Cellulose: The Plant Polysaccharide Our Body Can't Digest

December 7, 2025 •

3 min read

Over 90% of cotton fiber and 50% of wood consists of cellulose, a complex carbohydrate that forms the structural component of plant cell walls. While other polysaccharides like starch are a key energy source for humans, cellulose passes through our digestive system untouched, playing a crucial role as dietary fiber.

Quick Summary

Cellulose is a plant-based polysaccharide that humans cannot digest due to the lack of the necessary enzyme, cellulase. Instead of being broken down for energy, cellulose acts as dietary fiber, which is vital for promoting healthy digestion, regulating bowel movements, and supporting overall gut health.

Key Points

Indigestible Polysaccharide: The primary polysaccharide in plants that humans cannot digest is cellulose.
Enzyme Deficiency: Humans lack the enzyme cellulase, which is necessary to break the beta-1,4-glycosidic bonds in cellulose.
Dietary Fiber Role: Undigested cellulose acts as insoluble dietary fiber, adding bulk to stool and promoting regular bowel movements.
Gut Health Support: While indigestible by us, cellulose is fermented by beneficial gut bacteria, producing short-chain fatty acids that nourish the colon.
Energy vs. Structure: Unlike starch, which stores energy, cellulose provides structural rigidity to plant cell walls and offers no calories to humans.
Herbivore Adaptation: Animals like cows can digest cellulose because they host symbiotic microbes in their gut that produce cellulase.

The Science Behind Indigestible Cellulose

To understand why the human body cannot digest cellulose, we must examine its molecular structure and compare it to other carbohydrates that we can easily break down. Cellulose is a complex carbohydrate, or polysaccharide, made of thousands of glucose units linked together in long, straight chains. The key to its indigestibility lies in the specific type of chemical bond connecting these glucose molecules: the beta-1,4-glycosidic bond.

Our digestive system produces enzymes designed to break down other complex carbohydrates. For instance, the enzyme amylase efficiently breaks the alpha-1,4-glycosidic bonds found in starch, another plant polysaccharide. However, the human body does not produce the enzyme cellulase, which is required to cleave the beta-1,4-glycosidic bonds of cellulose. This fundamental enzymatic difference is why we can get energy from a potato (rich in starch) but not from a stalk of celery (rich in cellulose).

The Critical Role of Dietary Fiber

While we cannot digest cellulose for energy, its role in our diet is far from useless. In fact, it is a key component of dietary fiber, often referred to as 'roughage' or 'bulk'. As insoluble fiber, cellulose does not dissolve in water and remains mostly intact as it travels through our gastrointestinal tract. This bulk provides several critical health benefits:

Promotes Regular Bowel Movements: The bulk from cellulose adds volume and weight to stool, which helps it move more quickly and easily through the intestines. This aids in the prevention of constipation and hemorrhoids.
Supports Gut Microbiota: While humans don't have the enzyme to break down cellulose, our gut bacteria can ferment it. This fermentation process produces short-chain fatty acids (SCFAs), which nourish the cells lining the colon and contribute to a healthy gut microbiome.
Increases Satiety: High-fiber foods often require more chewing and can make you feel full and satisfied for longer. This can help with weight management by reducing overall calorie intake.
Improves Digestive Health: A high-fiber diet is associated with a reduced risk of colon cancer and diverticular disease.

Comparison Table: Starch vs. Cellulose


Feature	Starch	Cellulose
Molecular Structure	Branched chains of α-glucose units.	Long, straight chains of β-glucose units.
Glycosidic Bond Type	Alpha-1,4 and Alpha-1,6.	Beta-1,4.
Human Digestibility	Easily digested by human enzymes (amylase) into glucose.	Indigestible by human enzymes due to lack of cellulase.
Primary Function in Plant	Energy storage.	Structural support in cell walls.
Primary Function in Human Diet	Main source of readily available energy.	Insoluble dietary fiber, adding bulk to stool.
Effect on Blood Sugar	Rapidly broken down, causing a spike in blood sugar.	Undigested, so has no direct impact on blood sugar.

Why Can Some Animals Digest Cellulose?

Unlike humans, many herbivores, such as cows, sheep, and horses, possess specialized digestive systems or host symbiotic microorganisms that produce the necessary cellulase enzymes. For example, ruminants have a multi-chambered stomach (including the rumen) where billions of microbes ferment the ingested cellulose. This allows them to break down grass and other tough plant matter for energy, a strategy that offers a significant survival advantage for their lifestyle.

The Benefits of a Plant-Rich Diet

Incorporating a wide variety of plant-based foods into your diet is crucial for good health, even if some parts are indigestible. The insoluble fiber from cellulose works in tandem with soluble fiber (found in oats, apples, and beans) to create a comprehensive benefit profile for the digestive system. Soluble fiber absorbs water to form a gel-like substance that can help lower cholesterol and glucose levels, while insoluble fiber ensures everything moves smoothly through the gut. A balanced intake of both types of fiber is key to maximizing these nutritional benefits.

Conclusion

In conclusion, the plant polysaccharide our body is not able to digest is cellulose, the primary component of plant cell walls. This is due to the absence of the specific enzyme, cellulase, in our digestive system. However, this indigestible nature is not a drawback but a defining feature of dietary fiber. Cellulose provides crucial bulk that supports intestinal health, regulates bowel movements, and promotes a healthy gut microbiome by acting as a prebiotic. By understanding the science behind cellulose, we can better appreciate the vital role of dietary fiber in maintaining overall digestive wellness.

Visit the National Institutes of Health for more information on the role of polysaccharides in diet and health.

Frequently Asked Questions

The specific polysaccharide in plants that humans cannot digest is cellulose. It is the primary component of plant cell walls and acts as an insoluble fiber in the human diet.

Humans cannot digest cellulose because our digestive system lacks the enzyme cellulase, which is required to break the unique beta-1,4-glycosidic bonds that link its glucose units.

Yes, absolutely. As dietary fiber, cellulose is crucial for promoting good digestive health. It adds bulk to stool, aids in regulating bowel movements, and supports a healthy gut microbiome.

The main difference is the chemical bond type. Humans have enzymes like amylase to break down the alpha-glycosidic bonds in starch for energy, but we lack enzymes to break the beta-glycosidic bonds found in cellulose.

Yes, many animals like ruminants (cows, sheep) and termites can digest cellulose. They achieve this with the help of symbiotic microorganisms in their digestive tracts that produce the necessary cellulase enzymes.

When we eat cellulose, it passes through our stomach and small intestine mostly intact. It then moves to the large intestine where it adds bulk to the stool, helping to prevent constipation before being excreted.

Common food sources of cellulose include the skins of fruits and vegetables, whole grains, nuts, and legumes. It is a component of all plant-based foods, especially the fibrous parts.