Why Is Fibre Not Considered a Source of Carbohydrates in the Diet?

November 11, 2025 •

4 min read

While both are chemically classified as carbohydrates, the human body cannot break down fibre for energy in the same way it does with other carbs. This fundamental difference in how our digestive system processes it is why fibre is uniquely treated in the context of dietary intake and energy calculations.

Quick Summary

Fibre is an indigestible carbohydrate, meaning human enzymes cannot break it down into absorbable sugar molecules like glucose. This causes it to pass through the body largely intact, providing no net calories or energy, which fundamentally distinguishes it from digestible carbohydrates.

Key Points

Indigestible Nature: Fibre is a carbohydrate that cannot be broken down by human digestive enzymes, unlike starches and sugars.
No Energy Contribution: Because it is not absorbed, fibre provides little to no calories or energy, passing through the digestive system largely intact.
Blood Sugar Regulation: Fibre helps manage blood sugar levels by slowing down the absorption of other carbohydrates into the bloodstream.
The 'Net Carb' Distinction: The concept of 'net carbs' allows dieters to subtract fibre from total carbohydrates, as it is the indigestible portion.
Crucial Health Benefits: Despite not being an energy source, fibre is essential for digestive health, managing cholesterol, and promoting satiety.
Supports Gut Health: Soluble fibre is fermented by gut bacteria, producing beneficial short-chain fatty acids.

The Chemical and Physiological Difference Between Fibre and Digestible Carbohydrates

From a chemical perspective, fibre is a type of carbohydrate, meaning it is made of sugar molecules. The key distinction lies not in its chemical composition but in how the human body interacts with it. The human digestive tract lacks the necessary enzymes to break the bonds of fibre molecules, particularly the complex polysaccharides found in plant cell walls.

When we consume foods containing digestible carbohydrates like starches and sugars, enzymes such as amylase in our saliva and intestines rapidly break them down into simple sugars, primarily glucose. This glucose is then absorbed into the bloodstream and used by the body's cells for energy. This process is why standard carbohydrates contribute to your caloric intake and can cause fluctuations in blood sugar levels.

In contrast, fibre, which is made up of indigestible complex carbohydrates like cellulose, hemicellulose, and pectin, passes through the stomach and small intestine mostly intact. It isn't absorbed into the bloodstream as glucose. Instead, it travels to the large intestine, where some types of fibre are fermented by gut bacteria. This fermentation process can produce short-chain fatty acids (SCFAs), which offer various health benefits but do not contribute significant calories or impact blood sugar in the same way as digestible carbohydrates.

Soluble vs. Insoluble Fibre: Different Roles, Same Indigestible Classification

Understanding why fibre is not a calorie source is further clarified by looking at its two main types: soluble and insoluble fibre. Both are indigestible by human enzymes, but they behave differently in the digestive system.

Soluble Fibre

Dissolves in water, forming a gel-like substance in the digestive tract.
Sources include oats, peas, beans, apples, and citrus fruits.
As the gel moves through the digestive system, it slows down digestion.
This delayed digestion can help lower blood cholesterol and regulate blood sugar levels, which is a key reason for its health benefits.
It is fermented by beneficial gut bacteria in the large intestine, producing health-promoting SCFAs.

Insoluble Fibre

Does not dissolve in water.
Sources include whole-wheat flour, wheat bran, nuts, beans, and vegetables.
Adds bulk to the stool and helps food pass more quickly through the stomach and intestines.
This bulking action supports regular bowel movements and can be helpful for preventing constipation.
It is less fermentable than soluble fibre and remains largely unchanged as it passes through the body.

The Concept of 'Net Carbs'

The indigestibility of fibre is the basis for the dietary concept of "net carbs". Nutrition labels often list "Total Carbohydrates," which includes both digestible carbohydrates and fibre. For individuals counting digestible carbohydrates for metabolic management, like those following ketogenic diets, the common practice is to subtract the fibre from the total carbohydrate count. The remaining number represents the 'net carbs,' which is the amount of carbohydrates the body can actually break down and absorb for energy.


