Understanding What Can't the Human Body Digest and Why

October 15, 2025 •

5 min read

Over 90% of the nutrients we consume are absorbed in the small intestine, yet some food components, like dietary fiber, pass through largely intact. Understanding what can't the human body digest is key to appreciating the complex nutritional landscape of our daily meals and the vital role of our gut microbiota.

Quick Summary

Several food components and substances, including certain fibers, complex sugars, and artificial ingredients, cannot be broken down by human digestive enzymes. Instead, they travel to the large intestine where they are fermented by gut bacteria, contributing to gut health or, in some cases, causing digestive discomfort.

Key Points

Enzyme Deficiency Causes Indigestion: The human body cannot digest certain food components like cellulose because it lacks the specific enzymes (e.g., cellulase and alpha-galactosidase) required to break down their molecular bonds.
Fiber is Not Digested by Humans: Dietary fiber, both soluble and insoluble, is a type of carbohydrate that passes through the body largely undigested and is a prime example of an indigestible food component.
Gut Microbiota Breaks Down Undigestible Food: Our trillions of gut bacteria possess the enzymes to ferment indigestible materials, producing beneficial short-chain fatty acids (SCFAs) that are crucial for gut health.
Indigestible Foods Offer Health Benefits: These undigested materials, such as prebiotics and fiber, feed the healthy bacteria in our gut, contributing to a diverse microbiome, improved immunity, and reduced inflammation.
Distinguish from Food Intolerance: True indigestibility (like cellulose) differs from food intolerance (like lactose), which is an inability to process a specific food component due to an enzyme deficiency, causing gastrointestinal distress.
Certain Additives and Sugars are Undigestible: Artificial sweeteners like sorbitol and some viscous gums are not fully absorbed, potentially leading to digestive issues when fermented by gut bacteria.
Common Undigested Food Items: It is normal to see remnants of high-fiber foods like corn, nuts, and seeds in stool, as our bodies cannot break down their tough outer casings or structures.

The Core Reason: A Lack of Specific Enzymes

The inability of the human body to digest certain substances primarily stems from a lack of the necessary digestive enzymes. While we have a robust set of enzymes to break down proteins, fats, and most carbohydrates, some molecular bonds in specific compounds are simply resistant to our enzymatic arsenal. This is a crucial distinction between humans and herbivores like cows, who harbor specialized bacteria in their stomachs to produce the enzymes needed to break down tough plant matter.

Cellulose: The Ubiquitous Plant Fiber

One of the most prominent examples of an indigestible substance is cellulose, a type of insoluble fiber that forms the structural component of plant cell walls. This is why eating fibrous foods like celery, corn kernels, and leafy greens often results in seeing partially digested pieces in stool. Our bodies lack the enzyme cellulase, which is required to cleave the beta-glycosidic linkages that hold cellulose together.

Raffinose-Family Oligosaccharides (RFOs)

Another class of carbohydrates that humans struggle with are raffinose-family oligosaccharides (RFOs). These are found abundantly in legumes (beans, lentils) and certain vegetables like Brussels sprouts and cabbage. The human digestive system does not produce the enzyme alpha-galactosidase, which is needed to break down RFOs. When these sugars reach the large intestine, they are fermented by gut bacteria, producing gases like carbon dioxide and hydrogen, which can lead to bloating and flatulence.

Undigestible Artificial and Additive Ingredients

Beyond natural plant matter, several artificial and processed ingredients are also indigestible or poorly absorbed by the human body.

Artificial Sweeteners: Some non-nutritive sweeteners, such as sorbitol and mannitol, are not fully absorbed in the small intestine. They travel to the large intestine where they can be fermented, potentially causing gas, bloating, and diarrhea.
Stabilizing Gums: Ingredients like guar gum and xanthan gum are often added to processed foods for thickening and stabilizing. These are types of polysaccharides that our bodies do not have the enzymes to break down.

The Paradox: Benefits of Indigestible Foods

While some compounds are indigestible by us, they are essential for the health of our gut microbiome. This ecosystem of trillions of microorganisms plays a crucial role in our overall well-being. The indigestible carbohydrates, especially dietary fiber, serve as a food source for these beneficial bacteria, acting as prebiotics.

Fermentation and Short-Chain Fatty Acids (SCFAs)

When gut bacteria ferment dietary fiber and other undigested materials, they produce beneficial compounds called short-chain fatty acids (SCFAs), such as butyrate. SCFAs are a primary energy source for the cells lining the colon, helping to maintain a healthy gut barrier and reduce inflammation.

Indigestible vs. Hard-to-Digest vs. Food Intolerance

It is important to distinguish between substances that are completely indigestible due to lacking enzymes, those that are simply harder to digest, and a food intolerance.


