Nutrition Diet: Which carbohydrate doesn't digest in your body?

October 7, 2025 •

3 min read

According to the American Diabetes Association, dietary fiber is the type of carbohydrate that doesn't get broken down into sugar molecules and instead passes through the body undigested. This indigestible carb, along with another called resistant starch, is vital for gut health and overall well-being, despite not providing energy directly.

Quick Summary

This article explores the types of carbohydrates, specifically dietary fiber and resistant starch, that your body cannot fully digest in the small intestine. It details how these components are fermented by bacteria in the large intestine to produce beneficial compounds and improve digestive and metabolic health.

Key Points

Dietary Fiber is a Key Indigestible Carb: Fiber, found in plants, passes through the body undigested and comes in soluble and insoluble forms with different health benefits.
Resistant Starch Resists Digestion: This special type of starch passes through the small intestine largely intact, acting like fiber in the large intestine.
Indigestible Carbs Feed Your Microbiome: In the colon, beneficial bacteria ferment these carbs, producing short-chain fatty acids (SCFAs), including butyrate.
Health Benefits are Numerous: Increased intake can improve digestive health, aid in weight management, lower cholesterol, and regulate blood sugar.
'Cook and Cool' Increases Resistant Starch: For foods like potatoes and rice, cooling after cooking can increase their resistant starch content.
Gradual Increase is Advised: To avoid side effects like gas and bloating, it is best to gradually increase the amount of fiber and resistant starch in your diet.
Look for Plant-Based Sources: The best way to get both fiber and resistant starch is by consuming a varied diet of whole grains, legumes, vegetables, and fruits.

Understanding Indigestible Carbohydrates

Most carbohydrates, including simple sugars and starches, are broken down by enzymes in the small intestine and absorbed into the bloodstream as glucose for energy. However, some carbohydrates resist this process and pass into the large intestine, where they are fermented by beneficial bacteria. This fermentation supports a healthy gut microbiome.

The Two Main Types: Fiber and Resistant Starch

The primary indigestible carbohydrates are dietary fiber and resistant starch. They are not broken down by human enzymes but benefit the body in different ways and are found in various plant foods.

Dietary Fiber: The Indigestible Plant Material

Dietary fiber is plant material that remains undigested and is categorized into soluble and insoluble types.

Soluble Fiber: Dissolves in water to form a gel, which can help regulate blood sugar and cholesterol. It also acts as a prebiotic, feeding gut bacteria. Examples include oats, beans, and apples.

Insoluble Fiber: Doesn't dissolve in water and adds bulk to stool, promoting regularity. Sources include whole wheat, nuts, and the skins of fruits and vegetables.

Resistant Starch: A Fermentable Powerhouse

Resistant starch (RS) is a starch that avoids digestion in the small intestine and is fermented in the large intestine, similar to fiber. This fermentation produces short-chain fatty acids (SCFAs) like butyrate, which is vital for colon cell health.

Types of resistant starch vary by source: RS Type 1 in seeds and legumes, RS Type 2 in unripe bananas and raw potatoes, RS Type 3 in cooked and cooled starches, and RS Type 4 in chemically modified starches. Reheating cooked and cooled starches does not eliminate the resistant starch.

The Health Benefits of Indigestible Carbohydrates

Indigestible carbohydrates are crucial for health:

Promotes Digestive Health: Fiber aids regularity and can help prevent conditions like diverticulitis.
Nourishes the Gut Microbiome: They act as prebiotics, feeding beneficial bacteria, supporting immune function, and producing beneficial metabolites.
Aids in Weight Management: These carbs can increase feelings of fullness, potentially reducing calorie intake.
Manages Blood Sugar: Soluble fiber and resistant starch can slow sugar absorption, preventing sharp blood glucose spikes.
Lowers Cholesterol: Soluble fiber can bind to bile acids, leading to lower LDL cholesterol.

Comparison of Indigestible Carbs


Feature	Soluble Fiber	Insoluble Fiber	Resistant Starch
Digestion Process	Dissolves in water, forms gel.	Remains undissolved, adds bulk.	Resists digestion in small intestine.
Fermentation	Fermented by gut bacteria.	Fermented to a lesser extent or not at all.	Fermented by gut bacteria, yielding SCFAs.
Primary Function	Slows digestion, regulates blood sugar, lowers cholesterol.	Promotes bowel regularity, adds stool bulk.	Feeds gut bacteria, produces butyrate.
Common Sources	Oats, beans, apples, carrots, barley.	Whole grains, nuts, seeds, fruit/veg skins.	Unripe bananas, cooked and cooled rice/potatoes, legumes.
Key Benefits	Better blood sugar control, lower cholesterol.	Prevents constipation.	Stronger gut barrier, improved insulin sensitivity.
Potential Side Effects	Gas and bloating with sudden high intake.	Gas and bloating with sudden high intake.	Gas and bloating with sudden high intake.

How to Incorporate More Indigestible Carbs

Increase your intake gradually by adding legumes, choosing whole grains, eating fruits and vegetables with skins, and using the 'cook and cool' method for starches. Unripe bananas are also a good source of resistant starch.

A Word of Caution: Potential Side Effects

A sudden increase in indigestible carbs can cause gas and bloating due to fermentation. Introduce these foods gradually and drink plenty of fluids.

Conclusion

Indigestible carbohydrates, primarily dietary fiber and resistant starch, are essential for good nutrition, supporting gut health, regulating blood sugar and cholesterol, and promoting regularity. A varied diet rich in plant-based foods is the best way to incorporate these beneficial components for overall health.

For more on resistant starch and gut health, CSIRO provides resources.

Frequently Asked Questions

Yes, dietary fiber is a complex carbohydrate found in plants that the human body cannot break down or absorb in the small intestine. It passes through mostly intact, providing numerous health benefits.

Soluble fiber dissolves in water to form a gel-like substance that can lower cholesterol and blood sugar, while insoluble fiber does not dissolve and adds bulk to stool, helping with regularity.

Resistant starch is a type of starch that resists digestion and ferments in the large intestine. It is important because it feeds beneficial gut bacteria, leading to the production of short-chain fatty acids that support gut and overall health.

Good sources include whole grains (oats, barley), legumes (beans, lentils), unripe bananas, cooked and cooled starches (potatoes, rice), nuts, seeds, and many fruits and vegetables.

Yes, introducing a large amount of indigestible carbohydrates too quickly can cause gas and bloating due to the fermentation process by gut bacteria. It is best to increase intake gradually.

Soluble fiber and resistant starch can slow down the absorption of glucose into the bloodstream, which helps prevent sudden spikes in blood sugar, benefiting individuals with or at risk for type 2 diabetes.

No, reheating cooked and cooled starchy foods like potatoes or rice does not remove the resistant starch that has been formed during the cooling process.