What Does Our Body Not Digest? Understanding Indigestible Foods

October 28, 2025 •

4 min read

While our digestive system is remarkably efficient at breaking down many foods, humans cannot produce the necessary enzymes to break down cellulose, a complex carbohydrate and a major component of plant cell walls. This biological limitation directly influences what does our body not digest, primarily impacting how we process plant-based foods.

Quick Summary

The human body cannot fully digest certain complex carbohydrates and tough food components, like dietary fiber and seeds, due to a lack of specific enzymes. These indigestible parts pass through the digestive tract largely intact, playing a vital and beneficial role in gut health.

Key Points

Dietary Fiber Is Indigestible: The body cannot break down fiber, such as cellulose, because it lacks the necessary enzymes.
Undigested Food Is Often Normal: Seeing fragments of high-fiber foods like corn, seeds, and nuts in stool is common and not usually a cause for concern.
Fiber is Beneficial for Gut Health: Despite not being digested for energy, fiber is crucial for regulating bowel movements, adding bulk to stool, and supporting a healthy gut microbiome.
Chew Your Food Thoroughly: Incomplete chewing can result in larger pieces of food passing through the system undigested, so proper chewing is important.
Watch for Accompanying Symptoms: If undigested food is combined with chronic diarrhea, weight loss, or pain, it may indicate a deeper digestive issue and a doctor should be consulted.
Some Proteins and Sweeteners Can Be Indigestible: Certain food components like lactose (in intolerant individuals) and artificial sweeteners like sorbitol are also poorly absorbed and can cause digestive issues.
Cooking Can Aid Digestion: For some tough-skinned vegetables, cooking can soften the fibrous parts, allowing for easier breakdown and nutrient release.

The Science Behind Undigested Food

Digestion is a complex process involving mechanical and chemical breakdown, transforming food into smaller components that the body can absorb for energy, growth, and repair. This journey begins in the mouth with chewing and ends with waste elimination. However, not everything we consume is fully broken down. For many, seeing fragments of food in their stool can cause concern, but it is often a normal occurrence, particularly when certain types of food are involved. The primary reason for this is a lack of the specific enzymes needed to break down certain molecular bonds.

The Indigestible Giant: Dietary Fiber

The most prominent example of an indigestible food component is dietary fiber. Found in fruits, vegetables, whole grains, and legumes, fiber includes the parts of plant foods that the body cannot digest or absorb. Instead of being broken down into sugar molecules like other carbohydrates, fiber passes through the stomach, small intestine, and colon relatively intact.

There are two main types of fiber, and both are indigestible by human enzymes:

Insoluble Fiber: Often referred to as "roughage," this fiber does not dissolve in water. It adds bulk to stool and helps speed up the passage of food and waste through the digestive system. Examples include wheat bran, vegetables, and the skins of many fruits.
Soluble Fiber: This type of fiber dissolves in water to form a gel-like material. It helps slow down digestion, which can assist in controlling blood sugar and lowering cholesterol levels. Sources include oats, peas, beans, and apples.

Despite being indigestible, fiber is far from useless. It is crucial for a healthy digestive system, aiding bowel regularity, lowering cholesterol, and supporting a healthy gut microbiome.

Why We Can't Digest Cellulose

Cellulose is a type of insoluble fiber and is the most abundant organic polymer on Earth, forming the structural component of plant cell walls. Humans cannot digest it because we do not produce the enzyme cellulase, which is required to break the beta-glycosidic bonds in the cellulose molecule. This is in contrast to herbivores like cows, which have symbiotic bacteria in their digestive tracts that produce this enzyme for them.

Other Common Indigestible Culprits

Besides fiber, several other food items and components are known to pass through the digestive system without being fully broken down:

Seeds and Nuts: Many seeds, like sesame and chia, have tough outer coats containing phytic acid or cellulose that are difficult for the body to break down, especially if not chewed thoroughly.
Corn: The kernel of a corn cob is protected by an outer layer of cellulose, which our bodies cannot digest. While we do absorb the starchy nutrients from the inside of the kernel, the shell often appears intact in stool.
Cruciferous Vegetables: Broccoli, cabbage, and kale contain a complex carbohydrate called raffinose, which human enzymes cannot break down. Gut bacteria ferment raffinose in the colon, producing gas and causing bloating.
Artificial Sweeteners: Sugar alcohols like sorbitol and mannitol are not fully absorbed by the small intestine and can cause digestive issues like bloating and diarrhea when fermented by gut bacteria in the large intestine.
Lactose (in lactose intolerance): For individuals with lactose intolerance, a lack of the enzyme lactase means they cannot break down the sugar in dairy products. This leads to symptoms like bloating, gas, and cramps.

