulin sensitivity, resulting in improved glucose metabolism, ultimately helping to lower the risk of type 2 diabetes. 

Originally, the definition of dietary fiber was restricted to the sum of plant compounds that are not digestible by the secretions of the human digestive tract. But, this definition is vague because it depends on an exact understanding of what exactly is not digestible. Dietary fiber refers to the components of the plant cell wall as well as the indigestible residues. The composition of the plant cell wall varies according to the species of plants. In general, most plant cell walls contain 35 percent insoluble fiber, 45 percent soluble fiber, 15 percent lignans, 3 percent protein, and 2 percent ash. It is important to recognize that dietary fiber is a complex of these constituents, so the supplementation of a single component cannot substitute for a diet rich in high-fiber food.