Fiber, Resistant Starch, and Oligosaccharides: What is an indigestible carbohydrate called?

December 13, 2025 •

4 min read

According to The Whole Grains Council, certain carbohydrates found in whole grains resist digestion and are fermented in the lower colon, creating beneficial compounds. This process is carried out by an indigestible carbohydrate, a vital nutrient that includes fiber, resistant starches, and other resistant polysaccharides.

Quick Summary

Indigestible carbohydrates, commonly referred to as dietary fiber, are essential nutrients that pass through the digestive system largely intact. They are fermented by beneficial gut bacteria in the large intestine, supporting the microbiome and aiding overall digestive functions. Other types include resistant starch and specific oligosaccharides.

Key Points

Dietary Fiber is the Name: The most common term for an indigestible carbohydrate is dietary fiber, a broad category of plant-based nutrients that includes various types.
Two Main Types of Fiber: Dietary fiber is categorized into soluble fiber, which dissolves in water and lowers cholesterol, and insoluble fiber, which adds bulk to stool and promotes regularity.
Resistant Starch is Key: Resistant starch is another type of indigestible carbohydrate that passes through the small intestine and ferments in the large intestine, feeding beneficial gut bacteria.
Feed Your Gut Microbiome: The fermentation of indigestible carbohydrates by gut bacteria produces short-chain fatty acids (SCFAs), which are crucial for maintaining a healthy gut wall and overall health.
Improve Multiple Health Outcomes: A diet rich in these carbohydrates offers numerous benefits, including better weight management, regulated blood sugar, lower cholesterol, and improved digestive health.
Find it in Whole Foods: Excellent sources of indigestible carbohydrates include whole grains, legumes, vegetables, and fruits, particularly those with edible skins or seeds.

What Defines an Indigestible Carbohydrate?

An indigestible carbohydrate is a type of carbohydrate that our bodies cannot break down and absorb in the small intestine due to a lack of the necessary digestive enzymes. Instead, these complex molecules travel to the large intestine where they are fermented by beneficial bacteria in the gut microbiome. This process is responsible for many of the health benefits associated with a high-fiber diet. The term 'dietary fiber' is the most common name for this category, but it encompasses a variety of sub-types, each with distinct properties and functions in the body. Understanding these differences is key to appreciating their role in maintaining optimal health.

Types of Indigestible Carbohydrates

Beyond the general term of fiber, there are several key classifications of indigestible carbohydrates, including soluble and insoluble fiber, resistant starches, and certain oligosaccharides.

Soluble Fiber: This type of fiber dissolves in water to form a gel-like substance.
- Health Benefits: Helps to lower blood cholesterol and glucose levels.
- Food Sources: Found in oats, peas, beans, apples, carrots, and citrus fruits.
Insoluble Fiber: This fiber does not dissolve in water and remains mostly intact as it moves through the digestive tract.
- Health Benefits: Adds bulk to stool, promoting regularity and helping to prevent constipation.
- Food Sources: Found in whole-wheat flour, wheat bran, nuts, beans, and vegetables like green beans and cauliflower.
Resistant Starch: As the name suggests, resistant starch is a type of starch that resists digestion in the small intestine. It is fermented in the large intestine and acts as a prebiotic, feeding good bacteria. There are several types of resistant starch, categorized by their source and how they resist digestion.
Oligosaccharides: These are carbohydrates made up of a small number of simple sugar units (typically 3-10) that are not digested in the human small intestine. Instead, they travel to the colon where they are fermented by gut bacteria, often producing gas. A well-known example is inulin, found in foods like chicory root and onions.

The Function and Fermentation Process

When indigestible carbohydrates enter the large intestine, they are not simply waste. They become a critical food source for the trillions of bacteria that make up our gut microbiome. These microbes ferment the carbohydrates, producing short-chain fatty acids (SCFAs) like butyrate, acetate, and propionate. These SCFAs are then absorbed by the body and provide a range of health benefits, acting as a link between diet, gut health, and overall systemic well-being. Butyrate, for instance, serves as the primary energy source for the cells lining the colon, helping to maintain a healthy gut wall. This fermentation process is why a sudden increase in fiber intake can cause gas and bloating, as the gut bacteria adjust to their new fuel source.

The Health Benefits of Indigestible Carbohydrates

Incorporating a variety of indigestible carbohydrates into your diet can lead to significant health improvements.

