What Digests Starch into Simple Sugar? The Complete Guide

November 18, 2025 •

3 min read

Did you know that your mouth begins to break down starch into smaller sugars the moment you start chewing? This is thanks to a key enzyme in saliva that, along with others later in the digestive process, is responsible for digesting starch into simple sugar, a vital source of energy for your body.

Quick Summary

The digestion of starch into simple sugars is a multi-step enzymatic process. It begins with amylase in the mouth and continues in the small intestine with pancreatic amylase and other brush border enzymes like maltase, which complete the conversion to glucose for absorption.

Key Points

Amylase is the Primary Enzyme: The enzyme amylase, produced in the salivary glands and pancreas, is the main catalyst that digests starch into smaller sugar molecules like maltose and maltotriose.
Two-Stage Digestion: Starch digestion occurs in two main phases—the initial stage in the mouth using salivary amylase and the major stage in the small intestine with pancreatic amylase.
Maltase for Final Conversion: The enzyme maltase, located on the brush border of the small intestine, is responsible for breaking down maltose into individual glucose units, the form the body can absorb.
Specific Enzymes for All Linkages: While amylase targets specific glycosidic bonds in starch, enzymes like glucoamylase and isomaltase are necessary to cleave the final, more complex bonds to ensure complete digestion.
Optimal pH Matters: The activity of these enzymes is highly dependent on pH; salivary amylase is active in the neutral mouth, while pancreatic and intestinal enzymes function optimally in the alkaline environment of the small intestine.
Resistant Starch is Different: Some starch resists digestion by human enzymes and passes to the large intestine, where it is fermented by gut bacteria, providing additional health benefits.

The First Step: Salivary Amylase

Your journey to digesting starch begins the moment food enters your mouth. Salivary amylase, also known as ptyalin, is an enzyme secreted by the salivary glands. Its purpose is to begin the chemical breakdown of carbohydrates while you chew. As you masticate starchy foods like rice, potatoes, or bread, salivary amylase starts hydrolyzing (breaking down with water) the long polysaccharide chains of starch into smaller chains and disaccharides, primarily maltose. This is why starchy foods may taste slightly sweeter the longer you chew them.

Once swallowed, the food bolus travels down the esophagus to the stomach. Here, the low, acidic pH of the stomach quickly inactivates the salivary amylase, halting its digestive activity. While mechanical digestion continues in the stomach, significant carbohydrate digestion pauses until the food moves into the small intestine.

The Main Event: Pancreatic Amylase and Brush Border Enzymes

Upon leaving the stomach, the partially digested food (now called chyme) enters the duodenum, the first part of the small intestine. This is where the majority of carbohydrate digestion occurs. The pancreas releases pancreatic amylase into the small intestine, where it encounters a slightly alkaline environment, the optimal condition for its function.

Pancreatic amylase continues the work started in the mouth, breaking down the remaining starch into smaller carbohydrate units, such as maltose, maltotriose, and short chains of glucose called limit dextrins. However, these are still too large to be absorbed by the body. The final step requires a group of enzymes known as brush border enzymes, which are embedded in the microvilli lining the small intestine wall.

Brush Border Enzymes and Final Conversion

Among the most important brush border enzymes for starch digestion are:

Maltase: Converts maltose (a disaccharide) into two molecules of glucose.
Glucoamylase: Breaks down the short glucose chains (limit dextrins) into individual glucose molecules.
Isomaltase: Specifically targets and breaks the alpha-1,6 glycosidic bonds found at the branching points of starch, ensuring complete digestion.

Through the combined action of these intestinal enzymes, the complex starch molecule is completely dismantled into its simplest form: glucose. This single-unit sugar can then be absorbed by the cells lining the small intestine and transported into the bloodstream.

A Comparison of Starch-Digesting Enzymes

Understanding the different enzymes involved clarifies their specific roles throughout the digestive tract.


Feature	Salivary Amylase	Pancreatic Amylase	Maltase	Glucoamylase	Isomaltase
Source	Salivary Glands	Pancreas	Intestinal Wall (Brush Border)	Intestinal Wall (Brush Border)	Intestinal Wall (Brush Border)
Substrate	Starch	Starch (smaller polysaccharides)	Maltose	Limit Dextrins	Alpha-1,6 Bonds
Product	Maltose and smaller polysaccharides	Maltose, maltotriose, and limit dextrins	Glucose	Glucose	Glucose
Optimal pH	Neutral (approx. 6.7-7.0)	Alkaline (approx. 7.0-8.5)	Alkaline (approx. 7.0-8.5)	Alkaline (approx. 7.0-8.5)	Alkaline (approx. 7.0-8.5)

What About Undigested Starch?

Not all starch is digestible by human enzymes. Resistant starch is a type of dietary fiber that resists digestion in the small intestine and passes through to the large bowel. It is fermented by gut bacteria, which produce beneficial short-chain fatty acids like butyrate. Resistant starch can be found in legumes, some whole grains, and starchy foods that have been cooked and then cooled, such as cold rice or potatoes.

Conclusion: A Symphony of Digestion

The process of digesting starch into simple sugar is a well-coordinated sequence of events involving multiple enzymes at different stages of the human digestive tract. It starts in the mouth with salivary amylase, pauses in the acidic stomach, and is completed by a series of enzymes in the small intestine, including pancreatic amylase and brush border enzymes like maltase and glucoamylase. This multi-step breakdown is essential for converting complex carbohydrates into absorbable glucose, which is the body's primary energy source. While the majority of starch is digested, resistant starch offers additional health benefits by feeding beneficial gut microbes. For those interested in a deeper look into the enzymes involved, the NCBI's StatPearls offers a detailed overview of amylase and its role in human physiology.

Frequently Asked Questions

The primary enzyme that begins the breakdown of starch is amylase, which is first secreted in the saliva and later released by the pancreas into the small intestine.

No, significant starch digestion does not happen in the stomach. The acidic environment of the stomach inactivates salivary amylase, halting carbohydrate digestion until the food moves into the small intestine.

Amylase breaks down the long chains of starch into smaller sugar molecules, such as the disaccharide maltose, the trisaccharide maltotriose, and short glucose chains called limit dextrins.

Maltose, produced by the action of amylase, is converted into two glucose molecules by the enzyme maltase. Maltase is a brush border enzyme located on the wall of the small intestine.

After starch is completely broken down into monosaccharides (simple sugars) like glucose, they are absorbed across the intestinal wall of the small intestine and transported into the bloodstream.

Resistant starch is starch that resists digestion in the small intestine. Instead of being broken down by human enzymes, it is fermented by bacteria in the large intestine, where it acts as a prebiotic.

A deficiency in amylase can impair the body's ability to properly digest carbohydrates. This may lead to undigested carbohydrates causing symptoms like diarrhea and bloating.

Yes, plants also produce amylase. For example, during the ripening of fruit, an amylase called beta-amylase breaks down starch into maltose, contributing to the fruit's sweet flavor.