Which of the following carbohydrates is indigestible in the human body?

December 23, 2025 •

3 min read

Over 90% of the fiber we consume is made up of cellulose, a carbohydrate that the human body cannot digest. While many carbohydrates, like starch and sugars, are broken down for energy, cellulose and other dietary fibers pass through the digestive system largely intact. This indigestible nature is not a fault but a fundamental feature that provides significant health benefits.

Quick Summary

The human body cannot digest cellulose due to the absence of the enzyme cellulase, which breaks down its chemical bonds. This indigestible carbohydrate, known as dietary fiber, promotes digestive health and bowel regularity as it passes through the system unchanged.

Key Points

Cellulose is the Answer: The carbohydrate that humans cannot digest is cellulose, the primary structural component of plant cell walls.
Enzymes are the Key: Humans lack the specific enzyme, cellulase, required to break the β(1→4)-glycosidic bonds that link the glucose units in cellulose.
Fiber is Essential: The indigestible cellulose functions as dietary fiber, which is crucial for promoting bowel regularity and preventing constipation.
Gut Bacteria Help: While humans can't digest it, some beneficial gut bacteria can ferment cellulose, producing short-chain fatty acids that nourish the colon.
Not Just Waste: Indigestible carbohydrates play a vital role in digestive health by adding bulk to stool and stimulating peristalsis.
Herbivores Adapt Differently: Animals like cows and termites can digest cellulose due to symbiotic relationships with microbes that produce cellulase in their specialized digestive systems.

The Indigestible Carbohydrate: Cellulose

When discussing which of the following carbohydrates is indigestible in the human body, the answer is definitively cellulose. This is a fundamental concept in biology and human nutrition. While carbohydrates like starch, glycogen, and various simple sugars are readily broken down into glucose by digestive enzymes, cellulose is different due to its unique chemical structure. Cellulose is a polysaccharide composed of long, unbranched chains of glucose units linked together by β(1→4)-glycosidic bonds. Humans do not produce the necessary enzyme, cellulase, to break these specific beta linkages. In contrast, digestible carbohydrates like starch and glycogen use alpha-glycosidic bonds, which human enzymes can easily hydrolyze.

The Role of Dietary Fiber

Since it cannot be broken down, cellulose passes through the human stomach and small intestine undigested. It is a major component of insoluble dietary fiber, or "roughage," found in the cell walls of plants. This indigestible bulk plays a crucial role in maintaining digestive health, despite not providing any calories or nutrients directly.

The Functions of Fiber in the Digestive Tract:

Adds Bulk to Stool: Insoluble fiber, like cellulose, absorbs water and adds volume to waste material. This makes stools softer and easier to pass, which helps prevent constipation.
Promotes Regularity: By adding bulk, fiber helps stimulate peristalsis—the muscular contractions that move food and waste through the intestines. This helps ensure the smooth and timely passage of food.
Supports Gut Health: Although humans cannot break down cellulose, some of the gut bacteria in the large intestine can ferment it. This process produces beneficial short-chain fatty acids (SCFAs), which nourish the gut lining and play a role in overall health.
May Aid in Satiety: High-fiber foods often require more chewing and can create a feeling of fullness, which can help with weight management.

Comparison of Digestible vs. Indigestible Carbohydrates


Feature	Digestible Carbohydrates (e.g., Starch, Glycogen)	Indigestible Carbohydrates (e.g., Cellulose, Fiber)
Energy Source	Primary source of energy, broken down into glucose.	Not a direct energy source, passes through undigested.
Chemical Bond	Contains α-glycosidic bonds.	Contains β-glycosidic bonds.
Enzyme Required	Amylase and other digestive enzymes produced by humans.	Cellulase, an enzyme humans lack.
Digestion Site	Primarily in the mouth (initial) and small intestine.	Not digested by human enzymes; fermented by gut bacteria in the large intestine.
Health Function	Fuels the body's cells and nervous system.	Provides dietary fiber, promoting bowel regularity and gut health.
Found in	Breads, rice, potatoes, pasta, and muscle/liver tissue (glycogen).	Plant cell walls, including fruits, vegetables, whole grains, and legumes.

The Importance of Indigestible Fiber

The misconception that indigestible carbohydrates are simply 'waste' overlooks their critical role. While they don't provide energy, they are fundamental to a healthy digestive system. Without sufficient fiber, conditions like constipation become more common as the intestinal muscles are not adequately stimulated. A high-fiber diet has been linked to a reduced risk of colon cancer, as it helps shorten the transit time of waste through the colon. Furthermore, the production of SCFAs through bacterial fermentation has been shown to have a variety of beneficial effects on overall health. It's a symbiotic relationship: the indigestible fiber nourishes our gut microbiome, which, in turn, supports our health.

How Other Animals Digest Cellulose

It's a common point of comparison that many herbivores, like cows, sheep, and termites, are able to extract nutrients from cellulose. This is not because they produce the cellulase enzyme themselves, but because they have a symbiotic relationship with microorganisms in their digestive tract. These microbes, such as bacteria and protozoa, produce the cellulase needed to break down cellulose into usable compounds. For example, ruminant animals have a specialized multi-chambered stomach where extensive microbial fermentation occurs. This co-evolutionary partnership allows them to thrive on diets rich in plant material, a strategy that humans do not possess.

Conclusion

In summary, the carbohydrate that is indigestible in the human body is cellulose. The inability to digest cellulose is due to the lack of the specific enzyme required to break its beta-glycosidic bonds. Far from being useless, this property makes cellulose a vital component of dietary fiber, promoting intestinal regularity, bulk formation, and supporting a healthy gut microbiome. A balanced diet, rich in this indigestible carbohydrate from plant-based foods, is essential for overall digestive well-being. By understanding why our bodies process cellulose differently, we gain a deeper appreciation for the complex and beneficial roles that various carbohydrates play in our health.

More information on dietary fiber:

National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)

Frequently Asked Questions

Both starch and cellulose are polysaccharides made of glucose units. The key difference lies in the chemical bonds: starch has alpha-glycosidic bonds that human enzymes can break, while cellulose has beta-glycosidic bonds that human enzymes cannot break.

The human body cannot digest cellulose because it does not produce the enzyme cellulase, which is necessary to break the specific beta-glycosidic linkages that hold the cellulose polymer together.

Even though it is indigestible, cellulose is an essential part of the human diet because it acts as insoluble dietary fiber. It adds bulk to stool, aids in bowel movements, and promotes overall digestive health.

No, cellulose does not provide direct energy for humans. It passes through the digestive system undigested because it cannot be broken down into absorbable glucose molecules.

Cellulose is found in the cell walls of all plants. Excellent sources include fruits, vegetables, whole grains, nuts, seeds, and legumes.

Animals like cows, known as ruminants, have a symbiotic relationship with microorganisms in their digestive tract. These microbes produce the cellulase enzyme required to break down cellulose, allowing the animal to derive energy from it.

A sudden, significant increase in dietary fiber can cause temporary discomfort, such as bloating or gas, as the body adjusts. It is recommended to increase fiber intake gradually and drink plenty of fluids to avoid these effects.