What is a plant substance that your body cannot digest? Dietary fiber explained

December 11, 2025 •

4 min read

According to studies, most people in the United States consume only about half of the recommended daily fiber intake, which is a plant substance that your body cannot digest. This indigestible material, known as dietary fiber, passes through your system largely intact, playing a vital role in digestive and overall health. Understanding why your body cannot break it down reveals the secret to its many benefits.

Quick Summary

Dietary fiber is a plant-based carbohydrate that passes through the body undigested due to a lack of specific human enzymes. It comes in soluble and insoluble forms, both of which are crucial for maintaining digestive regularity and supporting the gut microbiome. Consuming adequate fiber helps regulate blood sugar, lower cholesterol, and aid in weight management.

Key Points

Cellulose and Fiber are Indigestible: The body cannot digest complex plant carbohydrates like cellulose because it lacks the specific enzymes needed to break their beta-glycosidic bonds.
Soluble vs. Insoluble Fiber: Fiber is categorized into two types: soluble, which forms a gel and regulates blood sugar, and insoluble, which adds bulk to stool and promotes regularity.
Essential for Digestive Health: Indigestible fiber is crucial for a healthy digestive system, preventing constipation and lowering the risk of conditions like diverticular disease and hemorrhoids.
Supports Gut Microbiome: While we can't digest fiber, our gut bacteria can ferment it, producing beneficial short-chain fatty acids that nourish the colon lining and reduce inflammation.
Boosts Overall Health: A high-fiber diet is linked to lower cholesterol, better blood sugar control, and weight management, reducing the risk of chronic diseases like heart disease and type 2 diabetes.
Most People Are Fiber Deficient: The majority of people, particularly in Western countries, do not consume the recommended daily amount of fiber, often due to a diet high in processed foods.

Why Humans Cannot Digest Fiber

Dietary fiber, which includes complex carbohydrates like cellulose, is a plant substance that your body cannot digest because humans lack the necessary enzymes. Our digestive enzymes are designed to break down starches and sugars, which are linked by alpha-glycosidic bonds. However, fiber's glucose units are linked by beta-glycosidic bonds, which the human body cannot break. This inability to break down fiber is the very reason it offers such significant health benefits, as it travels through the digestive tract mostly intact.

The Two Primary Types of Dietary Fiber

Dietary fiber is broadly categorized into two types, based on how they interact with water in the digestive system. Both are essential for a healthy diet and work together to support digestive function.

Soluble Fiber: This type of fiber dissolves in water to form a gel-like substance in the stomach. This gel helps slow down digestion, which aids in blood sugar regulation and prolongs the feeling of fullness. As it moves into the large intestine, gut bacteria ferment it, producing beneficial short-chain fatty acids. Sources include oats, barley, nuts, seeds, apples, and many legumes.
Insoluble Fiber: Unlike soluble fiber, this type does not dissolve in water. It passes through the digestive tract largely unchanged, adding bulk to stool and promoting regular bowel movements. By speeding up the passage of food and waste, insoluble fiber helps prevent constipation and other digestive issues. Key sources include whole grains, wheat bran, and the skins of fruits and vegetables.

Health Benefits of a High-Fiber Diet

Even though our bodies cannot digest it, fiber is a critical component of a healthy diet. A high-fiber diet has been linked to numerous positive health outcomes.

Improved Digestive Health: By adding bulk and speeding up transit time, insoluble fiber promotes regularity and reduces the risk of constipation, hemorrhoids, and diverticular disease. Soluble fiber also helps regulate bowel movements, which can be beneficial for conditions like irritable bowel syndrome (IBS).
Lower Cholesterol Levels: Soluble fiber can help lower low-density lipoprotein (LDL) or "bad" cholesterol by binding to bile acids and removing them from the body, forcing the liver to use more cholesterol to produce new bile.
Blood Sugar Regulation: Soluble fiber's ability to slow down digestion can prevent sharp spikes in blood sugar after a meal, a crucial benefit for people with diabetes. Research also indicates a protective effect of fiber against developing type 2 diabetes.
Weight Management: Fiber-rich foods tend to be more filling and have a lower energy density, meaning they provide fewer calories for the same volume of food. This can help curb appetite and reduce overall calorie intake.
Enhanced Gut Microbiome: The gut's beneficial bacteria feed on fermentable fiber, producing short-chain fatty acids (SCFAs) like butyrate. These SCFAs nourish the colon lining, help maintain immune function, and reduce inflammation throughout the body.