The Role of Chewing and Transit Time

The degree to which food is digested also depends on how it's prepared and consumed. Chewing food thoroughly is the first step of digestion, and eating too quickly can lead to larger chunks of food entering the stomach, making them harder to break down. The transit time of food through the digestive tract also plays a role. If food passes through too quickly due to factors like diarrhea, stress, or certain medical conditions, it can appear undigested in stool.

Indigestible vs. Digestible Carbohydrates


Feature	Indigestible Carbohydrates (Fiber)	Digestible Carbohydrates (Starch)
Molecular Structure	Long, complex chains with strong beta-glycosidic bonds.	Long or short chains with alpha-glycosidic bonds.
Breakdown	Resists breakdown by human digestive enzymes.	Easily broken down by enzymes like amylase.
Enzyme Required	Requires cellulase (which humans lack).	Uses amylase, produced in the mouth and pancreas.
Energy Yield	Provides no direct calories or energy to the body.	Provides glucose for energy.
Passage	Passes through digestive tract relatively intact.	Absorbed in the small intestine.
Effect on Bowels	Adds bulk and promotes regularity.	Does not contribute to stool bulk directly.

When to Be Concerned About Undigested Food

While seeing some undigested food, especially fiber, is normal, it can occasionally signal an underlying health issue. If undigested food is accompanied by persistent diarrhea, significant weight loss, or changes in bowel habits, it may indicate a malabsorption problem. Conditions like inflammatory bowel disease (IBD), celiac disease, or pancreatic insufficiency can affect the body's ability to properly break down and absorb nutrients. If you experience these accompanying symptoms, it is important to consult a healthcare provider.

Conclusion

Understanding what does our body not digest is key to appreciating the complex and efficient nature of our digestive system. Indigestible components like dietary fiber, cellulose, and some seeds are not simply waste; they are essential for regulating bowel movements, feeding beneficial gut bacteria, and managing aspects of metabolic health. By embracing these indigestible elements through a balanced diet rich in fruits, vegetables, and whole grains, we support our gut health and overall well-being, even as they pass through our system without being absorbed.

For more information on dietary fiber, consult resources like the Mayo Clinic's detailed guide.

Frequently Asked Questions

Yes, it is very common and normal to see corn kernels, seeds, or nuts in your stool. This is because their tough outer shells are made of cellulose, which the human body cannot digest. As long as you don’t have other symptoms like diarrhea or weight loss, there is no need for concern.

Humans cannot digest cellulose because our bodies do not produce the enzyme called cellulase. This enzyme is required to break the chemical bonds in the cellulose molecule. Animals like cows have specific gut bacteria that produce this enzyme for them.

Not necessarily. In the case of high-fiber foods like corn, your body digests the nutrients inside the kernel, even if the outer cellulose shell remains intact. However, if undigested food appears with symptoms like chronic diarrhea, it might indicate a malabsorption issue.

Yes, chewing your food thoroughly is the first and a very important step in digestion. Breaking down food into smaller pieces makes it easier for your stomach and enzymes to do their work, leading to more complete digestion and nutrient absorption.

Indigestible fiber plays a crucial role in a healthy diet. It adds bulk to stool to promote regular bowel movements, acts as a prebiotic to feed beneficial gut bacteria, and can help control blood sugar and cholesterol levels.

Some sugar alcohols, a type of artificial sweetener, are not fully absorbed in the small intestine. They can be fermented by bacteria in the large intestine, which may cause gas, bloating, and diarrhea.

You should see a doctor if seeing undigested food is accompanied by other symptoms such as persistent diarrhea, unexplained weight loss, or changes in your bowel habits, as this could signal an underlying medical condition.