Which is not digestive by humans? An in-depth look

November 15, 2025 •

3 min read

According to the National Institutes of Health, while humans can't digest certain plant-based materials like cellulose, they still play a crucial role in our diet. We will explore the various components that are not digestive by humans and the biological reasons behind this fascinating nutritional fact.

Quick Summary

Humans cannot digest specific food components, primarily certain types of dietary fiber, due to the lack of necessary enzymes in our digestive system. These indigestible substances, like cellulose, lignin, and resistant starches, pass through the gut largely intact, acting as crucial bulk or prebiotics for our gut bacteria. The inability to break down these molecules is a key characteristic of human digestion.

Key Points

Inability to digest: Humans lack the specific enzymes, such as cellulase, required to break down the chemical bonds in many complex carbohydrates and polymers.
Cellulose: Found in plant cell walls, it is indigestible by humans and functions as insoluble fiber, adding bulk to stool.
Lignin: This non-carbohydrate polymer provides rigidity to plants and passes through the human digestive system completely intact due to its complex structure.
Resistant starch: Escapes digestion in the small intestine to be fermented by gut bacteria in the large intestine, where it acts as a prebiotic.
Chitin: Found in insect exoskeletons and fungi, it is mostly indigestible but may be partially broken down by human-produced chitinase enzymes.
Dietary benefits: Despite being indigestible, these substances are vital for digestive regularity, feeding beneficial gut bacteria, and producing beneficial short-chain fatty acids.

What is not digestive by humans?

While our digestive system is remarkably efficient at breaking down proteins, fats, and simple carbohydrates, it is ill-equipped to handle certain complex molecules. The primary reason for this is the absence of specific enzymes required to break the chemical bonds holding these substances together. Instead of being absorbed for energy, these compounds travel through the gastrointestinal tract mostly unchanged and are later expelled.

Cellulose: The most common indigestible component

Cellulose is a polysaccharide and the main structural component of plant cell walls. It is an abundant organic compound found in fruits, vegetables, and whole grains. Although chemically composed of glucose units, the bonds between these units are different from those found in digestible starches.

Humans lack the enzyme cellulase, which is necessary to break the beta-1,4-glycosidic bonds of cellulose. This means that when we eat a plant, we cannot extract the energy stored within its cellulose structure. Instead, the undigested cellulose becomes a form of insoluble dietary fiber, or roughage. This fiber adds bulk to our stool, promoting regular bowel movements and preventing constipation.

Lignin: The tough, non-carbohydrate fiber

Lignin is a complex polymer found in the cell walls of plants that provides rigidity and strength. Unlike cellulose, it is not a carbohydrate but a complex phenolic polymer with a random and highly resistant structure. Humans cannot digest lignin, and neither can the microorganisms in our gut. Because of its extremely complex and irregular structure, digestive enzymes cannot effectively break it down, and it passes through the digestive system entirely intact.

Resistant starches: Starch that escapes digestion

Resistant starch is a type of starch that is not digested in the small intestine but instead passes through to the large intestine. Here, it acts as a prebiotic, serving as a food source for the beneficial bacteria in our gut microbiome. This bacterial fermentation process produces short-chain fatty acids, which have numerous health benefits, including supporting a healthy gut lining and aiding immune function. Resistant starches are naturally found in foods like green bananas, raw potatoes, and legumes. They can also be created by cooling cooked starchy foods like rice or potatoes.

Chitin: Found in insects and fungi

Chitin is another type of indigestible polysaccharide, similar in structure to cellulose but containing nitrogen. It is the primary component of the exoskeletons of insects and crustaceans, as well as the cell walls of fungi. While once thought to be completely indigestible by humans, recent studies suggest that our immune system can trigger the production of chitinase enzymes to help break down chitin, particularly after ingesting large quantities. Nonetheless, it remains largely undigested and provides significant dietary fiber.

Comparison of indigestible substances


Feature	Cellulose	Lignin	Resistant Starch	Chitin
Type of Molecule	Polysaccharide	Phenolic Polymer	Polysaccharide	Polysaccharide
Biological Source	Plant cell walls	Plant cell walls	Plant-based foods	Exoskeletons of arthropods, fungi
Digestibility by Humans	Indigestible due to lack of cellulase enzyme.	Indigestible due to complex, random structure.	Partially digestible; resists digestion in the small intestine.	Partially digestible due to potential chitinase production.
Role in Diet	Adds insoluble bulk/fiber to stool.	A non-carbohydrate component of dietary fiber.	Serves as a prebiotic for gut bacteria.	Provides fiber and may engage the immune system.
Example Food Sources	Fruits, vegetables, whole grains.	Mature plants, bran.	Green bananas, cooked and cooled potatoes.	Insects, mushrooms.

The importance of indigestible foods

Although these substances cannot be digested for energy, they are far from useless. Insoluble fiber, like cellulose and lignin, acts as a bulking agent that promotes digestive regularity and can reduce the risk of certain colon diseases. Soluble fiber and resistant starches, once fermented by gut bacteria, produce short-chain fatty acids that nourish the colon lining and reduce inflammation. These prebiotic effects support a healthy and diverse microbiome, which is linked to a wide range of benefits, from improved immunity to mental health. Therefore, incorporating these indigestible components into our diet is essential for overall health.

Conclusion

In conclusion, our digestive limitations are a result of our evolutionary history as omnivores, lacking the specialized gut microbes and enzymes of herbivores to digest complex polysaccharides like cellulose and chitin, or complex polymers like lignin. While this means we cannot extract energy from these sources, they provide crucial dietary fiber and prebiotics that support our gut microbiome and overall digestive health. Understanding which foods are not digestible by humans helps us appreciate the complexity of our nutritional needs and the vital roles that dietary fiber plays in our bodies.

Frequently Asked Questions

Humans cannot digest cellulose because they do not produce the necessary enzyme called cellulase. This enzyme is required to break the beta-1,4-glycosidic bonds that link the glucose molecules in cellulose.

No vertebrate animals produce cellulase. Ruminants like cows and horses have a special digestive tract that harbors symbiotic bacteria, which produce the enzymes necessary to break down cellulose.

Indigestible foods, such as fiber, pass through the stomach and small intestine mostly unchanged. In the large intestine, some are fermented by gut bacteria (like resistant starches), while others, like insoluble fiber, simply add bulk to the stool and are excreted.

Resistant starch acts as a prebiotic, a food source for beneficial gut bacteria. The fermentation of resistant starch by these bacteria produces short-chain fatty acids that nourish the gut lining and offer various health benefits.

Chitin is not considered harmful to humans. While it is mostly indigestible and adds dietary fiber, some human cells produce a chitinase enzyme that can break it down to some extent.

The difference lies in the chemical bonds. Digestible carbohydrates like starch have alpha-glycosidic bonds that human enzymes can break. Indigestible carbohydrates like cellulose have beta-glycosidic bonds that human enzymes cannot break down.

Dietary fiber is crucial for health because it promotes bowel regularity, prevents constipation, and, in the case of fermentable fiber, nourishes the beneficial bacteria in the gut. This supports overall digestive and immune health.