The Role of the Gut Microbiome in Fiber Breakdown

While humans lack the enzymes to digest fiber, our gut microbiome—the complex ecosystem of bacteria in our large intestine—is perfectly equipped for the task. These microbes produce the enzymes necessary to ferment certain types of fiber, including some soluble fibers and resistant starch. This fermentation process is not just a form of recycling; it is a critical symbiotic relationship. The bacteria get their energy, and we benefit from the SCFAs they produce.

How Fiber Differs from Other Plant Carbohydrates

Not all plant carbohydrates are indigestible. Starches, for instance, are made of glucose units linked by alpha-glycosidic bonds, which our bodies can easily break down into energy. This is why starchy foods like potatoes and bread have calories. Fiber's beta-glycosidic bonds, however, make it impervious to our enzymes, a key structural difference that determines its nutritional role.

Comparison: Soluble vs. Insoluble Fiber


Feature	Soluble Fiber	Insoluble Fiber
Primary Function	Dissolves in water, forms a gel, slows digestion	Does not dissolve in water, adds bulk, speeds up transit
Effect on Stool	Softens stool, makes it easier to pass	Increases bulk and weight of stool
Mechanism	Fermented by gut bacteria in the large intestine.	Passes through largely intact, attracting water.
Key Health Benefits	Lowers cholesterol, regulates blood sugar, promotes satiety, nourishes gut bacteria.	Prevents constipation, aids regularity, lowers risk of diverticular disease.
Common Sources	Oats, peas, beans, apples (pulp), citrus fruits.	Whole grains, wheat bran, nuts, seeds, fruit skins.

The Consequences of Low Fiber Intake

With an estimated 95% of Americans not meeting their daily fiber requirements, the health implications are significant. A diet low in fiber is a known risk factor for several chronic diseases. It can lead to an increase in constipation, inflammation, and an imbalanced gut microbiome, which in turn elevates the risk for heart disease, type 2 diabetes, and certain cancers, including colorectal cancer. The reliance on processed foods that are stripped of their natural fiber contributes directly to this deficiency, highlighting the importance of consuming whole plant foods.

Conclusion: More Than Just 'Roughage'

In conclusion, the plant substance that your body cannot digest is dietary fiber, including complex carbohydrates like cellulose, hemicellulose, and pectin. This is not a weakness of our digestive system but a key to a healthier one. The inability to break it down allows fiber to perform vital functions, from regulating blood sugar and lowering cholesterol to promoting digestive regularity and fostering a healthy gut microbiome. By incorporating a variety of high-fiber plant foods—both soluble and insoluble—into your daily diet, you harness this indigestible material for significant, long-term health benefits. It is a powerful reminder that some of the most beneficial nutrients are the ones our bodies can't absorb. For more information on dietary fiber and its role in health, you can consult reliable resources like the Harvard T.H. Chan School of Public Health's guide to fiber.

Frequently Asked Questions

The human body cannot digest fiber because it does not produce the necessary digestive enzymes, such as cellulase, that are capable of breaking the beta-glycosidic bonds in cellulose and other plant fibers.

Indigestible fiber passes through the stomach and small intestine largely intact. In the large intestine, some types of soluble fiber are fermented by gut bacteria, while insoluble fiber adds bulk to stool and is eventually expelled from the body.

Yes, dietary fiber is very good for you. Its indigestibility is the source of many of its health benefits, including promoting digestive health, lowering cholesterol, regulating blood sugar, and feeding beneficial gut bacteria.

Soluble fiber dissolves in water to form a gel, slowing digestion and helping lower cholesterol and blood sugar. Insoluble fiber does not dissolve and adds bulk to stool, promoting regular bowel movements.

Good sources include fruits, vegetables, whole grains (like oats and brown rice), legumes (beans, peas, and lentils), nuts, and seeds. Eating a variety of these foods ensures you get both soluble and insoluble fiber.

While fiber supplements can be helpful, it is generally recommended to get fiber from whole food sources. Supplements do not provide the same range of vitamins, minerals, and other beneficial compounds found in high-fiber foods.

A low-fiber diet can increase the risk of digestive issues like constipation, hemorrhoids, and diverticular disease. It is also a risk factor for chronic conditions such as heart disease and type 2 diabetes.