Is fiber Cannot be used by the body for energy True or False?

December 22, 2025 •

4 min read

According to the Mayo Clinic, most Americans consume significantly less fiber than the recommended daily amounts, which has a tangible impact on digestive and overall health. Many people also wonder if fiber cannot be used by the body for energy, a common but complex misconception.

Quick Summary

The statement that humans cannot use fiber for energy is an oversimplification; while our enzymes can't break it down, gut bacteria ferment it into short-chain fatty acids (SCFAs) that provide energy.

Key Points

Indirect Energy Source: While humans can't directly digest fiber for energy, our gut bacteria ferment it into short-chain fatty acids (SCFAs), which we absorb for a small amount of energy.
Soluble Fiber Fermentation: Soluble fiber, found in oats and beans, is highly fermentable by gut microbes, making it a significant source of energy via SCFAs.
Insoluble Fiber's Role: Insoluble fiber, or 'roughage' from whole grains and vegetables, is minimally fermented and provides little energy, but is crucial for digestive regularity and adding bulk to stool.
Beyond Energy Benefits: Fiber's most important contributions are its non-caloric functions, including promoting gut health, managing blood sugar and cholesterol, and aiding in weight control.
Gut Health Booster: The SCFAs produced from fiber fermentation nourish the cells of the colon and support a healthy gut microbiome by inhibiting pathogenic bacteria.

The Surprising Answer: Why 'False' Isn't So Simple

The short answer is more nuanced than a simple 'true' or 'false.' The statement is technically false, but for a reason that most people don't consider. The human body, with its own digestive enzymes, cannot break down dietary fiber for energy. This is the basis for the 'true' side of the argument, as fiber passes through the upper digestive tract largely intact. However, the human body exists in a symbiotic relationship with its gut microbiota.

In the large intestine, trillions of bacteria and other microorganisms get to work on the fiber that our bodies couldn't digest. This is where the 'false' part of the statement comes in. These friendly gut bacteria, which we depend on for many aspects of our health, can ferment the fiber. This process produces beneficial metabolites called short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate. The human body can then absorb these SCFAs and use them as a source of energy. For every gram of fiber consumed, we may receive up to two calories of energy in the form of these SCFAs, though the exact amount varies. Therefore, while we don't directly digest fiber for energy like we do with other carbohydrates, our bodies indirectly gain energy from it with the help of our gut microbes.

The Two Faces of Fiber

Dietary fiber is broadly categorized into two types, and their fermentability—and thus, their energy contribution—differs significantly.

Soluble Fiber

What it is: Dissolves in water to form a gel-like substance in the gut.
How it works: This gel slows digestion and the absorption of nutrients, including glucose from other carbohydrates. This helps manage blood sugar levels and can create a feeling of fullness.
Fermentability: Soluble fiber is highly fermentable by gut bacteria, making it a primary source for SCFA production.
Sources: Oats, peas, beans, apples, carrots, and citrus fruits are all good sources of soluble fiber.

Insoluble Fiber

What it is: Does not dissolve in water. Often referred to as 'roughage.'
How it works: It adds bulk to stool and helps food pass more quickly through the digestive system. This promotes regularity and helps prevent constipation.
Fermentability: Insoluble fiber is much less fermentable and passes mostly intact through the large intestine. It contributes minimally to SCFA production and energy gain via fermentation.
Sources: Whole-wheat flour, wheat bran, nuts, green beans, and potatoes with the skin are excellent sources of insoluble fiber.

Comparison: Soluble vs. Insoluble Fiber


Feature	Soluble Fiber	Insoluble Fiber
Dissolves in Water?	Yes	No
Forms Gel?	Yes	No
Fermentable?	Highly	Poorly
Digestion Speed	Slows digestion	Speeds things up
Primary Role	Regulates blood sugar & cholesterol	Adds bulk, promotes regularity
Energy from SCFAs	Yes, significant	Minimal
Examples	Oats, apples, beans, nuts	Wheat bran, nuts, root vegetables

The Many Benefits Beyond Energy

While the energy from fermented fiber is modest, fiber's broader benefits are significant and essential for human health. The SCFAs produced by gut bacteria not only provide energy but also have a crucial role in maintaining colon health, reducing inflammation, and potentially lowering the risk of various diseases.

Gut Health: SCFAs, particularly butyrate, serve as the preferred energy source for the cells lining the colon. This helps maintain the integrity of the gut lining, which is vital for overall health. A healthy gut microbiome also helps crowd out harmful pathogens.
Blood Sugar Control: Soluble fiber's ability to slow nutrient absorption helps prevent sharp blood sugar spikes after meals, which is beneficial for managing and preventing type 2 diabetes.
Heart Health: Certain types of soluble fiber can bind to cholesterol in the digestive tract, preventing it from being absorbed and helping to lower LDL ('bad') cholesterol levels.
Weight Management: Both types of fiber can aid in weight control. Soluble fiber promotes feelings of fullness by creating a gel, while insoluble fiber adds bulk to food, helping to reduce overall calorie intake.

Conclusion

The claim that fiber cannot be used by the body for energy is a misconception, though an understandable one. While humans lack the enzymes for direct digestion, we rely on our gut bacteria to ferment certain types of fiber into short-chain fatty acids, which we then absorb for a small amount of energy. However, focusing solely on this energy contribution misses the point. Fiber is a cornerstone of a healthy diet because of its critical, non-caloric functions, from regulating blood sugar and cholesterol to fostering a thriving gut microbiome. The intricate and symbiotic process of fiber fermentation highlights its importance far beyond simple energy provision. For more in-depth nutritional information on fiber, visit The Nutrition Source from Harvard T.H. Chan School of Public Health.

Frequently Asked Questions

No, the human stomach and small intestine cannot digest fiber because our bodies lack the necessary enzymes. Fiber passes through these parts of the digestive tract largely unchanged.

In the large intestine, the gut microbiota ferments dietary fiber. This process breaks down the fiber and produces beneficial short-chain fatty acids (SCFAs) as a byproduct.

The SCFAs produced by the fermentation of fiber are absorbed by the body, particularly by the cells lining the colon, and can be used for energy.

Yes. Soluble fiber is highly fermentable by gut bacteria and thus provides a more significant amount of energy via SCFAs. Insoluble fiber is poorly fermented and offers minimal energy in this manner.

While fiber is not broken down into calories like other carbohydrates, the fermentation of soluble fiber by gut bacteria results in SCFAs that are absorbed and provide a small caloric benefit, estimated at about 2 calories per gram.

Fiber is crucial for maintaining digestive health, promoting bowel regularity, feeding beneficial gut bacteria, and regulating blood sugar and cholesterol levels. Its health benefits go far beyond its minimal energy contribution.

Excellent sources of fermentable, or soluble, fiber include oats, barley, beans, lentils, apples, and bananas. These foods provide the fuel for your beneficial gut bacteria to produce SCFAs.