Why is fibre difficult to digest? The Digestive Science Explained

December 26, 2025 •

4 min read

Despite being a vital part of a healthy diet, humans lack the specific enzymes to break down dietary fiber. This complex carbohydrate passes mostly intact through the stomach and small intestine, unlike other nutrients such as proteins and starches which our bodies can readily absorb.

Quick Summary

Fibre is difficult for humans to digest because we lack the necessary enzymes. Instead, our gut microbiome ferments it in the large intestine, producing beneficial compounds.

Key Points

Missing Enzymes: The primary reason humans cannot digest fibre is the lack of specific enzymes, like cellulase, required to break down its complex structure.
Gut Microbiome is Key: Instead of human enzymes, bacteria in our large intestine ferment dietary fibre, which is a crucial part of our digestive process.
Two Types of Fibre: Soluble fibre forms a gel and is fermented by gut microbes, while insoluble fibre adds bulk to stool and remains largely intact.
Beneficial Byproducts: Fermentation produces short-chain fatty acids (SCFAs) that provide energy and support a healthy gut lining.
Manages Digestion: Insoluble fibre aids in bowel regularity and prevents constipation by adding bulk to stool.
Start Slow: Increasing fibre intake too rapidly can cause bloating and gas; it's best to introduce it gradually and with sufficient water.

The primary reason why is fibre difficult to digest for the human body lies in our biology. Unlike other carbohydrates that are easily broken down into glucose by our digestive enzymes, fiber's chemical structure is resistant to this process. This fundamental difference is key to understanding its unique journey through the gastrointestinal tract and the surprising health benefits it provides.

The Missing Enzyme: Cellulase

Most carbohydrates, like starch, consist of sugar molecules linked together in a way that our body's enzymes, such as amylase, can easily cleave. Fibre, which is predominantly a non-starch polysaccharide like cellulose, has a different type of chemical bond (beta-glucosidic linkages) that human digestive enzymes cannot break down. The enzyme required to do so, cellulase, is naturally produced by many plant-eating animals and certain microorganisms, but not by humans. This is why eating a high-fibre plant-based diet doesn't provide us with a significant amount of direct caloric energy, even though it is packed with nutrients.

The Two Faces of Fibre: Soluble and Insoluble

Not all fibre is created equal. The way it behaves in the digestive system depends on whether it is soluble or insoluble in water. While both types are difficult for humans to digest, their impact on the body is quite different.

Insoluble Fibre

Insoluble fibre, often called "roughage," does not dissolve in water. It passes through the digestive tract largely unchanged, adding bulk to stool. This is crucial for maintaining regular bowel movements and preventing constipation. It acts like a broom, sweeping waste through the colon. Good sources include whole-wheat flour, wheat bran, and many vegetables.

Soluble Fibre

Soluble fibre, on the other hand, dissolves in water to form a gel-like substance. This gel can slow down digestion, which helps to regulate blood sugar levels and can assist in lowering cholesterol. As it moves into the large intestine, it becomes a food source for our gut bacteria. Excellent sources are oats, beans, apples, and citrus fruits.

The Unsung Heroes: Our Gut Microbiome

While our own digestive system may struggle with fibre, we have billions of microscopic helpers that thrive on it: our gut bacteria, collectively known as the microbiome. In the large intestine, these beneficial microbes ferment the fibre that we cannot digest ourselves.

Fermentation is not digestion: The fermentation process by gut bacteria is what truly unlocks the value of fibre for human health. Through this process, the microbes produce beneficial compounds known as short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate. These SCFAs are then absorbed by the body and provide a range of health benefits, from nourishing the cells of the gut lining to potentially influencing immune function. This symbiotic relationship is why eating fibre is so important, even if we can't digest it ourselves.

The Difference Between Soluble and Insoluble Fibre

This table outlines the key differences in how the two primary types of fibre are processed in the body.


Characteristic	Soluble Fibre	Insoluble Fibre
Dissolves in Water?	Yes	No
Effect on Digestion	Forms a gel, slows digestion	Adds bulk, speeds up transit
Primary Function	Feeds gut bacteria, lowers cholesterol	Promotes regularity, prevents constipation
Fermentation by Gut Bacteria	Readily fermented	Minimally fermented
Main Food Sources	Oats, beans, apples, carrots	Whole grains, nuts, wheat bran

Potential Side Effects and How to Manage Them

While the digestion of fibre is a beneficial process, increasing your intake too quickly can lead to digestive discomfort such as bloating, gas, and cramping. This is often due to the increased fermentation by gut bacteria and the time your digestive system needs to adapt. To avoid these issues, it is recommended to increase fibre intake gradually over a few weeks and ensure you drink plenty of water, as fibre works best when it can absorb fluids.

Conclusion: The Ultimate Gut Collaboration

In conclusion, fibre is difficult for humans to digest because we simply do not possess the necessary enzymes. However, this isn't a flaw but rather a brilliant collaboration with our gut microbiome. This relationship allows for a crucial fermentation process that produces short-chain fatty acids, contributing to our overall health in numerous ways. The undigested fibre also provides essential bulk and regularity, highlighting that its value comes not from being directly absorbed, but from its unique journey through our digestive system. By understanding this process, we can better appreciate the vital role that different types of fibre play in maintaining a healthy and balanced body.

For more detailed information on dietary fiber, consult the Harvard School of Public Health's dedicated section on the topic.

Frequently Asked Questions

Humans lack the specific enzymes, like cellulase, to break down the complex chemical bonds in fibre. This is a normal part of our evolutionary biology, and instead, our gut bacteria evolved to handle this task.

Soluble fibre dissolves in water to form a gel, is readily fermented by gut bacteria, and helps regulate blood sugar and cholesterol. Insoluble fibre does not dissolve, passes largely intact through the digestive tract, and adds bulk to stool to aid in regularity.

Short-chain fatty acids (SCFAs) are beneficial compounds produced when gut bacteria ferment fibre. They serve as an energy source for our gut lining cells, support immune function, and contribute to overall metabolic health.

Yes, specifically insoluble fibre helps with constipation by absorbing water and adding bulk to your stool, making it softer and easier to pass through the digestive tract.

Yes, consuming an excessive amount of fibre or increasing intake too quickly can cause digestive discomfort such as bloating, gas, and cramping. It is important to increase intake gradually.

Insoluble fibre largely passes through the large intestine unchanged, where it absorbs water and adds weight and bulk to the stool, promoting regular bowel movements.

Increase your fibre intake slowly over several weeks. Add new fibre-rich foods in small quantities, spread your fibre consumption throughout the day, and drink plenty of water to help it move through your system smoothly.