How the Small Intestine Breaks Down Carbohydrates

January 6, 2026 •

3 min read

The majority of carbohydrate digestion occurs in the small intestine, following initial breakdown in the mouth. It is here that complex carbohydrates are transformed into simple sugars, a critical process for fueling the body. This transformation is orchestrated by a powerful team of enzymes and specialized structures that ensure efficient nutrient absorption.

Quick Summary

The small intestine uses pancreatic amylase and brush border enzymes to convert complex carbohydrates and disaccharides into monosaccharides like glucose, fructose, and galactose. Specialized transporters in the intestinal lining then absorb these simple sugars into the bloodstream for energy.

Key Points

Enzymatic Action: Pancreatic amylase and brush border enzymes are the primary players responsible for breaking down carbohydrates in the small intestine.
Two-Phase Process: Pancreatic amylase first reduces complex starches into smaller polysaccharides and disaccharides, while brush border enzymes then convert these into absorbable monosaccharides.
Final Products: The end products of carbohydrate digestion in the small intestine are glucose, fructose, and galactose.
Efficient Absorption: The small intestine's brush border, with its numerous microvilli, provides a vast surface area for the efficient absorption of monosaccharides.
Specialized Transport: Different monosaccharides are absorbed via distinct mechanisms; glucose and galactose use active transport with sodium, while fructose uses facilitated diffusion.
Lactose Intolerance: A deficiency of the brush border enzyme lactase can lead to lactose intolerance, causing digestive issues when dairy is consumed.

From Mouth to Monosaccharide: The Carbohydrate Digestion Journey

While the digestion of carbohydrates begins in the mouth with salivary amylase, this process is brief and minimal due to the short time food spends there. Once swallowed, the acidic environment of the stomach deactivates salivary amylase, halting carbohydrate digestion temporarily. The real work of breaking down carbohydrates into absorbable monosaccharides begins and is completed in the small intestine.

The Role of Pancreatic Amylase

As partially digested food, now called chyme, enters the duodenum, the first part of the small intestine, it is met with pancreatic amylase. The pancreas secretes this powerful enzyme into the duodenum, where it continues the breakdown of starches and complex carbohydrates. Pancreatic amylase specifically cleaves the $\alpha$-1,4 glycosidic bonds within these molecules, breaking them down into smaller polysaccharides, maltose (a disaccharide), and maltotriose (a trisaccharide).

This enzymatic activity is crucial because complex carbohydrates are too large to be absorbed by the intestinal lining. Pancreatic amylase reduces these large molecules into smaller, more manageable units, setting the stage for the final digestive steps.

The Brush Border Enzymes: The Final Stage of Digestion

The final and most critical stage of carbohydrate digestion happens at the 'brush border,' the dense layer of microvilli that lines the small intestine's surface. These microvilli are studded with specific enzymes, known collectively as brush border enzymes, which complete the hydrolysis of disaccharides and other remaining saccharides into their simplest forms.

Common brush border enzymes include:

Maltase: Breaks down maltose into two molecules of glucose.
Sucrase: Splits sucrose into one molecule of glucose and one molecule of fructose.
Lactase: Hydrolyzes lactose into one molecule of glucose and one molecule of galactose.
α-Dextrinase: Breaks down the $\alpha$-1,6 glycosidic bonds in dextrins and starch fragments, releasing individual glucose molecules.

This final enzymatic action ensures that all digestible carbohydrates are converted into monosaccharides (glucose, fructose, and galactose), the only forms that can be absorbed across the intestinal wall.

Absorption into the Bloodstream

With digestion complete, the monosaccharides are ready for absorption. The small intestine is lined with a vast surface area of villi and microvilli, maximizing the efficiency of nutrient uptake. The absorption mechanism varies slightly for each monosaccharide:

Glucose and Galactose: These are absorbed via secondary active transport. They are co-transported with sodium ions into the enterocyte (the intestinal lining cell) by the sodium-glucose cotransporter (SGLT1).
Fructose: This monosaccharide enters the enterocyte through facilitated diffusion using a different protein transporter (GLUT5).

After entering the enterocytes, all three monosaccharides exit the cell via a transporter called GLUT2 on the basolateral membrane and enter the capillaries within the villi. They are then transported via the hepatic portal vein to the liver, which can convert galactose and fructose into glucose or store glucose as glycogen.

Table of Carbohydrate Digestion Enzymes and Functions


Enzyme	Source	Action	Substrates	Products
Pancreatic Amylase	Pancreas	Breaks down $\alpha$-1,4 glycosidic bonds	Starch, Dextrins	Maltose, Maltotriose, Oligosaccharides
Maltase	Brush Border	Breaks down maltose	Maltose	2 Glucose molecules
Sucrase	Brush Border	Breaks down sucrose	Sucrose	1 Glucose, 1 Fructose molecule
Lactase	Brush Border	Breaks down lactose	Lactose	1 Glucose, 1 Galactose molecule
α-Dextrinase	Brush Border	Breaks down $\alpha$-1,6 glycosidic bonds	Dextrins	Glucose molecules

Factors Affecting Carbohydrate Digestion

Several factors can influence the efficiency of this digestive process. The amount of fiber in a meal, for instance, can affect the glycemic index by slowing down the digestion and absorption of carbohydrates. Conversely, highly processed foods can be digested and absorbed more rapidly. Certain medical conditions can also impair this process, leading to malabsorption issues. For example, individuals with lactose intolerance lack sufficient lactase enzyme, causing lactose to pass undigested into the large intestine and lead to digestive discomfort.

Conclusion

The small intestine is the central hub for breaking down carbohydrates. Through the coordinated action of pancreatic amylase and a specialized team of brush border enzymes, large carbohydrates are meticulously broken down into absorbable monosaccharides. These simple sugars are then absorbed into the bloodstream to provide energy for the body's cells. This elegant and efficient process is a testament to the sophistication of the human digestive system, ensuring that we can effectively harness the energy stored in the foods we eat. For further details on the mechanics of nutrient transport, including the specific transporters involved, Colorado State University offers an insightful resource.

Frequently Asked Questions

The small intestine is the primary site where the majority of carbohydrate digestion occurs, completing the process that begins briefly in the mouth.

Two main groups of enzymes are involved: pancreatic amylase, secreted by the pancreas, and brush border enzymes like maltase, sucrase, and lactase, which are embedded in the intestinal lining.

Pancreatic amylase breaks down complex starches and carbohydrates that enter the small intestine into smaller polysaccharides and disaccharides, such as maltose and maltotriose.

Brush border enzymes are located on the microvilli of the intestinal wall and are responsible for the final conversion of disaccharides into individual monosaccharides (glucose, fructose, and galactose) right before absorption.

Glucose and galactose are absorbed via active transport with sodium, while fructose is absorbed through facilitated diffusion. They then enter the bloodstream and travel to the liver.

Indigestible fibers are not broken down in the small intestine. They pass into the large intestine where they can be fermented by gut bacteria or are eliminated from the body.

The microvilli form the 'brush border' and dramatically increase the surface area of the small intestine. This expanded surface is essential for housing brush border enzymes and maximizing the absorption of digested nutrients.