What Cannot Be Digested by the Human Body?

November 15, 2025 •

4 min read

Over 65% of the world's population has a reduced ability to digest lactose found in milk. While most nutrients are absorbed, many common substances and food components actually cannot be digested by the human body and instead pass through the digestive tract largely intact. Understanding these indigestible elements is key to comprehending your digestive health.

Quick Summary

An overview of common foods and substances the human body cannot digest, including dietary fiber, cellulose, certain dairy products, and artificial sweeteners. It explains why these items remain undigested and their effects on the digestive system.

Key Points

Dietary Fiber Is Indigestible: The human body lacks the enzymes to break down dietary fiber, which passes through largely intact but is essential for gut health.
Cellulose From Plants is Unbreakable: The tough outer layers of vegetables like corn kernels are made of cellulose, which humans cannot digest, though the nutrients inside are absorbed.
Artificial Sweeteners Can Cause Bloating: Some sugar alcohols like sorbitol and mannitol are poorly absorbed, leading to fermentation by gut bacteria and causing gas and bloating.
Lactose Affects Many People: A large portion of the global population cannot properly digest lactose due to a lack of the lactase enzyme, resulting in digestive discomfort.
Some Seeds and Skins are Tough: The fibrous skins of bell peppers and certain seeds can resist digestion, sometimes leading to undigested pieces appearing in stool.
Indigestible Components Aid Digestion: Despite not being absorbed for energy, substances like fiber are vital for promoting regular bowel movements and feeding beneficial gut bacteria.
Proper Chewing is Crucial: Chewing food thoroughly, especially fibrous items, helps break it down mechanically and mixes it with saliva to begin the digestive process properly.

The Primary Indigestible: Dietary Fiber

Dietary fiber is perhaps the most well-known substance that the human body cannot fully digest. Unlike other carbohydrates that are broken down into glucose, the human digestive system lacks the necessary enzymes to process fiber completely. This indigestible nature is actually a core reason for fiber's health benefits. There are two main types of fiber: soluble and insoluble.

Soluble Fiber dissolves in water to form a gel-like substance in the stomach, which helps slow down digestion. This can help regulate blood sugar levels and lower cholesterol. Good sources include oats, beans, apples, and carrots.

Insoluble Fiber does not dissolve in water and instead adds bulk to stool, which promotes healthy and regular bowel movements. It helps move material through the digestive system more quickly. It can be found in whole-wheat flour, nuts, and many vegetables.

Why We Cannot Digest Cellulose and Other Plant Structures

Cellulose is a type of insoluble fiber and a major component of plant cell walls. This complex carbohydrate is what gives plants their structure. Humans do not have the enzyme cellulase needed to break down cellulose, which is why eating corn often results in kernels appearing intact in stool. While the body digests the nutrients within the corn kernel, the tough outer skin of cellulose remains unaffected.

Another plant component that is largely indigestible is lignin. Found in the tough, woody parts of plants, lignin is an even more resilient polymer than cellulose and passes through the digestive tract almost completely unaltered.

Indigestible Components and Digestive Sensitivity

Beyond fiber, other substances and compounds can also pass through the digestive system without being fully processed, sometimes causing issues for sensitive individuals.

Artificial Sweeteners: Sugar alcohols like sorbitol and mannitol are not fully absorbed in the small intestine. When they reach the large intestine, they are fermented by gut bacteria, which can cause bloating, gas, and diarrhea, especially when consumed in large amounts.
Lactose (for the intolerant): Lactose, the sugar in milk, requires the enzyme lactase to be broken down. Individuals with lactose intolerance do not produce enough lactase, so the lactose remains undigested, leading to discomfort, bloating, and gas when it is fermented by gut bacteria in the colon.
Certain Seeds and Skins: The tough outer coats of some seeds (like those on berries and some nuts) and vegetables (like bell pepper skin) can be difficult to break down. Soaking seeds can sometimes help remove compounds like phytic acid that interfere with digestion, but for many, these parts remain indigestible.

Comparison of Digestible vs. Indigestible Components

To better understand what cannot be digested, let's compare some common food components.


Component	Source	Digestible?	Why/Why Not	Role in Digestion
Dietary Fiber (Cellulose)	Plant cell walls, corn	No	Humans lack the enzyme cellulase.	Adds bulk to stool, aids in waste removal.
Dietary Fiber (Pectin)	Apples, oats, beans	Partially (soluble)	Dissolves in water but is not absorbed.	Slows digestion, manages blood sugar/cholesterol.
Lactose	Dairy products	No (for intolerant)	Insufficient production of the enzyme lactase.	For those with sufficient lactase, provides energy.
Fats	Meat, oils, butter	Yes	Broken down by bile and enzymes (lipases).	Energy storage, nutrient absorption.
Protein	Meat, legumes, nuts	Yes	Broken down by stomach acid and enzymes (pepsin).	Cellular repair and growth.
Starches	Potatoes, bread	Yes	Broken down into glucose by amylase.	Primary source of energy.

Understanding the Implications

Knowing what cannot be digested helps us make informed dietary choices and understand our bodies better. For instance, the reason fiber is so important for gut health is precisely because it's indigestible. It provides food for the beneficial bacteria in the large intestine and keeps the digestive system moving properly. However, consuming too much of these tough-to-digest components, especially high-fiber foods, can cause discomfort, bloating, or gas if the digestive system isn't used to it. The key is to increase intake gradually and ensure proper hydration.

Moreover, conditions like lactose intolerance illustrate how individual biological differences can affect what is and isn't digestible. For those with a sensitivity to lactose or certain carbohydrates, avoiding or limiting these foods can significantly improve digestive comfort and overall well-being. It is important to remember that not all discomfort after eating is a sign of an issue; some of it is a normal part of the digestive process involving indigestible components. An excellent resource for more information on the digestive process is the NCBI Bookshelf's Physiology, Digestion entry.

Conclusion

While it might seem counterintuitive, substances that cannot be digested play a crucial role in maintaining a healthy digestive system. From the insoluble fiber that provides bulk to the soluble fiber that helps regulate blood sugar, these components are far from useless. Factors like individual enzyme production, as seen with lactose intolerance, can also change what the body is able to process. By understanding what cannot be digested, you gain a deeper appreciation for the complex workings of your own body and how to optimize your diet for better health. If you consistently experience symptoms of poor digestion, consulting a healthcare professional is always the best course of action.

Frequently Asked Questions

Humans lack the specific enzymes required to break down dietary fiber, which is a type of carbohydrate found in plants. Instead of being absorbed, it passes through the digestive tract and aids in bowel regularity.

Some people are lactose intolerant, meaning they do not produce enough of the enzyme lactase needed to break down lactose, the sugar found in dairy products. This results in the lactose fermenting in the gut and causing symptoms like gas and bloating.

Yes, it is often normal to see small bits of undigested food in stool, especially items with a fibrous outer layer like corn or seeds. This is typically not a cause for concern unless accompanied by other symptoms.

Many artificial sweeteners, particularly sugar alcohols, are not fully absorbed by the body. They travel to the large intestine where they are fermented by bacteria, potentially causing digestive issues.

Cellulose is a complex carbohydrate that forms the structural part of plant cell walls. Humans lack the enzyme cellulase to break it down, so it passes through the system unchanged and is a major component of insoluble fiber.

Yes, cooking can help soften and break down the fibrous components of some foods, such as cruciferous vegetables, making them easier on the digestive system for many people.

No, it is perfectly normal to see corn kernels in your stool, as the body cannot digest the tough outer cellulose layer. The nutrients inside are still absorbed, but the casing remains intact.