Improved Gut Health: By serving as prebiotics, these carbohydrates promote the growth of beneficial gut flora, which can improve digestion and nutrient absorption. A balanced gut microbiome is also linked to better immune function and mood regulation.
Weight Management: Fiber-rich foods promote a feeling of fullness (satiety), which can help manage appetite and reduce overall calorie intake.
Blood Sugar Regulation: Soluble fiber slows the absorption of sugar into the bloodstream, preventing sharp spikes in blood glucose levels. This is particularly beneficial for individuals with or at risk for type 2 diabetes.
Lowered Cholesterol: Soluble fiber can bind to cholesterol particles in the digestive tract and remove them from the body, helping to lower LDL ('bad') cholesterol levels.
Cardiovascular Health: By lowering cholesterol and improving blood sugar control, a diet rich in these carbohydrates helps to reduce the risk of heart disease.

How to Increase Your Intake

For many, increasing fiber intake can seem like a challenge, but it is easily accomplished by focusing on whole foods. The average person consumes far less fiber than the recommended 25-35 grams per day. Start small to allow your digestive system to adapt, and be sure to drink plenty of water as you increase your intake.

Incorporate more whole grains like oats, barley, quinoa, and brown rice.
Add legumes, such as lentils, chickpeas, and beans, to soups and salads.
Choose fresh fruits over fruit juices to benefit from the intact fiber.
Eat a variety of vegetables, especially those with skins like potatoes and apples.
Consider overnight oats for breakfast, as cooking and cooling can increase resistant starch levels.
Opt for whole-wheat bread and pasta instead of refined options.

Comparison of Indigestible Carbohydrate Types


Feature	Soluble Fiber	Insoluble Fiber	Resistant Starch	Oligosaccharides
Dissolves in Water?	Yes	No	Varies (most types)	Yes (partially)
Gel-Forming?	Yes	No	Varies	Yes (some types)
Fermentability	Highly fermentable	Less fermentable	Highly fermentable	Highly fermentable
Primary Function	Slows digestion, lowers cholesterol	Adds bulk, promotes regularity	Acts as a prebiotic	Acts as a prebiotic
Main Source Examples	Oats, beans, apples	Whole grains, nuts, vegetables	Green bananas, cooked and cooled potatoes	Onions, chicory root, legumes

Conclusion: More Than Just 'Roughage'

In summary, an indigestible carbohydrate is more accurately and comprehensively referred to as dietary fiber, which itself is a broad category including soluble fiber, insoluble fiber, resistant starch, and oligosaccharides. These compounds pass through the upper digestive tract untouched by human enzymes, reaching the large intestine where they are fermented by gut bacteria. This fermentation is not a waste process, but a crucial biological interaction that produces short-chain fatty acids and promotes a healthy microbiome. Beyond just aiding regularity, these carbohydrates offer robust benefits for cardiovascular health, blood sugar control, and weight management. By increasing your consumption of a diverse range of whole plant foods, you can ensure a higher intake of these invaluable nutrients and support your overall well-being. For more detailed information on dietary fiber, the Harvard T.H. Chan School of Public Health provides an excellent resource on its Nutrition Source website.

Frequently Asked Questions

The main difference is how the body processes them. Digestible carbohydrates are broken down into glucose for energy in the small intestine, while indigestible carbohydrates, or fiber, pass through the small intestine undigested and are fermented in the large intestine.

Soluble fiber can be found in oats, beans, lentils, peas, apples, bananas, carrots, and citrus fruits. It helps to lower blood glucose and cholesterol levels.

Insoluble fiber is abundant in whole-wheat products, nuts, seeds, and vegetables like cauliflower and green beans. It helps promote bowel regularity by adding bulk to your stool.

Yes, resistant starch functions as a prebiotic, feeding the 'good' bacteria in your large intestine. This fermentation process promotes a healthy gut microbiome, which is vital for overall health.

You can increase your resistant starch intake by eating green bananas, raw potatoes, or cooked and cooled starchy foods like rice, potatoes, and pasta. Cooking and cooling retrogrades some starches, making them resistant to digestion.

Adequate intake provides a range of benefits, including improved digestive health and regularity, better blood sugar control, lower cholesterol, and enhanced satiety, which can assist with weight management.

While fiber is essential, consuming too much too quickly can lead to gastrointestinal discomfort, such as gas, bloating, and cramping. It is best to increase your intake gradually and drink plenty of water to help your body adjust.