Feature	Indigestible by nature	Hard-to-Digest	Food Intolerance
Mechanism	The body lacks the specific enzyme to break down a molecular bond, e.g., cellulase for cellulose.	Caused by high fat content, certain proteins, or rapid consumption that overwhelm the digestive system.	An inability to digest or absorb certain foods, often due to an enzyme deficiency, e.g., lactase in lactose intolerance.
The Trigger	Natural plant compounds like cellulose, lignin, and some oligosaccharides.	Fried foods, highly processed items, and large quantities of raw vegetables or fats.	A specific component like lactose (dairy) or gluten (in celiac disease).
Symptoms	Often mild, like undigested pieces in stool; can cause gas or bloating if fermented.	Heartburn, stomach pain, or slower gastric emptying.	Gastrointestinal distress (gas, bloating, diarrhea, cramping), but not a life-threatening immune response.
Management	No need for concern, as fiber is beneficial. Manage fermentation symptoms with diet changes.	Reduce portion size, cook food thoroughly, or chew more slowly.	Avoidance or moderation of the specific food; some can use enzyme supplements.

The Role of Gut Microbiota

Our gut bacteria are the true experts at digesting the food we can't. They possess a diverse array of enzymes that can break down complex carbohydrates like dietary fiber, resistant starches, and prebiotics. This fermentation process is not just a side effect but a cornerstone of a healthy digestive system. Different types of fiber and other undigested components promote the growth of different beneficial bacteria, contributing to a diverse and robust gut ecosystem. The health of this microbial community has far-reaching effects on our immune function, metabolism, and even mental health. For example, the fermentation of certain prebiotics can produce compounds that influence metabolic regulation and reduce inflammation.

Conclusion

The things our body can't digest, such as dietary fiber, complex sugars like raffinose, and certain artificial additives, are not always a negative. The indigestible components play a critical, symbiotic role with our gut microbiome, feeding beneficial bacteria and leading to the production of healthful compounds like SCFAs. While a food intolerance or simply eating hard-to-digest foods can cause discomfort, the presence of undigested components from healthy plant foods is a normal part of the digestive process and a sign of a high-fiber diet. By focusing on a diet rich in a variety of plant-based foods, we can support a diverse and thriving internal ecosystem that contributes to our overall health and well-being. For personalized nutrition advice, it is always recommended to consult with a healthcare provider or a registered dietitian.

Tips for Better Digestive Health

Eat slowly and chew food thoroughly: This aids the initial stages of digestion and can help prevent larger, undigested food pieces from causing issues.
Hydrate adequately: Drinking plenty of water helps fiber move smoothly through your digestive tract.
Increase fiber gradually: Adding too much fiber too quickly can cause gas and bloating. Add high-fiber foods to your diet slowly to give your gut time to adjust.
Consider enzyme supplements for intolerances: If you know you have an issue like lactose or raffinose intolerance, specific enzyme supplements can help, but they are not a cure-all.
Focus on whole, unprocessed foods: These foods contain natural fibers and nutrients that are beneficial for your gut microbiome.
Balance soluble and insoluble fiber: Soluble fiber (oats, beans) dissolves in water and helps regulate blood sugar, while insoluble fiber (whole grains, vegetable skins) adds bulk to stool.

Frequently Asked Questions

Humans cannot digest cellulose because our bodies do not produce the enzyme cellulase, which is necessary to break down the tough molecular bonds that form cellulose in plant cell walls.

Yes, it is often normal. High-fiber foods like corn, beans, and seeds, which have tough outer layers of cellulose, frequently pass through the digestive system partially or fully intact. This is not typically a cause for concern unless accompanied by other symptoms.

The gut microbiota ferments the undigested carbohydrates, like fiber and prebiotics, that reach the large intestine. This fermentation process produces short-chain fatty acids (SCFAs) which provide energy for colon cells and have anti-inflammatory effects.

Raffinose-family oligosaccharides (RFOs) are complex sugars found in legumes and vegetables. They are hard to digest because humans lack the enzyme alpha-galactosidase needed to break them down, which often results in gas and bloating as gut bacteria ferment them.

Many artificial sweeteners, such as sorbitol and mannitol, are poorly absorbed by the body. They travel to the large intestine where they are fermented by bacteria, which can cause symptoms like bloating, gas, and diarrhea.

Indigestible fiber is essential for health. It adds bulk to stool, promoting regular bowel movements and preventing constipation. It also acts as a prebiotic, nourishing the beneficial bacteria in your gut, which is vital for a healthy microbiome.

An indigestible food, like cellulose, simply cannot be broken down by human enzymes, and this is normal. A food intolerance is a digestive issue where the body struggles to process a specific component (e.g., lactose) due to an enzyme deficiency, leading to discomfort.