Which Type of Carbohydrates We Cannot Digest?

October 23, 2025 •

4 min read

Over 95% of Americans do not consume the recommended daily intake of fiber, the most prominent indigestible carbohydrate. Understanding which type of carbohydrates we cannot digest is key to appreciating their role in our diet, as these non-digestible components are vital for a healthy gut and overall well-being.

Quick Summary

Certain carbohydrates, including various forms of dietary fiber and resistant starch, are not broken down by human digestive enzymes. Instead, they pass to the large intestine, where they are fermented by beneficial gut bacteria, promoting digestive regularity and gut health.

Key Points

Dietary Fiber Is Indigestible: The most common carbohydrates we cannot digest are various forms of dietary fiber, including both soluble and insoluble types.
Specific Enzyme Deficiency: Humans cannot break down the beta-glycosidic bonds found in molecules like cellulose because our bodies lack the necessary enzymes.
Gut Bacteria Do the Work: Indigestible carbohydrates act as food for beneficial gut bacteria, which ferment them to produce health-promoting short-chain fatty acids (SCFAs).
Resistant Starch is Key: Some starches, particularly when cooked and cooled, become 'resistant' and pass through the small intestine undigested, feeding colon bacteria.
Prebiotics for Gut Health: Certain oligosaccharides, like inulin found in onions and chicory root, are also non-digestible and selectively fuel beneficial gut microbes.
Health Benefits Abound: Consuming indigestible carbohydrates improves bowel function, helps regulate blood sugar, lowers cholesterol, and supports weight management.
Diverse Food Sources: Incorporating a variety of whole plant foods, including legumes, whole grains, nuts, and vegetables, is the best way to get different types of indigestible carbs.

What Makes a Carbohydrate Indigestible?

Most carbohydrates, such as starches and sugars found in white bread and sugary drinks, are easily broken down by enzymes like amylase in our digestive tract and absorbed as glucose. However, some carbohydrates are built with different chemical bonds that our body's enzymes cannot break. The two primary reasons humans cannot digest certain carbohydrates are the type of glycosidic bonds and the physical structure of the molecule. For example, cellulose, found in plant cell walls, is composed of glucose units linked by beta-glycosidic bonds, which human enzymes lack the ability to cleave.

The Major Categories of Indigestible Carbohydrates

Indigestible carbohydrates are most commonly known as dietary fiber, but the category also includes resistant starches and some oligosaccharides.

Dietary Fiber: This is the most recognized form of indigestible carbohydrate. It includes a variety of plant-based polysaccharides and lignin that resist human digestive enzymes. Fiber is further classified into two main types:
- Insoluble Fiber: This type does not dissolve in water and adds bulk to the stool, promoting regularity and preventing constipation. Examples include cellulose and hemicellulose found in wheat bran, nuts, and many vegetables.
- Soluble Fiber: This type dissolves in water to form a gel-like substance in the digestive tract. It can help lower cholesterol and blood sugar levels by slowing down digestion. Sources include oats, barley, beans, and many fruits.
Resistant Starch: This is a type of starch that escapes digestion in the small intestine and functions like soluble fiber in the large intestine. The resistance can occur due to different factors, such as being physically trapped within food (like in whole grains) or changing its structure after cooking and cooling (like in cooked potatoes or rice). Fermentation of resistant starch by gut bacteria produces short-chain fatty acids (SCFAs), which have numerous health benefits.
Oligosaccharides: These are carbohydrate polymers containing a small number of simple sugars, typically three to ten. The human body lacks the enzymes to digest many of these, including fructooligosaccharides (FOS) found in onions and asparagus, and galactooligosaccharides (GOS) in legumes and human milk. These act as prebiotics, selectively feeding beneficial bacteria in the colon.

The Crucial Role of Gut Bacteria

When indigestible carbohydrates reach the large intestine, our native gut bacteria, or microbiota, take over. These microbes possess the necessary enzymes to ferment these carbohydrates, a process that produces a range of beneficial compounds, most notably short-chain fatty acids (SCFAs). SCFAs like butyrate, propionate, and acetate are then absorbed and used by the body for energy and other functions. Butyrate, for instance, is the primary energy source for the cells lining the colon, helping to maintain a healthy gut barrier.

