Tough Complex Carbs the Body Cannot Digest: Fiber and Resistant Starch

December 2, 2025 •

3 min read

According to the American Diabetes Association, fiber is a type of carbohydrate that the human body is unable to digest. While many carbohydrates are broken down into glucose for energy, this category of tough complex carbs the body cannot digest passes through the digestive system largely intact, providing significant health benefits.

Quick Summary

Certain complex carbohydrates, like dietary fiber and resistant starch, are not broken down by the human digestive system due to a lack of specific enzymes. Instead, they travel to the large intestine, where they provide numerous benefits for gut health and overall well-being, acting as prebiotics and bulking agents.

Key Points

Dietary Fiber and Resistant Starch: These are the main types of tough complex carbs the body cannot digest, unlike simple sugars and digestible starches.
Two Kinds of Fiber: Fiber comes in two forms: soluble, which dissolves in water and regulates blood sugar, and insoluble, which adds bulk to stool and prevents constipation.
Prebiotic Role: When indigestible carbohydrates reach the large intestine, they are fermented by gut bacteria, producing beneficial short-chain fatty acids.
Resistant Starch Sources: Resistant starch is found in green bananas, legumes, and cooked and cooled starches like potatoes and rice.
Essential for Health: These indigestible carbs are vital for gut health, blood sugar control, heart health, and weight management, despite not providing calories directly.
Incorporating into Diet: Gradually increasing intake of diverse plant-based foods, along with sufficient hydration, helps maximize the benefits of indigestible carbs.

Understanding Indigestible Carbohydrates

Indigestible carbohydrates are a vital component of a healthy diet, playing a crucial role in digestive health, blood sugar regulation, and overall wellness. Unlike simple carbohydrates (sugars) and most starches, which are broken down into glucose and absorbed in the small intestine, these tough carbohydrates, primarily known as dietary fiber and resistant starch, proceed to the large intestine without being digested. There, they are fermented by beneficial gut bacteria, leading to the production of short-chain fatty acids (SCFAs) that nourish the colon and provide various systemic health benefits.

Dietary Fiber: The Indigestible Structural Component

Dietary fiber is a broad term for plant-based carbohydrates that our bodies cannot digest. It is primarily made of the structural parts of plants, such as cellulose in cell walls. There are two main types of dietary fiber, each with distinct properties and functions.

Soluble Fiber

Soluble fiber dissolves in water to form a gel-like material in the digestive tract. This gel helps to slow down digestion, which can assist in regulating blood sugar levels and lowering blood cholesterol. Sources include oats, barley, beans, lentils, apples, and carrots. Its functions include moderating glucose absorption and binding to bile acids.

Insoluble Fiber

Insoluble fiber does not dissolve in water and remains mostly unchanged as it moves through the digestive system. It provides bulk to the stool, promoting regularity and helping to prevent constipation. Sources include whole wheat products, nuts, seeds, beans, leafy greens, and fruit and vegetable skins. It increases stool bulk and may lower the risk of diverticular disease and colorectal cancer.

List of Common High-Fiber Foods

Legumes: Lentils, chickpeas, black beans, and kidney beans are excellent sources.
Whole Grains: Oats, barley, quinoa, and brown rice provide significant amounts.
Vegetables: Leafy greens, broccoli, Brussels sprouts, carrots, and avocados are high in fiber.
Fruits: Berries, apples, pears, and bananas are great sources.
Nuts and Seeds: Flaxseeds, chia seeds, almonds, and walnuts contribute significantly.

Resistant Starch: The Third Type of Dietary Fiber

Resistant starch (RS) is another type of tough complex carb the body cannot digest. It resists digestion in the small intestine and functions like a fermentable fiber in the large intestine, acting as a prebiotic that feeds beneficial gut bacteria.

Types of Resistant Starch

There are four main types of resistant starch:

RS1 (Physically Inaccessible): Found in seeds, legumes, and unprocessed whole grains.
RS2 (Native Starch): Found in raw potatoes and unripe (green) bananas.
RS3 (Retrograded Starch): Forms when starchy foods are cooked and then cooled.
RS4 (Chemically Modified): Artificially created and not found naturally.

Benefits of Indigestible Carbs

Gut Health: Fermentation produces short-chain fatty acids like butyrate, which fuels colon cells.
Blood Sugar Control: Soluble fiber and resistant starch slow digestion and glucose absorption.
Weight Management: They add bulk, increasing fullness and potentially reducing calorie intake.
Heart Health: Soluble fiber can help lower LDL cholesterol.

Comparison Table: Soluble vs. Insoluble Fiber and Resistant Starch


Feature	Soluble Fiber	Insoluble Fiber	Resistant Starch
Effect in Water	Dissolves to form a gel	Does not dissolve	Can be soluble or insoluble
Digestive Pathway	Fermented in the large intestine	Passes through mostly intact	Fermented in the large intestine
Main Function	Slows digestion, regulates blood sugar & cholesterol	Adds bulk, promotes regularity	Acts as a prebiotic, feeds gut bacteria
Satiety Effect	Promotes feelings of fullness	Provides a bulking sensation	Increases satiety hormones
Primary Sources	Oats, barley, nuts, seeds, apples, citrus fruits	Whole wheat, wheat bran, nuts, green beans, potatoes	Green bananas, legumes, cooked & cooled starches
Effect on Stool	Can soften stool	Adds bulk and speeds passage	Adds bulk

Incorporating Indigestible Carbs into Your Diet

Aim for 25 to 35 grams daily. Increase intake slowly to avoid discomfort and drink plenty of water. Eating a variety of plant-based foods ensures a good mix of fiber and resistant starch.

Conclusion

Tough complex carbs like dietary fiber and resistant starch are vital for health. They support gut bacteria, aid blood sugar control, help with weight management, and benefit heart health. Increase your intake by eating diverse plant foods.

For more in-depth information on dietary fiber and its health benefits, visit Harvard's T.H. Chan School of Public Health resource: Fiber: The Nutrition Source.

Frequently Asked Questions

These carbs pass through the stomach and small intestine undigested due to a lack of specific enzymes. They then travel to the large intestine where they are fermented by beneficial gut bacteria.

Yes, the two main types are dietary fiber and resistant starch. Dietary fiber includes both soluble and insoluble types, which have different effects on the digestive system.

Excellent sources include legumes, whole grains, nuts, seeds, fruits, and vegetables. Specifically, green bananas, cooked and cooled potatoes, and lentils are high in resistant starch.

Yes, they can. Indigestible carbohydrates like fiber add bulk to food and promote feelings of fullness, which can reduce appetite and overall calorie intake.

Soluble fiber dissolves in water to form a gel and helps lower cholesterol and regulate blood sugar. Insoluble fiber does not dissolve and adds bulk to stool to prevent constipation.

While the body does not absorb them for energy, some fermentation by gut bacteria does produce small amounts of short-chain fatty acids that can be used for energy. However, the caloric contribution is minimal compared to digestible carbs.

Resistant starch acts as a prebiotic, serving as food for beneficial gut bacteria. The fermentation of resistant starch produces short-chain fatty acids like butyrate, which is crucial for colon health.

Increase your intake gradually to allow your digestive system to adjust, and make sure to drink plenty of water. Abruptly adding a large amount can cause gas, bloating, and other digestive discomforts.