What Breaks Down Carbohydrates to Simple Sugars?

December 7, 2025 •

3 min read

The human body is an incredible biological machine, with the digestion of carbohydrates beginning the very moment food enters the mouth. This multi-stage enzymatic process converts complex carbohydrates into simple sugars, which are the body's primary fuel source. Understanding this process provides insight into how our bodies create energy from the foods we eat.

Quick Summary

The digestion of carbohydrates into simple sugars is a multi-step enzymatic process. It begins with salivary amylase in the mouth, continues with pancreatic amylase in the small intestine, and concludes with brush border enzymes. These enzymes break down complex carbohydrates into monosaccharides for absorption.

Key Points

Enzymatic Digestion: Digestive enzymes, specifically carbohydrases, are the proteins responsible for breaking down carbohydrates into simple sugars.
Oral Digestion: The process begins in the mouth with salivary amylase, which starts breaking down complex starches into smaller molecules.
Intestinal Powerhouse: The small intestine is where the bulk of carbohydrate digestion occurs, driven by pancreatic amylase and brush border enzymes.
Specific Enzymes: Key brush border enzymes like maltase, sucrase, and lactase are responsible for breaking down specific disaccharides into monosaccharides.
Absorption: Once broken down into simple sugars, these molecules are absorbed through the intestinal wall into the bloodstream for energy.
Indigestible Fiber: Fiber is a type of carbohydrate that human digestive enzymes cannot break down, passing largely intact into the large intestine.
Energy Production: The simple sugars are transported to cells and converted into ATP, the primary energy source for the body.

The Step-by-Step Digestion of Carbohydrates

The breakdown of carbohydrates into simple sugars is a fascinating and crucial biological process that powers our bodies. This conversion is driven by a specific class of proteins known as digestive enzymes. The journey of a carbohydrate-rich food, such as a piece of bread, through the digestive system showcases a well-coordinated effort by different enzymes at different stages. This systematic breakdown ensures that carbohydrates are reduced to their smallest possible components, simple sugars like glucose, which can then be absorbed and used for energy.

Phase 1: In the Mouth

Digestion of carbohydrates starts immediately when you begin chewing. The mechanical action of chewing breaks the food into smaller pieces, increasing the surface area for enzymes to act on. Simultaneously, your salivary glands release saliva, which contains the enzyme salivary amylase.

Salivary Amylase: This enzyme initiates the hydrolysis of starch, a complex carbohydrate, by breaking the long polysaccharide chains into smaller disaccharides (like maltose) and oligosaccharides. The activity of salivary amylase, while important, is limited by the short time food spends in the mouth and the acidic environment of the stomach.

Phase 2: The Stomach and Small Intestine

After being swallowed, the food travels down the esophagus and enters the stomach, where the highly acidic conditions deactivate salivary amylase. The primary digestion of carbohydrates pauses here as the stomach focuses on protein digestion. The next major step in carbohydrate breakdown occurs in the small intestine, the main site of absorption.

Pancreatic Amylase: As the partially digested food, now called chyme, moves from the stomach into the small intestine, the pancreas secretes pancreatic amylase. This potent enzyme continues the job that salivary amylase started, breaking down remaining starches into maltose.
Brush Border Enzymes: Lining the walls of the small intestine are tiny, hair-like projections called microvilli, which are rich in specialized enzymes known as brush border enzymes. These enzymes are responsible for the final conversion of disaccharides into absorbable monosaccharides.

The Final Breakdown

Several key brush border enzymes complete the digestion of various disaccharides:

Maltase: Splits maltose into two molecules of glucose.
Sucrase: Breaks down sucrose (table sugar) into glucose and fructose.
Lactase: Hydrolyzes lactose (milk sugar) into glucose and galactose.

Comparison of Key Carbohydrate-Digesting Enzymes


Enzyme	Location of Production	Site of Action	Substrates	End Products
Salivary Amylase	Salivary Glands	Mouth	Starches (Polysaccharides)	Smaller Polysaccharides, Maltose
Pancreatic Amylase	Pancreas	Small Intestine	Starches, Glycogen	Maltose, Oligosaccharides
Maltase	Small Intestine Lining	Small Intestine	Maltose	Glucose
Sucrase	Small Intestine Lining	Small Intestine	Sucrose	Glucose, Fructose
Lactase	Small Intestine Lining	Small Intestine	Lactose	Glucose, Galactose

Absorption and Energy Use

Once the carbohydrates have been completely broken down into monosaccharides (glucose, fructose, and galactose), they are small enough to be absorbed through the wall of the small intestine and enter the bloodstream. From there, they are transported to various cells throughout the body to be used for immediate energy or sent to the liver for storage as glycogen. The body's cells convert glucose into ATP (adenosine triphosphate), the primary energy currency.

The Fate of Fiber

Not all carbohydrates are broken down in the same manner. Dietary fiber, a type of carbohydrate, is indigestible by human enzymes. It passes largely intact through the small intestine and into the large intestine, where it provides bulk to stool and can be fermented by intestinal bacteria, contributing to gut health.

The Complexity of Digestion

The entire process of carbohydrate digestion highlights the body's efficiency and complexity. From the initial enzymatic action in the mouth to the final conversion and absorption in the small intestine, each step is crucial for providing the body with the necessary energy. While the journey is a continuous one, the specific enzymes at each stage play a unique and vital role in breaking down complex carbohydrates into the simple sugars our cells can readily use. This precise enzymatic action ensures proper nutrient absorption and energy production, underpinning overall metabolic health.

For additional scientific context on these processes, you can reference resources like the National Center for Biotechnology Information (NCBI).

Frequently Asked Questions

The main enzymes that break down carbohydrates are a group called carbohydrases. Amylase is a key carbohydrase that begins the digestion of starches in the mouth and continues in the small intestine.

Carbohydrate digestion begins in the mouth with salivary amylase. It pauses in the stomach and concludes in the small intestine, where monosaccharides are absorbed.

Simple sugars, or monosaccharides, are absorbed through the epithelial cells lining the microvilli of the small intestine. From there, they are transported into the bloodstream.

Human digestive enzymes cannot break down fiber. It passes through the small intestine largely undigested and is instead fermented by bacteria in the large intestine.

The pancreas releases pancreatic amylase into the small intestine. This powerful enzyme is crucial for continuing the breakdown of starches into maltose and other smaller sugars.

Brush border enzymes are specialized enzymes, including lactase, sucrase, and maltase, that are located on the surface of the microvilli in the small intestine. They complete the breakdown of disaccharides.

Without the specific enzymes to break down carbohydrates, the large molecules would not be able to pass into the bloodstream for absorption. The body would not be able to efficiently extract energy from carbohydrates.