What Polysaccharide Is Not Digested by Humans?

November 17, 2025 •

3 min read

Over 90% of the normal human gut is colonized by four major bacterial phyla, which ferment indigestible dietary polysaccharides that pass through our digestive tract. Cellulose, a complex carbohydrate and structural component of plant cell walls, is the primary polysaccharide that is not digested by humans.

Quick Summary

Cellulose is the key polysaccharide that the human body cannot break down. This is because humans lack the enzyme cellulase, which is necessary to hydrolyze the strong beta-glycosidic bonds within cellulose's structure. Instead of being a source of energy, cellulose acts as dietary fiber, promoting digestive health by adding bulk to stool and feeding beneficial gut bacteria.

Key Points

Cellulose is indigestible: The polysaccharide cellulose, the main component of plant cell walls, cannot be broken down by the human digestive system.
Lack of cellulase enzyme: Humans lack the enzyme cellulase, which is necessary to hydrolyze the strong $\beta$-1,4-glycosidic bonds that link cellulose's glucose molecules.
Source of dietary fiber: Instead of providing energy, cellulose acts as insoluble dietary fiber, adding bulk to stool and promoting regular bowel movements.
Feeds gut bacteria: While we can't digest it, our gut microbiota can ferment cellulose, producing beneficial short-chain fatty acids.
Promotes gut health: A diet rich in cellulose helps maintain bowel health, prevents constipation, and supports a healthy gut microbiome.
Differs from digestible carbs: Digestible polysaccharides like starch have different chemical bonds ($α$-linkages) that our enzymes can easily break down.

Cellulose: The Indigestible Polysaccharide

Cellulose is a complex carbohydrate, or polysaccharide, made of long, straight chains of glucose molecules. These glucose units are linked together by a specific type of bond known as a $\beta$-1,4-glycosidic bond. This differs structurally from the digestible polysaccharides like starch and glycogen, where glucose units are linked by $\alpha$-1,4- and $\alpha$-1,6-glycosidic bonds. The human digestive system, with enzymes such as amylase, is perfectly adapted to break down the alpha bonds found in starch, but we completely lack the enzyme, cellulase, that can break the beta bonds of cellulose.

Why the Inability to Digest?

This inability to produce cellulase is a fundamental biological difference between humans and many herbivores, such as cows, sheep, and termites. These animals have specialized digestive systems or symbiotic microorganisms (bacteria and protozoa) in their gut that produce cellulase, allowing them to extract energy from cellulose-rich plants like grass and wood. For humans, cellulose passes through the gastrointestinal tract largely intact.

The Indispensable Role of Cellulose in the Diet

Despite being indigestible, cellulose is a crucial component of a healthy diet, where it is classified as dietary fiber. It is an insoluble fiber, meaning it does not dissolve in water. This property, along with its resilience to human digestive enzymes, gives it several important functions:

Bulking Agent: Cellulose adds bulk to the stool, promoting regular bowel movements and preventing constipation.
Intestinal Transit: It stimulates peristalsis—the muscle contractions that move food along the digestive tract—which helps to efficiently and quickly move waste through the intestines.
Gut Microbiota Support: While humans cannot digest it, the beneficial bacteria in the large intestine can ferment some cellulose and other non-digestible carbohydrates. This process nourishes the gut microbiota and produces short-chain fatty acids (SCFAs), which have numerous health benefits.

Other Indigestible Polysaccharides

Cellulose is not the only polysaccharide that humans cannot digest. Other non-starch polysaccharides (NSPs) and some forms of resistant starch also escape digestion in the upper gastrointestinal tract.

Hemicellulose: A branched heteropolysaccharide found with cellulose in plant cell walls.
Pectin: A polysaccharide found in plant cell walls and fruits, acting as a gelling agent.
Inulin: A fructan polysaccharide, which acts as a prebiotic, feeding beneficial bacteria.
Resistant Starch: Some starches are resistant to human enzymes, passing to the large intestine for microbial fermentation.

Comparison of Digestible vs. Indigestible Polysaccharides


Feature	Digestible Polysaccharides (e.g., Starch, Glycogen)	Indigestible Polysaccharides (e.g., Cellulose)
Primary Function in Humans	Energy source	Dietary fiber, promotes gut health
Component Sugars	Alpha-glucose units	Beta-glucose units
Glycosidic Bonds	α-1,4 and α-1,6	β-1,4
Human Enzymes	Produced (e.g., amylase, maltase)	Not produced (lack cellulase)
Site of Major Digestion	Mouth and small intestine	Minimal; passes to large intestine
Role in Gut Microbes	Broken down before reaching large intestine	Fermented by beneficial bacteria in the colon
Effect on Blood Sugar	Rapid release of glucose, potential spikes	Slows absorption of other carbohydrates, moderates blood sugar

Fermentation and Health Benefits

When indigestible polysaccharides reach the colon, they become a food source for the resident microbiota, which ferments them into short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate. Butyrate, in particular, serves as the primary energy source for the cells lining the colon, helping to maintain their integrity and health. This fermentation process supports a healthy gut ecosystem, which is linked to a stronger immune system, better metabolic health, and protection against certain chronic diseases, including colorectal cancer.

Conclusion

The inability of humans to digest cellulose is not a deficiency but rather a symbiotic feature of our digestive system. While we cannot break down the $\beta$-1,4-glycosidic bonds to use cellulose for energy, this robust polysaccharide serves an essential function as insoluble dietary fiber. It promotes regularity, ensures smooth intestinal function, and nourishes the trillions of beneficial bacteria in our gut. Thus, incorporating cellulose-rich foods like vegetables, fruits, and whole grains is vital for maintaining optimal digestive and overall health.

Frequently Asked Questions

Humans cannot break down cellulose because our bodies do not produce the enzyme cellulase. This enzyme is required to cleave the specific chemical bonds, known as $\beta$-1,4-glycosidic bonds, that link the glucose units in cellulose.

When humans eat cellulose, it passes through the stomach and small intestine mostly intact. In the large intestine, some of it is fermented by beneficial gut bacteria, but the majority is excreted as a component of dietary fiber.

No, cellulose is not useless. Although it does not provide energy, it is an essential part of a healthy diet as insoluble dietary fiber. It adds bulk to stool, aids bowel regularity, and feeds the beneficial bacteria in our gut.

Cows and other ruminant animals can digest cellulose because they have a multi-chambered stomach, including a rumen that houses symbiotic microorganisms. These microorganisms produce the necessary cellulase enzyme to break down cellulose into usable energy sources.

Yes, cellulose-rich dietary fiber can aid in weight management. It contributes to a feeling of fullness and satiety, which can help control overall calorie intake. High-fiber foods also tend to be less energy-dense.

Excellent sources of cellulose include vegetables such as broccoli and carrots, whole grains like wheat bran, nuts and seeds, and the skins of many fruits, including apples and pears.

Yes, the microbial fermentation of indigestible carbohydrates like cellulose in the large intestine produces gases such as hydrogen, carbon dioxide, and methane. This is a natural part of the process and contributes to intestinal gas.