What is a substance that cannot be digested called?

October 28, 2025 •

3 min read

According to the National Institutes of Health, most Americans consume only about 15 grams of fiber per day, roughly half of the recommended amount. A substance that cannot be digested is called dietary fiber, also known as roughage or bulk. This plant-based material passes through the digestive system mostly intact, playing a vital role in maintaining digestive and overall health.

Quick Summary

This article explains that dietary fiber, a plant-based material, is the term for a substance the human body cannot digest. It details the types of fiber and their specific health benefits for gut and cardiovascular health.

Key Points

Dietary Fiber Is Indigestible: A substance that cannot be digested is called dietary fiber, or roughage, because humans lack the specific enzymes to break it down.
Two Types of Fiber Exist: Fiber is classified into two main types, soluble and insoluble, each offering distinct health benefits.
Feeds the Gut Microbiome: While undigested by humans, fiber is fermented by beneficial gut bacteria, which produce nourishing compounds called short-chain fatty acids.
Promotes Digestive Regularity: Insoluble fiber adds bulk to stool and speeds up the movement of waste through the intestines, preventing constipation.
Supports Heart and Blood Sugar Health: Soluble fiber helps lower LDL cholesterol and slows the absorption of sugar, which aids in managing blood sugar levels.
Found in Whole Foods: The best sources of both types of fiber are whole plant foods, including fruits, vegetables, whole grains, and legumes.
Contributes to Longevity: Increased fiber intake is linked to a reduced risk of several chronic conditions, including heart disease and type 2 diabetes.

What Exactly is Dietary Fiber?

Dietary fiber is a type of carbohydrate found in plants that the body's digestive enzymes cannot break down. Unlike fats, proteins, and other carbohydrates that are absorbed for energy, fiber remains largely intact as it travels through the stomach, small intestine, and colon. It is a critical component of a healthy diet, providing numerous health benefits even though it is not digested in the traditional sense.

There are two primary types of dietary fiber, each with unique properties and functions in the body: soluble fiber and insoluble fiber. A diverse diet including both is essential for optimal health.

Soluble Fiber

Soluble fiber dissolves in water and forms a gel-like substance in the digestive tract. This gel can help slow down digestion, which aids in blood sugar control and can lead to a longer feeling of fullness. As it passes through the intestines, soluble fiber also binds with fatty acids and cholesterol, helping to remove them from the body. This cholesterol-lowering effect is a key reason why a high-fiber diet is linked to better heart health.

Good sources of soluble fiber include:

Oats and oat bran
Beans, peas, and lentils
Apples, citrus fruits, and bananas
Carrots and barley

Insoluble Fiber

Insoluble fiber does not dissolve in water and remains mostly intact as it moves through the digestive system. Its main function is to add bulk to the stool, promoting regular bowel movements and preventing constipation. By speeding up the movement of waste through the intestines, insoluble fiber helps maintain bowel health and may reduce the risk of certain colorectal issues.

Excellent sources of insoluble fiber include:

Whole-wheat flour and wheat bran
Nuts and seeds
The skins of fruits and vegetables
Cauliflower, green beans, and potatoes

The Crucial Role of the Gut Microbiome

While human enzymes cannot break down fiber, the bacteria residing in our large intestine, known as the gut microbiome, can. These bacteria ferment certain types of fiber, a process that produces beneficial compounds called short-chain fatty acids (SCFAs). SCFAs are a vital energy source for the cells lining the colon and play a significant role in gut health.

This symbiotic relationship is essential for more than just digestion. A healthy gut microbiome, fueled by dietary fiber, can help train and support the body's immune system, which has important implications for overall health. A diet lacking in fiber can lead to an imbalance in the gut bacteria, potentially contributing to various digestive and inflammatory issues.

Why Your Body Can’t Digest Fiber

The inability of the human body to digest fiber comes down to a matter of enzymes. Specifically, humans lack the enzyme cellulase, which is required to break down cellulose, a key component of plant cell walls. Herbivores, in contrast, have evolved to host symbiotic bacteria that produce cellulase, allowing them to extract nutrients from plant matter. For humans, this undigested plant material serves a different but equally important purpose—it provides the structural support for a healthy digestive process and the raw material for our beneficial gut bacteria.

Comparison of Fiber Types: Soluble vs. Insoluble


Feature	Soluble Fiber	Insoluble Fiber
Dissolves in Water?	Yes, forms a gel-like substance	No, passes through mostly intact
Effect on Digestion	Slows down digestion and gastric emptying	Speeds up the transit of food through the gut
Effect on Stool	Makes stool softer and adds bulk	Increases stool bulk and regularity
Primary Health Benefits	Lowers cholesterol, regulates blood sugar, increases satiety	Prevents constipation, supports bowel regularity
Source Examples	Oats, beans, apples, carrots, psyllium	Whole grains, nuts, seeds, vegetable skins
Bacterial Fermentation	Highly fermentable by gut bacteria	Less fermentable by gut bacteria

Conclusion: More Than Just 'Roughage'

In conclusion, a substance that cannot be digested is called dietary fiber, a vital nutrient for human health. Far from being useless 'roughage,' this undigested plant matter is essential for maintaining a healthy digestive system, managing weight, and supporting heart health. By understanding the roles of both soluble and insoluble fiber, individuals can make informed dietary choices to boost their intake through whole foods like fruits, vegetables, and whole grains. The synergistic relationship between fiber and the gut microbiome highlights how non-digestible substances contribute to overall well-being. Ensuring adequate fiber intake is a straightforward and impactful way to invest in your long-term health.

Incorporating diverse, high-fiber foods into your diet promotes digestive regularity and feeds the beneficial bacteria in your gut, leading to a healthier you.

Frequently Asked Questions

Humans cannot digest dietary fiber primarily because our digestive system lacks the enzyme cellulase, which is necessary to break down the complex carbohydrates found in plant cell walls.

Yes, dietary fiber is categorized into two main types: soluble fiber, which dissolves in water and forms a gel, and insoluble fiber, which does not dissolve and remains largely intact.

While the body does not absorb fiber for energy directly, beneficial gut bacteria ferment certain fibers to produce short-chain fatty acids (SCFAs), which provide energy to colon cells.

A common example of an indigestible substance is cellulose, a major component of plant cell walls found in fruits, vegetables, and whole grains.

Adding too much fiber too quickly can cause digestive discomfort like gas, bloating, and cramping. It is recommended to increase fiber intake gradually and drink plenty of water.

High-fiber foods tend to be more filling than low-fiber foods, helping you feel fuller for longer. This can help reduce overall calorie intake and assist with weight management.

No, soluble fiber is known for lowering cholesterol and regulating blood sugar, while insoluble fiber is more effective for preventing constipation and promoting bowel regularity.