Understanding Which Carbohydrate is Not Absorbed by the Body

October 27, 2025 •

5 min read

Less than 50% of the dietary fiber levels recommended for good health are consumed by the average American. But what carbohydrate is not absorbed by the body? The answer lies in various non-digestible carbohydrates, primarily dietary fiber and resistant starch, that pass through the digestive tract largely intact.

Quick Summary

Dietary fiber, resistant starch, sugar alcohols, and non-digestible oligosaccharides are the carbohydrates not absorbed by the body. They are fermented by gut bacteria, which produces beneficial short-chain fatty acids for digestive health and beyond.

Key Points

Dietary Fiber and Resistant Starch: The primary carbohydrates not absorbed by the body are dietary fiber and resistant starch, which pass through the small intestine largely undigested.
Fuel for Gut Bacteria: In the large intestine, these non-absorbed carbohydrates are fermented by the gut microbiota, a process that produces beneficial short-chain fatty acids.
Improved Digestion and Regularity: Insoluble fiber adds bulk to stool, promoting regular bowel movements and preventing constipation.
Blood Sugar and Cholesterol Control: Soluble fiber and resistant starch slow sugar absorption and help lower blood cholesterol levels, improving metabolic health.
Enhanced Satiety and Weight Management: Fiber adds volume to meals and promotes feelings of fullness, which can aid in controlling weight and appetite.
The Power of Cooling: The amount of resistant starch in foods like rice, potatoes, and pasta can be increased simply by cooking and then cooling them.

The Digestive Journey of Carbohydrates

Most carbohydrates, like those found in bread and sugar, are broken down into simple sugars (glucose) by enzymes in the mouth and small intestine. This glucose is then absorbed into the bloodstream and used for energy. However, not all carbohydrates are created equal, and some bypass this process entirely. These are the non-digestible carbohydrates, and they include dietary fiber and resistant starch, which continue their journey to the large intestine where they are fermented by gut bacteria.

Dietary Fiber: The Indigestible Plant Material

Dietary fiber is a cornerstone of this category, consisting of the indigestible parts of plant materials like fruits, vegetables, nuts, and grains. There are two main types, and their impact on your body varies depending on their properties.

Soluble fiber: This type dissolves in water to form a gel-like substance. It slows down digestion, which helps control blood sugar levels, lower cholesterol, and keep you feeling full for longer. Excellent sources include oats, beans, apples, and carrots.
Insoluble fiber: As the name suggests, this fiber does not dissolve in water. It adds bulk to your stool and helps move food through the digestive system, which is crucial for preventing constipation and promoting regular bowel movements. Insoluble fiber is abundant in whole grains, wheat bran, and the skins of many fruits and vegetables.

Resistant Starch: The Unexpected Fiber

Resistant starch is a type of starch that, as its name implies, resists digestion in the small intestine, behaving much like soluble fiber. It passes to the large intestine where it is fermented by bacteria, producing beneficial compounds like short-chain fatty acids (SCFAs), particularly butyrate. Interestingly, the amount of resistant starch in food can be influenced by cooking and cooling.

There are four main types of resistant starch:

Type 1: Found in grains, seeds, and legumes where it is physically trapped within fibrous cell walls.
Type 2: Present in some raw starchy foods, such as unripe (green) bananas and raw potatoes.
Type 3: Formed when certain starchy foods, like potatoes or rice, are cooked and then cooled. The cooling process creates new resistant starch through a process called retrogradation.
Type 4: A man-made resistant starch created via a chemical process.

Other Non-Absorbed Carbohydrates

Beyond fiber and resistant starch, other carbohydrates are not fully absorbed by the body. These include non-digestible oligosaccharides (NDOs), such as fructooligosaccharides (FOS) and galactooligosaccharides (GOS), which are naturally present in some plants like chicory root, garlic, and onions. They are a type of prebiotic, selectively feeding beneficial bacteria in the gut. Additionally, sugar alcohols (polyols) like sorbitol and xylitol are used as reduced-calorie sweeteners and are incompletely absorbed by the body.

Health Benefits of Non-Absorbed Carbohydrates

These non-digestible carbohydrates offer a variety of health benefits that go far beyond simple digestive regularity. Their role as prebiotics—fuel for healthy gut bacteria—is particularly significant for overall well-being.

