What is the function of cellulose in the human body?

December 11, 2025 •

4 min read

Although humans cannot digest cellulose due to a lack of the necessary enzyme, this plant fiber plays a vital and impressive role in maintaining a healthy digestive system and supporting overall well-being. It is an essential component of dietary fiber found in all plant-based foods, providing benefits that are integral to human health.

Quick Summary

Cellulose is an indigestible insoluble fiber that promotes digestive health by adding bulk to stool, facilitating bowel movements, and reducing gut transit time. It nourishes beneficial gut bacteria and aids in managing weight and stabilizing blood sugar levels indirectly.

Key Points

Indigestible for humans: Humans lack the enzyme cellulase to break down cellulose, so it is not absorbed as a nutrient.
Functions as insoluble fiber: Its primary role is to act as insoluble dietary fiber, adding bulk and speeding up the passage of waste through the digestive system.
Promotes bowel regularity: By absorbing water and bulking stool, it helps prevent constipation and supports regular bowel movements.
Nourishes gut bacteria: It serves as a food source for beneficial gut bacteria, which ferment it to produce health-promoting short-chain fatty acids.
Supports weight management: The bulk it adds to food can increase feelings of fullness, helping to reduce overall calorie intake.
Contributes to gut health: It helps maintain the integrity of the colon lining and reduces transit time, which is linked to a lower risk of colon cancer.

Cellulose: Indigestible Yet Indispensable

Cellulose is a complex carbohydrate and the most abundant organic polymer on Earth, forming the primary structural component of plant cell walls. Unlike herbivores, humans do not produce the enzyme cellulase needed to break down the beta-1,4-glycosidic bonds that link its glucose units. Consequently, cellulose remains largely intact as it travels through the digestive tract, where it functions as an insoluble dietary fiber.

The Role of Cellulose as Insoluble Fiber

As it passes through the gastrointestinal tract, cellulose's fibrous nature provides several key functions:

Adds bulk to stool: The physical structure of cellulose fibers absorbs water and adds volume to waste material. This bulkier, softer stool is easier for the intestinal muscles to move, promoting regularity and preventing constipation.
Stimulates peristalsis: The added bulk helps stimulate the rhythmic muscle contractions known as peristalsis, which moves food and waste efficiently through the intestines.
Reduces transit time: By accelerating the movement of waste through the colon, cellulose helps decrease the time that potentially harmful substances and toxins remain in contact with the intestinal wall. This is linked to a reduced risk of colon cancer.
Supports gut integrity: A high-cellulose diet helps maintain the structural health of the gut by providing beneficial support to the mucosal lining.

The Gut Microbiome and Short-Chain Fatty Acids

While humans cannot directly digest cellulose, certain gut bacteria possess the enzymes to ferment it in the large intestine. This microbial fermentation process is a significant function of cellulose in the human body, as it produces beneficial metabolites called short-chain fatty acids (SCFAs), primarily butyrate, propionate, and acetate.

Nourishes colon cells: Butyrate is the main energy source for colonocytes, the cells lining the colon, helping to maintain the integrity of the intestinal wall.
Influences gene expression: SCFAs can regulate gene transcription and have anti-inflammatory effects by inhibiting histone deacetylases.
Promotes healthy bacteria: The presence of cellulose provides a food source for specific bacteria, such as Alistipes and Akkermansia muciniphila, which are associated with improved intestinal barrier function and anti-inflammatory responses.

Cellulose and Overall Health

Beyond its immediate impact on the digestive system, cellulose and the broader category of dietary fiber are linked to a number of systemic health benefits:

Weight Management: The bulking effect of cellulose contributes to a feeling of fullness or satiety after meals. This can lead to a lower overall calorie intake, which is beneficial for weight control and combating obesity.
Blood Sugar Control: High-fiber diets can help stabilize blood glucose levels by slowing down the absorption of sugars into the bloodstream. This is particularly important for managing type 2 diabetes.
Cardiovascular Health: Regular consumption of dietary fiber has been associated with a decreased risk of cardiovascular disease. While soluble fiber is more directly linked to lowering cholesterol, the overall benefits of a high-fiber diet impact weight, blood sugar, and blood pressure, all of which contribute to heart health.
Diverticular Disease: The bulking action of cellulose can help reduce the pressure inside the colon, which may alleviate the symptoms and lower the risk of diverticulitis, a condition where small pouches in the colon become inflamed.

Comparison of Insoluble (Cellulose) vs. Soluble Fiber

It is important to understand that not all dietary fiber is the same. While both are crucial for health, they behave differently in the body.


Feature	Insoluble Fiber (Cellulose)	Soluble Fiber	Examples
Interaction with water	Does not dissolve in water	Dissolves in water to form a gel-like substance	Cellulose, wheat bran, whole grains
Digestive effect	Adds bulk to stool and promotes regular bowel movements	Delays gastric emptying and slows digestion	Pectin (apples), beta-glucans (oats), psyllium
Impact on cholesterol	Has limited or no direct effect on lowering blood cholesterol	More effective at binding cholesterol and reducing its absorption	Pectin, beta-glucans, guar gum
Effect on blood sugar	Helps with regularity, supporting general metabolic health	Gels can slow the absorption of sugar, helping to stabilize blood glucose	Pectin, guar gum

How to Increase Your Cellulose Intake

To benefit from the function of cellulose in the human body, focus on a diet rich in whole plant foods. Good sources include fruits (especially with edible skins like apples and pears), leafy greens (spinach, kale), root vegetables (carrots, potatoes), legumes (beans, peas), and whole grains. A balanced diet with a variety of these foods will ensure adequate intake of both insoluble and soluble fiber. For more information on dietary fiber recommendations, visit the Harvard T.H. Chan School of Public Health website.

Conclusion

While cellulose passes through the human body undigested, its function as an insoluble dietary fiber is profoundly important. It acts as an essential catalyst for a healthy and regular digestive system by adding bulk and stimulating intestinal transit. Furthermore, it supports the gut microbiome, which, in turn, produces valuable short-chain fatty acids that nourish our cells and modulate inflammatory responses. By including a variety of cellulose-rich foods in our diet, we can harness these benefits to improve gut health, manage weight, and contribute to a lower risk of metabolic diseases.

Frequently Asked Questions

No, the human body cannot digest cellulose. We lack the enzyme, cellulase, which is required to break down the specific chemical bonds in the cellulose molecule.

Cellulose passes through the stomach and small intestine largely intact. When it reaches the large intestine, some of it is fermented by gut bacteria, but most continues on to add bulk to stool before being excreted.

No, since humans cannot digest cellulose, it does not provide any calories or energy. Its primary benefit comes from its function as a fiber rather than as a nutrient.

As an insoluble fiber, cellulose adds bulk and weight to stool by absorbing water. This softens the stool and helps stimulate the intestinal muscles to move waste more efficiently, preventing constipation.

For most people, increased cellulose intake through whole foods is safe. However, consuming too much fiber too quickly can cause gas, bloating, and cramping. Individuals with certain conditions like Crohn's disease or a history of bowel obstruction may need to limit intake.

Cellulose is found in all plant foods. Excellent sources include fruits with edible skins (like apples and pears), vegetables (leafy greens, celery, root vegetables), legumes, nuts, seeds, and whole-grain products.

While not as fermentable as soluble fiber, cellulose can be broken down by certain gut bacteria. This process helps feed and diversify the gut microbiome, which is essential for overall digestive and immune function.