What Breaks Down Carbs Into Sugar? A Detailed Look at Digestion

November 13, 2025 •

3 min read

Over 90% of carbohydrate digestion occurs in the small intestine, but the process begins much earlier. The primary substances responsible for this transformation are a group of specialized proteins called enzymes, which break down complex carbohydrates into simple sugars for energy. This detailed look at what breaks down carbs into sugar will guide you through the process.

Quick Summary

The digestive system uses a series of enzymes, including salivary and pancreatic amylase, to break down complex carbohydrates into simple sugars like glucose, fructose, and galactose. This chemical process begins in the mouth and is completed in the small intestine, where the monosaccharides are absorbed into the bloodstream for energy.

Key Points

Salivary and pancreatic amylase: These enzymes are the primary culprits behind breaking down complex starches into smaller glucose units during digestion.
Mouth and small intestine: Digestion of carbohydrates begins in the mouth with salivary amylase but is completed in the small intestine by pancreatic and brush border enzymes.
Simple sugars: The final products of carbohydrate digestion are monosaccharides—namely glucose, fructose, and galactose—which are absorbed into the bloodstream.
Fiber is indigestible: Unlike other carbohydrates, dietary fiber cannot be broken down by human enzymes and passes through the digestive system mostly intact.
Multiple enzymes: A series of enzymes, including amylase, maltase, lactase, and sucrase, work sequentially to ensure full carbohydrate breakdown.
Regulation by hormones: Hormones like insulin help regulate what happens after digestion, storing excess glucose and controlling blood sugar levels.

The intricate process of converting carbohydrates into usable sugar is a symphony of chemical reactions, orchestrated by digestive enzymes. These enzymes act as biological catalysts, accelerating the breakdown of large, complex carbohydrates into smaller, absorbable simple sugars, primarily glucose. The entire process is a systematic journey through the digestive tract, with different enzymes taking the lead in various stages.

The Journey Begins: Oral Digestion

Carbohydrate digestion starts the moment food enters your mouth. As you chew, salivary glands release saliva, which contains the enzyme salivary amylase. This enzyme begins the initial hydrolysis, or breakdown, of long-chain starches (complex carbohydrates) into smaller glucose chains, like maltose and dextrins. However, this action is brief, as the acidic environment of the stomach quickly deactivates salivary amylase once the food is swallowed.

The Stomach: A Pause in Chemical Breakdown

Upon reaching the stomach, the chewed food, now called chyme, is subjected to the stomach's strong acids. While mechanical digestion continues, chemical digestion of carbohydrates temporarily halts because salivary amylase cannot function in the highly acidic pH of the stomach. The stomach's role at this stage is primarily to mix and sterilize the food mass before it moves on.

The Small Intestine: Where Most Action Happens

The bulk of carbohydrate digestion occurs in the small intestine, where the environment is alkaline, allowing enzymes to work efficiently.

Pancreatic Amylase

As the chyme enters the duodenum (the first part of the small intestine), the pancreas releases pancreatic amylase. This enzyme continues the breakdown of starches that salivary amylase started, reducing them into maltose and smaller glucose chains (oligosaccharides).

The Brush Border Enzymes

Finally, a set of specialized enzymes located on the microvilli (the 'brush border') of the small intestine's lining completes the digestion.

Maltase breaks down maltose into two molecules of glucose.
Sucrase breaks down sucrose (table sugar) into one molecule of glucose and one of fructose.
Lactase breaks down lactose (milk sugar) into one molecule of glucose and one of galactose.
Alpha-dextrinase digests the remaining smaller glucose chains into individual glucose units.

The Fate of Fiber

It's important to note that not all carbohydrates are broken down in this way. Dietary fiber, a type of carbohydrate, is resistant to digestive enzymes. This means it passes through the small intestine largely undigested and reaches the large intestine. Here, some fiber is fermented by beneficial gut bacteria, but it is not converted into a usable form of sugar for energy.

What Happens After Digestion?

Once broken down into simple sugars, the body absorbs them through the wall of the small intestine and into the bloodstream. From there, they are transported to the liver and then to the body's cells, where the primary sugar, glucose, is used for immediate energy or converted to glycogen for storage. Hormones like insulin and glucagon regulate this process, ensuring blood glucose levels remain stable.

Comparison of Key Digestive Enzymes for Carbohydrates


Enzyme	Origin	Primary Location of Action	Carbohydrates Digested	Products of Digestion
Salivary Amylase	Salivary glands	Mouth	Starches	Smaller glucose chains (dextrins, maltose)
Pancreatic Amylase	Pancreas	Small Intestine	Remaining starches	Maltose, maltotriose, and dextrins
Maltase	Small Intestine (Brush Border)	Small Intestine	Maltose	Glucose
Sucrase	Small Intestine (Brush Border)	Small Intestine	Sucrose	Glucose and Fructose
Lactase	Small Intestine (Brush Border)	Small Intestine	Lactose	Glucose and Galactose
Alpha-dextrinase	Small Intestine (Brush Border)	Small Intestine	Dextrins	Glucose

Conclusion

The breakdown of carbohydrates into sugar is a sophisticated process involving multiple enzymatic steps throughout the digestive system. From the initial action of salivary amylase in the mouth to the final enzymatic hydrolysis by brush border enzymes in the small intestine, each enzyme plays a critical role in converting complex carbohydrates into the simple sugars our bodies use for energy. The efficiency of this process is a testament to the body's remarkable metabolic machinery, highlighting the importance of a balanced diet rich in both complex carbohydrates for sustained energy and fiber for digestive health. To learn more about the role of enzymes in metabolism, explore reputable resources like the article on protein function by Britannica.

Frequently Asked Questions

The main enzymes that break down carbohydrates are amylase, produced in both the salivary glands and the pancreas. Salivary amylase begins the process in the mouth, while pancreatic amylase completes most of the work in the small intestine.

The majority of carbohydrate digestion occurs in the small intestine. Here, pancreatic amylase and various brush border enzymes complete the breakdown of starches and other sugars into monosaccharides.

Carbohydrate digestion begins in the mouth. Chewing breaks down food mechanically, and salivary amylase, secreted in saliva, begins the chemical breakdown of starches into smaller carbohydrates.

Dietary fiber cannot be broken down by human digestive enzymes. It passes through the stomach and small intestine largely undigested, reaching the large intestine where it can be fermented by gut bacteria.

Complex carbohydrates like starches are long chains of sugar molecules that require more enzymatic action (from amylase) to be broken down. Simple carbohydrates, such as disaccharides, are broken down more quickly by specific brush border enzymes like sucrase and lactase.

After carbohydrates are broken down into simple sugars (monosaccharides) in the small intestine, they are absorbed into the bloodstream. They are then transported to cells for immediate energy or sent to the liver and muscles for storage as glycogen.

Enzymes are highly specific. Each enzyme, such as lactase for lactose or sucrase for sucrose, has a unique shape that fits with a specific substrate (the carbohydrate it breaks down). This ensures that each type of carbohydrate is broken down efficiently.