Improved Gut Health: By fermenting non-digestible carbs, gut bacteria produce SCFAs, such as butyrate. Butyrate is the primary energy source for the cells lining the colon and plays a crucial role in maintaining a healthy gut barrier.
Better Blood Sugar Control: Soluble fiber and resistant starch slow down the absorption of sugar, preventing sudden spikes in blood glucose after meals. This is beneficial for managing blood sugar levels and improving insulin sensitivity.
Weight Management: Both soluble and insoluble fiber add bulk to your diet, helping you feel full faster and for longer. This increased satiety can help manage appetite and reduce overall calorie intake.
Lower Cholesterol Levels: Soluble fiber can bind to bile acids in the digestive tract, which the body then excretes. To replace these, the liver must use cholesterol, which helps lower overall blood cholesterol levels.
Reduced Risk of Chronic Disease: A diet rich in non-digestible carbohydrates is associated with a lower risk of heart disease, type 2 diabetes, and certain types of cancer. The anti-inflammatory effects and gut health benefits contribute to this protective effect.

Comparing Carbohydrate Types

To illustrate the differences, here is a comparison of how different carbohydrate types are processed by the body.


Feature	Digestible Carbohydrates (e.g., Simple Sugars, Starch)	Non-Digestible Carbohydrates (e.g., Fiber, Resistant Starch)
Digestion	Enzymatically broken down into simple sugars in the mouth and small intestine.	Not broken down by human enzymes in the small intestine.
Absorption	Simple sugars (glucose) are absorbed into the bloodstream.	Passes through the small intestine largely intact to the large intestine.
Energy	Provides calories (4 kcal/g) directly to the body.	Contributes fewer calories (approx. 2 kcal/g for resistant starch) and nutrients via fermentation by gut bacteria.
Effect in Large Intestine	Any remaining sugars are fermented, but most are absorbed before reaching this point.	Fermented by gut microbiota, producing beneficial short-chain fatty acids (SCFAs).
Health Impact	Can cause blood sugar spikes and contributes to fat storage if consumed in excess.	Regulates blood sugar, lowers cholesterol, improves gut health, and increases satiety.

How to Increase Your Intake

Increasing your intake of these beneficial carbohydrates is relatively simple and can be done by making small changes to your diet. Focus on whole, unprocessed plant foods, as these are the best natural sources.

Eat more whole grains: Choose whole-wheat bread, brown rice, barley, and oats instead of refined grains.
Add legumes: Incorporate beans, lentils, and peas into your meals. Legumes are a fantastic source of both fiber and resistant starch.
Snack on fruits and vegetables: Eat fruits with edible skins like apples and pears. Raw vegetables like carrots, broccoli, and leafy greens are also excellent sources of fiber.
Go for cold leftovers: To increase resistant starch, try eating cold cooked rice, pasta, or potatoes that have been previously cooked and cooled.
Try fermented foods: Include probiotic-rich fermented foods like yogurt with prebiotic fibers (e.g., inulin from chicory root).

Conclusion

Understanding what carbohydrate is not absorbed by the body reveals a critical aspect of nutrition that is often overlooked. These non-digestible carbohydrates, including dietary fiber and resistant starch, are not just passive filler. They are active participants in promoting digestive health, regulating blood sugar, and nourishing the trillions of bacteria that make up our gut microbiome. By intentionally incorporating more of these foods into your diet, you can support your overall health in significant ways. The benefits of consuming a diverse array of these carbohydrates work synergistically to support not only the digestive system but also the metabolic and immune functions of the body. This holistic approach to nutrition is essential for long-term well-being.

For more information on dietary fiber and its benefits, consider visiting the Harvard T.H. Chan School of Public Health's dedicated nutrition page: Fiber - The Nutrition Source.

Frequently Asked Questions

The human body lacks the specific enzymes necessary to break down the chemical bonds in dietary fiber and resistant starch. While other carbohydrates are digested, these particular types pass through to the large intestine undigested.

Dietary fiber is a broad category that includes both soluble and insoluble types of plant material. Resistant starch is a specific type of starch that resists digestion but acts similarly to soluble, fermentable fiber once it reaches the large intestine.

When certain starchy foods like rice, potatoes, and pasta are cooked and then cooled, a process called retrogradation occurs. This process rearranges the starch molecules into a structure that resists digestion, forming Type 3 resistant starch.

Sugar alcohols, or polyols, are a type of carbohydrate that is only partially absorbed by the body. Because of their incomplete absorption, they provide fewer calories than sugar and do not significantly raise blood sugar levels.

SCFAs are compounds like butyrate, propionate, and acetate produced by beneficial gut bacteria when they ferment non-digestible carbohydrates. They serve as a primary energy source for colon cells and have been linked to improved gut barrier function and reduced inflammation.

Excellent sources include legumes (beans, lentils), whole grains (oats, barley, brown rice), fruits with edible skins, most vegetables, nuts, seeds, and cooked and cooled potatoes or pasta.

While beneficial, a sudden significant increase in fiber intake can cause digestive discomfort such as bloating, gas, and cramping. It is best to increase your intake gradually and drink plenty of fluids.