The simplest definition of fiber is indigestible matter. Indigestible means that it survives digestion in the small intestine and reaches the large intestine.

There are 3 major fiber classifications¹:

Dietary Fiber – nondigestible carbohydrates and lignin that are intrinsic and intact in plants

Functional Fiber – isolated, nondigestible carbohydrates that have beneficial physiological effects in humans

Total Fiber – dietary fiber + functional fiber

The differences between dietary and functional fiber are compared in the table below:

Table 2.1531 Differences between dietary fiber and functional fiber

Dietary fiber is always intact in plants, whereas functional fiber can be isolated, extracted or synthesized. Functional fiber is only carbohydrates, while dietary fiber also includes lignins. Functional fiber can be from plants or animals, while dietary fiber is only from plants. Functional fiber must be proven to have a physiological benefit, while dietary fiber does not.

Polysaccharide fiber differs from other polysaccharides in that it contains beta-glycosidic bonds (as opposed to alpha-glycosidic bonds). To illustrate these differences, consider the structural differences between amylose and cellulose (type of fiber). Both are linear chains of glucose, the only difference is that amylose has alpha-glycosidic bonds, while cellulose has beta-glycosidic bonds as shown below.

Figure 2.1531 Structures of amylose and cellulose

The beta-bonds in fiber cannot be broken down by the digestive enzymes in the small intestine so they continue into the large intestine.

Fiber can be classified by its physical properties. In the past, fibers were commonly referred to as soluble and insoluble. This classification distinguished whether the fiber was soluble in water. However, this classification is being phased out in the nutrition community. Instead, most fibers that would have been classified as insoluble fiber are now referred to as nonfermentable and/or nonviscous and soluble fiber as fermentable, and/or viscous because these better describe the fiber’s characteristics². Fermentable refers to whether the bacteria in the colon can ferment or degrade the fiber into short chain fatty acids and gas. Viscous refers to the capacity of certain fibers to form a thick gel-like consistency. The following table lists some of the common types of fiber and provides a brief description about each.

Table 2.1532 Common types of nonfermentable, nonviscous (insoluble) fiber

Table 2.1533 Common types of fermentable, viscous (soluble) fiber

The following table gives the percentage of total dietary fiber in 5 foods.

Table 2.1534 Total dietary fiber (as percent of sample weight)³

The table below shows the amount of nonfermentable, nonviscous fiber in these same five foods.

Table 2.1535 Nonviscous fiber (as percent of sample weight)³

The table below shows the amount of fermentable, viscous fiber in these same five foods.

Table 2.1536 Viscous Fiber (as percent of sample weight)³

tr = trace amounts

Foods that are good sources of non fermentable, non viscous fiber include whole wheat, whole grain cereals, broccoli, and other vegetables. This type of fiber is believed to decrease the risk of constipation and colon cancer, because it increases stool bulk and reduces transit time⁴. This reduced transit time theoretically means shorter exposure to consumed carcinogens in the intestine, and thus lower cancer risk.

Fermentable, viscous fiber can be found in oats, rice, psyllium seeds, soy, and some fruits. This type of fiber is believed to decrease blood cholesterol and sugar levels, thus also lowering the risk of heart disease and diabetes, respectively⁴. Its viscous nature slows the absorption of glucose preventing blood glucose from spiking after consuming carbohydrates. It lowers blood cholesterol levels primarily by binding bile acids, which are made from cholesterol, and causing them to be excreted. As such, more cholesterol is used to synthesize new bile acids.

References & Links

1. DRI Book –  [Anonymous]. (2005) Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids. Washington, D.C.: The National Academies Press. https://www.nap.edu/read/10490/chapter/9

2. Dietary Reference Intakes: Proposed Definition of Dietary Fiber Food and Nutrition Board. 2001 https://www.nap.edu/read/10161/chapter/3

3. Marlett JA. (1992) Content and composition of dietary fiber in 117 frequently consumed foods. J Am Diet Assoc 92: 175-186.

4. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.