Where are most carbohydrates broken down? The answer is the small intestine

January 9, 2026 •

3 min read

According to the National Institutes of Health, the small intestine is where the vast majority of digestible carbohydrates are broken down and absorbed. While digestion begins in the mouth, it is the enzymes in the small intestine that perform the most extensive work, reducing complex carbohydrates into single sugar units for absorption.

Quick Summary

The majority of carbohydrate digestion happens in the small intestine. This process is driven by pancreatic amylase and specific brush border enzymes, which break down starches and other complex carbohydrates into absorbable monosaccharides.

Key Points

Primary Site: Most carbohydrate breakdown occurs in the small intestine, not the mouth or stomach.
Enzymatic Action: Digestion in the small intestine relies on pancreatic amylase and brush border enzymes.
Final Products: The goal is to break down complex carbs into absorbable monosaccharides like glucose, fructose, and galactose.
Stomach's Role: The stomach plays no part in the chemical digestion of carbohydrates; its acidic environment halts the process initiated in the mouth.
Fiber's Fate: Indigestible fiber passes through the small intestine and is fermented by bacteria in the large intestine, offering health benefits.
Absorption: Monosaccharides are absorbed into the bloodstream through the extensive surface area of the small intestine's microvilli.

The Journey of Carbohydrate Digestion

Carbohydrate digestion is a multi-step process that involves several organs and enzymes to convert complex carbohydrates into simple, absorbable sugar molecules, known as monosaccharides. While the journey begins in the mouth, the most critical phase takes place further along the digestive tract.

The Starting Point: The Mouth

Digestion of carbohydrates begins the moment food enters the mouth. As you chew your food, it is mixed with saliva secreted by the salivary glands. Saliva contains the enzyme salivary amylase, which starts the chemical breakdown of starches into smaller polysaccharides and maltose. However, this initial breakdown is limited due to the short time food spends in the mouth and is halted once the food reaches the acidic environment of the stomach.

In the Stomach: A Temporary Pause

After being swallowed, the food, now called chyme, passes into the stomach. Contrary to popular belief, no chemical digestion of carbohydrates occurs here. The highly acidic environment of the stomach deactivates the salivary amylase, effectively pausing the carbohydrate-digesting process. The stomach's role is primarily to mix the chyme and prepare it for its entry into the small intestine.

The Main Event: The Small Intestine

The small intestine is the primary site where are most carbohydrates broken down and absorbed. Once the chyme enters the duodenum (the first part of the small intestine), a powerful enzymatic process is initiated. This process involves two key sets of enzymes:

Pancreatic Amylase: The pancreas releases pancreatic juice containing the enzyme pancreatic amylase into the small intestine. This potent enzyme continues the work started by salivary amylase, breaking down the remaining large carbohydrate molecules, such as starch, into smaller saccharides like maltose and dextrins.
Brush Border Enzymes: The small intestine's inner lining is covered with microscopic, finger-like projections called microvilli, which are collectively known as the brush border. The membranes of these microvilli contain specialized enzymes called brush border enzymes, which complete the final stages of carbohydrate digestion. These include:
- Maltase: Breaks down maltose into two glucose molecules.
- Sucrase: Breaks down sucrose (table sugar) into glucose and fructose.
- Lactase: Breaks down lactose (milk sugar) into glucose and galactose.

This final enzymatic action breaks all digestible carbohydrates down into their simplest forms, or monosaccharides.

The Final Step: Absorption into the Bloodstream

Once broken down into monosaccharides, these single sugar units are absorbed through the walls of the small intestine. The surface of the small intestine is highly folded and covered with villi and microvilli, which greatly increases the surface area for absorption. The monosaccharides are transported across the intestinal cell membranes and enter the bloodstream, which carries them to the liver. From there, they can be distributed throughout the body to provide energy.

Indigestible Carbohydrates: What Happens to Fiber?

It is important to note that not all carbohydrates are broken down in the small intestine. Dietary fiber, a type of carbohydrate, is resistant to human digestive enzymes. Instead of being digested, it passes into the large intestine largely intact. Here, it is fermented by beneficial gut bacteria, producing short-chain fatty acids that can be used as an energy source by the cells lining the colon. Fiber also adds bulk to the stool and promotes healthy bowel movements.

Comparison of Carbohydrate Digestion Stages

To better understand the process, here is a comparison of carbohydrate digestion in different parts of the digestive system.


Digestive Location	Primary Action	Key Enzymes Involved	Status of Carbohydrate Digestion
Mouth	Mechanical chewing and initial enzymatic breakdown of starch.	Salivary Amylase	Partial digestion, producing smaller polysaccharides and maltose.
Stomach	Mechanical mixing; chemical digestion is halted.	None (salivary amylase is inactivated).	Digestion is stopped due to high acidity.
Small Intestine	Extensive chemical breakdown into monosaccharides and nutrient absorption.	Pancreatic Amylase, Brush Border Enzymes (Maltase, Sucrase, Lactase)	Complete digestion and absorption of digestible carbohydrates.
Large Intestine	Fermentation of undigestible fiber by gut bacteria.	Bacterial enzymes	Fermentation, not digestion, occurs for fiber.

Conclusion

While carbohydrate digestion begins in the mouth with salivary amylase, the extensive and most crucial phase of breakdown occurs in the small intestine. The combined action of pancreatic amylase from the pancreas and the specialized brush border enzymes on the intestinal wall is responsible for converting complex carbohydrates into absorbable monosaccharides. This efficient system ensures our bodies can readily access the energy contained within the foods we eat, with dietary fiber continuing its beneficial journey into the large intestine.

Further Reading: For a detailed look into the biological processes, visit NCBI's Bookshelf on Carbohydrate Digestion.

Frequently Asked Questions

In the mouth, mechanical chewing breaks down food, and the enzyme salivary amylase begins the chemical digestion of starches into smaller sugar chains. This is only a small portion of the total carbohydrate digestion.

No, carbohydrate digestion does not occur in the stomach. The high acidity of the stomach's environment deactivates salivary amylase, temporarily stopping the breakdown process.

The pancreas secretes pancreatic amylase into the small intestine, where it continues the breakdown of starches and other polysaccharides into smaller disaccharides and oligosaccharides.

Brush border enzymes are digestive enzymes located on the microvilli of the small intestine's lining. They are responsible for the final breakdown of disaccharides into absorbable monosaccharides like glucose, fructose, and galactose.

The final products, monosaccharides, are absorbed through the intestinal wall of the small intestine. They enter the capillaries and are transported via the bloodstream to the liver.

Fiber is a carbohydrate that humans cannot digest with enzymes. It passes mostly intact into the large intestine, where it is fermented by bacteria and contributes to digestive health.

The glucose absorbed from carbohydrate digestion is either used immediately by cells for energy, stored in the liver and muscles as glycogen, or converted to fat if there is an excess amount.