Health Benefits of Indigestible Carbohydrates

The inability to digest certain carbohydrates is not a limitation but a critical aspect of human nutrition. The fermentation process offers numerous health advantages:

Improved Digestive Health: Both soluble and insoluble fiber are vital for regulating bowel movements, preventing constipation, and lowering the risk of diverticulosis.
Blood Sugar Control: Soluble fiber and resistant starch slow the absorption of sugar, which helps prevent sharp spikes in blood glucose levels, benefiting individuals with or at risk of type 2 diabetes.
Cholesterol Reduction: Viscous soluble fibers, like those in oats and psyllium, can help reduce LDL ('bad') cholesterol levels by binding to cholesterol and bile acids in the gut.
Weight Management: Indigestible carbohydrates add bulk to food and promote satiety, helping you feel fuller for longer and reducing overall calorie intake.
Enhanced Gut Microbiome: As prebiotics, oligosaccharides and fermentable fibers stimulate the growth of beneficial bacteria like Bifidobacteria and Lactobacillus, which can improve gut health and immune function.

Comparison of Soluble vs. Insoluble Fiber


Feature	Soluble Fiber	Insoluble Fiber
Dissolves in Water?	Yes	No
Effect on Digestion	Slows digestion	Speeds up digestion
Primary Function	Forms a gel, helps lower cholesterol & glucose	Adds bulk, promotes bowel regularity
Fermentable by Gut Bacteria?	Yes, highly fermentable	Minimally fermentable
Health Benefits	Improved blood sugar control, lower cholesterol	Constipation relief, improved stool consistency
Food Sources	Oats, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables	Whole wheat flour, wheat bran, nuts, beans, cauliflower, green beans, potatoes

Practical Tips for Increasing Indigestible Carbohydrate Intake

To reap the benefits of indigestible carbohydrates, focus on whole, unprocessed foods:

Eat more legumes: Add lentils, chickpeas, and beans to your meals. They are excellent sources of both fiber and resistant starch.
Cool cooked starches: Allow cooked potatoes, rice, and pasta to cool. This process, known as retrogradation, increases their resistant starch content.
Choose whole grains: Opt for whole-grain versions of bread, pasta, and cereals over their refined counterparts. These often retain more cellulose and other fibers.
Enjoy a variety of fruits and vegetables: Eating fruits with edible skins (like apples) and a range of vegetables ensures a diverse intake of both soluble and insoluble fiber.
Don't forget the prebiotics: Incorporate foods like onions, garlic, and asparagus, which contain beneficial oligosaccharides.

Conclusion

While many carbohydrates provide energy, it's the specific types we cannot digest—dietary fibers, resistant starches, and oligosaccharides—that offer profound health advantages. These indigestible carbohydrates are essential for feeding our gut microbiota, which in turn produces vital compounds that benefit our digestive system, help manage blood sugar and cholesterol, and support overall health. Rather than viewing them as simply undigested waste, we should recognize their crucial role as functional components of a balanced diet. Prioritizing a wide variety of whole plant foods is the most effective way to ensure an adequate intake of these beneficial, non-digestible carbohydrates.

Visit the Mayo Clinic for more information on the health benefits of dietary fiber.

Frequently Asked Questions

The main reason is that humans lack the specific digestive enzymes needed to break down the chemical bonds in certain carbohydrate molecules. For example, we cannot break the beta-glycosidic bonds in cellulose, which is the main component of insoluble fiber.

No, indigestible carbohydrates fall into several categories, including dietary fiber (both soluble and insoluble), resistant starch, and certain oligosaccharides. They differ in their chemical structure and how they behave in the digestive system.

When indigestible carbohydrates reach the large intestine, they are fermented by gut bacteria. This process produces beneficial compounds like short-chain fatty acids (SCFAs) which are absorbed and used for energy, gut health, and other systemic functions.

Resistant starch is found in foods like legumes (beans, lentils), raw potatoes, and unripe bananas. It is also created when starchy foods like rice, potatoes, and pasta are cooked and then cooled.

While generally beneficial, increasing indigestible carbohydrate intake too quickly can cause digestive discomfort like bloating or gas as your gut microbiota adjusts. It is best to increase fiber and resistant starch intake gradually.

Cellulose is a specific type of insoluble dietary fiber. It is a major component of plant cell walls and is not digestible by humans, adding bulk to stool to aid in digestion.

Oligosaccharides are short-chain carbohydrates that are indigestible by human enzymes. They act as prebiotics, serving as food for beneficial bacteria in the colon. Fructooligosaccharides (FOS) and galactooligosaccharides (GOS) are common examples.