Feature	Digestible Carbohydrates (Starches, Sugars)	Fibre (Soluble and Insoluble)
Energy Contribution	Provides calories/energy to the body.	Provides little to no calories/energy.
Digestion by Humans	Easily broken down by human digestive enzymes.	Not broken down by human digestive enzymes.
Absorption	Broken down into simple sugars (glucose) and absorbed into the bloodstream.	Passes through the small intestine largely unabsorbed.
Blood Sugar Impact	Can cause a significant rise in blood sugar levels.	Helps regulate blood sugar levels by slowing digestion.
Fermentation	Does not undergo significant fermentation in the large intestine.	Soluble fibre is fermented by gut bacteria, producing SCFAs.
Nutritional Example	Potatoes, bread, table sugar, juice.	Whole grains, vegetables, fruits, legumes, nuts.

Health Benefits of Fibre Beyond Energy

While not a source of readily available energy, fibre offers numerous health benefits, making it a crucial component of a healthy diet.

Promotes Digestive Health: Both soluble and insoluble fibre are essential for maintaining a healthy digestive system. Insoluble fibre adds bulk to the stool, promoting regular bowel movements and preventing constipation.
Supports Gut Microbiome: Soluble fibre acts as a prebiotic, feeding the beneficial bacteria in the gut. A healthy and diverse gut microbiome is linked to various aspects of health, including improved immunity and mental well-being.
Manages Blood Sugar: By slowing down the absorption of sugar, soluble fibre helps regulate blood sugar levels and can improve insulin sensitivity. This is particularly beneficial for individuals with diabetes or those at risk of developing it.
Aids Weight Management: Fibre promotes a feeling of fullness or satiety, which can reduce overall calorie intake and assist with weight management.
Lowers Cholesterol: Soluble fibre can help lower low-density lipoprotein (LDL), or "bad" cholesterol levels, reducing the risk of heart disease.
Reduces Chronic Disease Risk: A high-fibre diet is associated with a reduced risk of several chronic diseases, including cardiovascular disease, type 2 diabetes, and certain types of cancer.

For more detailed information on dietary fibre, you can refer to authoritative sources like the Harvard T.H. Chan School of Public Health's dedicated page on the topic.

Conclusion: A Misconception Based on a Chemical Label

The reason fibre is not considered a source of carbohydrates in a nutritional context is rooted in human physiology, not chemical structure. While it is technically a carbohydrate, our body's inability to digest and absorb it for energy means it does not function as a fuel source. This physiological reality explains why it is excluded from 'net carb' calculations and highlights its distinct and vital role in promoting overall health. Unlike digestible carbs that provide a calorie boost, fibre's contribution is in regulating digestion, managing blood sugar, and feeding beneficial gut bacteria, making it an indispensable part of a balanced diet.

Frequently Asked Questions

Chemically, fibre is a type of carbohydrate, as it is composed of sugar molecules. However, from a nutritional and metabolic perspective, it is not treated as a carbohydrate source because the human body cannot digest and absorb it for energy.

Humans lack the specific enzymes required to break the strong bonds in fibre's molecular structure. Other carbohydrates, like starch, have different chemical bonds that our digestive enzymes can easily break down into simple sugars for energy.

Digestible carbohydrates (starches, sugars) are broken down and absorbed for energy, impacting blood sugar. Indigestible carbohydrates (fibre) cannot be broken down and pass through the body mostly intact, providing no usable calories.

Soluble fibre forms a gel in the digestive system that slows down the absorption of sugar. This prevents sharp spikes in blood glucose levels, helping to regulate them more effectively.

Net carbs refer to the total carbohydrates that the body can digest and use for energy. It is calculated by subtracting the grams of fibre from the total carbohydrate count listed on a nutrition label.

Yes, some soluble fibre is fermented by the bacteria in your large intestine. This process produces beneficial short-chain fatty acids (SCFAs), but does not contribute significant calories.

Yes, fibre comes in two main types: soluble and insoluble. Soluble fibre dissolves in water and can lower cholesterol and blood sugar, while insoluble fibre adds bulk to stool and promotes regularity.