21 2.153 Fiber
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 | Functional Fiber |
| Intact in plants | Isolated, extracted, or synthesized |
| Carbohydrates + lignins | Only carbohydrates |
| Only from plants | From plants or animals |
| No proven benefit | Must prove benefit |
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.
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
| Fiber | Description |
| Cellulose | Main component of plant cell walls |
| Hemicellulose | Surround cellulose in plant cell walls |
| Lignin | Noncarbohydrate found within “woody” plant cell walls |
Table 2.1533 Common types of fermentable, viscous (soluble) fiber
| Fiber | Description |
| Hemicellulose | Surro