Dietary fiber is often grouped with starches and sugars, leading to the common question of whether it is a polysaccharide. Fiber is defined as the component of plant-based foods that resists digestion by enzymes in the human small intestine. This indigestibility is the unifying characteristic that gives fiber its distinct health properties. The majority of what we call fiber is indeed composed of large, complex carbohydrate molecules.
Defining Polysaccharides and Carbohydrates
Carbohydrates are organic compounds built from carbon, hydrogen, and oxygen atoms that serve as the main source of energy for the body. They are chemically classified based on the number of sugar units they contain. Monosaccharides, like glucose and fructose, are single sugar units, while disaccharides, such as sucrose, are made of two units bonded together.
Polysaccharides are large, complex carbohydrate molecules formed when ten or more monosaccharides link together in long chains or branched structures. Common examples include starch, used by plants to store energy, and glycogen, used by animals for the same purpose.
The key difference between digestible polysaccharides like starch and dietary fiber lies in the chemical bond that connects their sugar units. Human digestive enzymes, specifically amylase, are designed to break the alpha-bonds found in starch, releasing glucose for energy. Fiber’s polysaccharide components, however, contain beta-bonds that human enzymes cannot cleave, meaning they pass through the small intestine largely intact.
The Chemical Composition of Dietary Fiber
The majority of dietary fiber is made up of Non-Starch Polysaccharides (NSPs), which are the primary structural components of plant cell walls. These NSPs include well-known substances like cellulose, hemicellulose, and pectins. Cellulose, for example, is a long, linear chain of glucose units, structurally similar to starch, but the beta-linkages make it resistant to human digestion.
Hemicellulose and pectins are more diverse NSPs, often containing a mixture of different sugar units, such as xylose, arabinose, and galactose, and exhibiting more branched structures. Their resistance to enzymatic hydrolysis is what makes them functionally different from digestible carbohydrates.
Lignin is a critical exception to the “polysaccharide” rule, as it is not a carbohydrate but is included in the definition of dietary fiber. Lignin is a highly complex, non-carbohydrate polymer made of phenylpropane units that provides rigidity to plant structures like wood and seeds. Since it is completely indigestible by human enzymes, it is grouped with the NSPs under the functional category of dietary fiber.
Structural Differences and Biological Function
The diverse chemical structures of the components of dietary fiber result in two distinct physical categories: soluble and insoluble fiber. Soluble fiber, which includes pectins, gums, and some hemicelluloses, dissolves in water to form a viscous, gel-like substance in the digestive tract. This gel slows the rate at which food moves through the digestive system, which can help stabilize blood sugar levels and lower LDL cholesterol by binding to bile acids.
In contrast, insoluble fiber, which is primarily made of cellulose and lignin, does not dissolve in water. These components largely retain their structure as they pass through the gut, acting as a bulking agent. This bulk stimulates the walls of the intestine, promoting regularity and aiding in the movement of waste material.
Both types of fiber influence the health of the large intestine. Soluble fibers are fermentable and serve as a food source for the gut microbiota in the colon. The bacteria break down these polysaccharides, producing beneficial short-chain fatty acids, which play a role in gut health. Insoluble fibers are less fermentable but are important for adding volume to the stool and supporting a healthy transit time.