What Carbs Can't Be Digested by Humans? An In-Depth Look

November 3, 2025 •

3 min read

While most carbohydrates are broken down into glucose for energy, a significant portion passes through our digestive system untouched. This is because these specific carbs can't be digested by humans due to a lack of the necessary enzymes. These indigestible carbohydrates, including dietary fiber and resistant starch, play a crucial role in gut health and overall wellness.

Quick Summary

This article explores the types of carbohydrates humans cannot digest, primarily dietary fiber and resistant starch. It details how they function in the body, their health benefits, and their food sources.

Key Points

Indigestible Carbs: Humans cannot digest certain carbs, like dietary fiber and resistant starch, due to a lack of specific enzymes.
Dietary Fiber's Role: Fiber adds bulk and softens stool (insoluble) or forms a gel that slows digestion (soluble), promoting regularity and controlling blood sugar and cholesterol.
Resistant Starch is Beneficial: Resistant starch, found in foods like cooled potatoes and green bananas, acts like fiber by feeding good gut bacteria and producing beneficial short-chain fatty acids.
Prebiotic Power: These carbs act as prebiotics, nourishing the gut microbiome and promoting a healthy balance of bacteria essential for digestive and overall health.
Boost Your Intake: Increase your consumption of whole grains, legumes, nuts, seeds, fruits, and vegetables to ensure a varied intake of different types of indigestible carbohydrates.

The Fundamental Reason We Can't Digest Certain Carbs

Our bodies produce a range of enzymes designed to break down macronutrients, such as amylases, lactase, sucrase, and maltase for starches and sugars. However, some carbohydrates have molecular structures, often featuring beta-glycosidic linkages, that our enzymes cannot break down. These undigested molecules then travel to the large intestine, where beneficial gut bacteria ferment them, leading to various health benefits.

Indigestible Carbs: The Pillars of Dietary Fiber

Dietary fiber, a well-known category of indigestible carbohydrates, consists of polysaccharides and lignin primarily found in plant cell walls. Fiber is categorized based on its solubility in water.

Soluble Fiber

This type dissolves in water, forming a gel in the digestive tract. It's easily fermented by large intestine bacteria, providing fuel for gut microbes.

Examples of soluble fiber include: pectins in apples, berries, and citrus fruits; beta-glucans in oats and barley; gums like guar gum; and inulin and oligofructose in onions, garlic, and chicory root.

Insoluble Fiber

Insoluble fiber doesn't dissolve in water and remains mostly intact through the digestive system. It adds bulk to stool, aiding regularity and preventing constipation. While less fermentable than soluble fiber, some fermentation can still occur.

Examples of insoluble fiber include: cellulose and hemicellulose in plant cell walls and whole grains, and lignin in nuts and seeds.

Resistant Starch: The Hidden Indigestible Carb

Resistant starch (RS) is a starch that avoids digestion in the small intestine and acts like dietary fiber. It is fermented by large intestine bacteria, feeding beneficial gut microbes. Resistant starch content can change with food preparation. There are five main types of resistant starch:

RS1: Starch trapped within whole grains, seeds, and legumes.
RS2: Starch granules in raw potatoes and green bananas.
RS3: Starch formed when cooked starchy foods are cooled. Reheating does not eliminate this.
RS4: Chemically modified starch in processed foods.
RS5: Amylose-lipid complexes from cooking starch with fats.

The Health Benefits of Indigestible Carbohydrates

Indigestible carbohydrates offer numerous health benefits, largely through their interaction with the gut microbiome. They act as prebiotics, feeding beneficial gut bacteria and promoting a diverse microbial community. This fermentation produces short-chain fatty acids (SCFAs) like butyrate, which fuel colon cells and reduce inflammation. These carbs also contribute to weight management by increasing fullness and satiety, regulate blood sugar by slowing glucose absorption, support cardiovascular health by helping lower cholesterol, and reduce constipation by adding bulk to stool.

Comparing Key Types of Indigestible Carbs


Feature	Soluble Fiber	Insoluble Fiber	Resistant Starch (RS3)
Dissolves in Water?	Yes	No	Varies (often soluble)
Viscosity (Gel-Forming)?	High	Low	Low to high
Fermentability	Highly fermentable	Minimally fermentable	Highly fermentable
Primary Function	Slows digestion, lowers cholesterol and blood sugar	Adds bulk to stool, aids regularity	Feeds gut bacteria, produces SCFAs
Food Sources	Oats, apples, beans, nuts, psyllium	Whole wheat, vegetables, legumes, nuts	Cooked and cooled rice, pasta, potatoes
Impact on Transit Time	Delays gastric emptying	Accelerates intestinal transit	Varies

Conclusion

Understanding what carbs can't be digested by humans highlights their vital role in a healthy diet. These indigestible carbohydrates, including various forms of dietary fiber and resistant starch, are crucial prebiotics for our gut microbiome. They provide numerous health benefits, from improved digestion and regularity to better blood sugar control and reduced risk of chronic diseases. Consuming a variety of plant-based foods like whole grains, legumes, fruits, and vegetables is key to getting enough of these essential compounds. Including these complex, non-digestible carbs is an important strategy for supporting gut health and overall well-being.

Supporting your gut with the right carbs

For more information on the critical role of gut bacteria, explore the connection between the gut microbiome and overall health at The American Gut Project.

Frequently Asked Questions

Humans cannot digest cellulose, a type of insoluble fiber, because our digestive systems lack the specific enzymes required to break down the beta-glycosidic bonds that link its glucose molecules.

Soluble fiber dissolves in water to form a gel, which slows digestion and helps regulate blood sugar and cholesterol. Insoluble fiber does not dissolve and adds bulk to stool, promoting regularity.

Yes, resistant starch is healthy. It acts as a prebiotic, fueling beneficial gut bacteria and producing short-chain fatty acids that support colon health and can increase insulin sensitivity.

Yes, you can. Cooking and then cooling starchy foods like rice, potatoes, and pasta can significantly increase their resistant starch content through a process called retrogradation. Reheating these foods will not undo this effect.

Indigestible carbohydrates are fermented by the gut microbiota in the large intestine. This process produces beneficial compounds known as short-chain fatty acids (SCFAs), which nourish the colon and have systemic health effects.

Excellent sources include a wide range of plant-based foods: whole grains (oats, barley), legumes (beans, lentils), nuts, seeds (chia, flax), and various fruits and vegetables.

Yes, for some individuals, the fermentation process of these carbohydrates in the colon can produce gas and cause bloating, especially when intake is increased too quickly. Gradually increasing fiber and fluid intake can help minimize these effects.