What part of food Cannot be digested by the body?

November 29, 2025 •

3 min read

Over 95% of ingested food is typically digested and absorbed by the body, but a specific component passes through almost entirely intact. This is the part of food that cannot be digested by the body, known as dietary fiber, which plays a critical role in our health despite not being a source of calories or nutrients.

Quick Summary

This guide explains the indigestible component of food, primarily dietary fiber, including its types, sources, and important functions in the digestive system. It covers why the body cannot break down fiber and its positive health effects.

Key Points

Dietary Fiber is Indigestible: The body cannot break down dietary fiber because it lacks the necessary enzymes, causing it to pass through the digestive system largely intact.
Two Types of Fiber: Soluble fiber forms a gel and helps regulate blood sugar, while insoluble fiber adds bulk and promotes regularity.
Corn Kernels and Seeds: Many undigested food fragments, like the outer skin of corn kernels and some seeds, are high in cellulose and insoluble fiber, which the body cannot break down.
Benefits of Indigestible Food: Fiber's indigestible nature is beneficial, aiding bowel movements, lowering cholesterol, and feeding good gut bacteria.
Healthy Digestion Requires Fiber: Consuming adequate fiber from whole grains, fruits, and vegetables is crucial for a well-functioning digestive system.

The Indigestible Role of Dietary Fiber

Dietary fiber, sometimes referred to as roughage or bulk, is the component of plant-derived foods that the human body cannot fully digest or absorb. Unlike carbohydrates, proteins, and fats, which are broken down by our digestive enzymes into absorbable nutrients, fiber moves through the stomach, small intestine, and colon largely intact. The reason our bodies cannot digest fiber is that we lack the specific enzymes required to break down complex carbohydrates like cellulose, a primary component of plant cell walls. Despite being indigestible, fiber is a vital part of a healthy diet, playing a significant role in digestive regularity and overall health.

Soluble vs. Insoluble Fiber: A Comparison

Fiber is not a single substance but is categorized into two main types based on its solubility in water. Both soluble and insoluble fiber are crucial for health, and most plant foods contain a combination of both.


Feature	Soluble Fiber	Insoluble Fiber
Water Solubility	Dissolves in water.	Does not dissolve in water.
Gastrointestinal Effect	Forms a gel-like substance, slowing digestion.	Adds bulk to stool, promoting regular bowel movements.
Health Benefits	Lowers cholesterol, stabilizes blood sugar levels.	Prevents constipation and promotes regularity.
Common Sources	Oats, peas, beans, apples, carrots, citrus fruits.	Whole-wheat flour, wheat bran, nuts, beans, leafy greens.

The Health Benefits of Indigestible Fiber

While fiber doesn't provide direct nutrients like other food components, its indigestible nature is precisely what makes it so valuable. For instance, insoluble fiber acts like a street sweeper for the intestines, helping to push waste through the digestive tract and preventing constipation. This process is critical for maintaining colon health. Soluble fiber, on the other hand, forms a gel that helps to lower blood cholesterol by binding to cholesterol particles and removing them from the body. This gel also helps slow the absorption of sugar, which can help control blood sugar levels in people with diabetes.

Why Corn Kernels Appear Undigested

One of the most common examples of undigested food is corn. While a corn kernel may appear to pass through the body unchanged, a portion of it is actually digested. The inner, starchy part of the kernel contains digestible nutrients, but its outer skin is made of cellulose, a type of insoluble fiber. Since humans lack the enzymes to break down cellulose, the outer casing remains intact and is expelled in the stool. This is a perfect illustration of how some parts of food cannot be digested by the body.

Other Hard-to-Digest Items

Beyond just corn, certain foods are known to pass through the digestive system relatively unmolested. Seeds, nuts, and skins from certain fruits and vegetables often fall into this category due to their high fiber and cellulose content. Some people may also notice undigested fragments of fibrous vegetables, like leafy greens, in their stool. However, this is not a cause for concern and is a normal part of how the digestive system processes fiber.

The Importance of Good Gut Bacteria

Though human enzymes cannot digest fiber, some of the bacteria in our large intestine can. These beneficial bacteria ferment certain types of fiber, producing short-chain fatty acids (SCFAs) that are beneficial for colon health and can be absorbed by the body. This process further demonstrates the importance of fiber not just for bowel function, but for nurturing a healthy gut microbiome, which has wide-ranging implications for overall health.

Conclusion

The parts of food that cannot be digested by the body, primarily dietary fiber, are far from useless. Instead, these components, which pass through the digestive system undigested, are vital for maintaining digestive health, controlling cholesterol and blood sugar, and feeding the beneficial bacteria in our gut. Incorporating a variety of fiber-rich foods like whole grains, fruits, vegetables, and legumes is one of the most effective ways to support a healthy digestive system and benefit from the indispensable role of this indigestible but essential food component.

Learn more about the benefits of dietary fiber from a trusted health resource.

Frequently Asked Questions

Dietary fiber is indigestible because the human digestive system lacks the enzymes, such as cellulase, needed to break down the complex carbohydrate structures found in plant cell walls.

Soluble fiber dissolves in water to form a gel-like substance that slows digestion, while insoluble fiber does not dissolve and adds bulk to stool, helping food pass through the digestive tract.

No, it is normal to see small pieces of undigested, high-fiber foods like corn or seeds in stool. It does not typically indicate a health problem, as these foods are simply passing through the system as intended.

No, many animals like cows and other herbivores have specialized digestive systems and symbiotic bacteria that can break down cellulose and other plant fibers that humans cannot digest.

Indigestible food, or fiber, passes through the stomach and small intestine largely intact, enters the large intestine, and is then expelled from the body. Some fermentable fibers are processed by gut bacteria in the colon.

Yes, eating indigestible fiber is highly beneficial. It helps prevent constipation, lowers cholesterol, controls blood sugar levels, and promotes a healthy gut microbiome by feeding beneficial bacteria.

The outer skin of a corn kernel is made of cellulose, a fiber that humans cannot digest. While the starchy inside is digested, the tough outer layer remains intact and is visible in stool.