Nutrition Diet: Understanding What Enzyme Absorbs Carbs

October 14, 2025 •

4 min read

Approximately 5% of carbohydrate digestion begins in the mouth, but the majority of the work happens later in the small intestine. For those asking what enzyme absorbs carbs?, the answer involves a multi-step process that differentiates digestion (breaking down food with enzymes) from actual nutrient absorption (transporting nutrients into the bloodstream).

Quick Summary

Carbohydrate digestion is a multi-step process driven by enzymes like amylase and brush border enzymes, which break down complex carbs into simple sugars. These monosaccharides are then absorbed into the bloodstream using specialized protein transporters in the small intestine.

Key Points

Enzymes Digest, Transporters Absorb: The key distinction is that enzymes like amylase and lactase break down large carbohydrate molecules, while specialized protein transporters on the intestinal wall, such as SGLT1 and GLUT2, are responsible for their absorption into the bloodstream.
Amylase is the Primary Initial Enzyme: Digestion starts with salivary amylase in the mouth and is continued by pancreatic amylase in the small intestine, breaking down starches into simpler sugar compounds.
Final Breakdown Occurs at the Brush Border: Specialized enzymes like maltase, sucrase, and lactase are located on the intestinal wall (the brush border) to perform the final hydrolysis of disaccharides into absorbable monosaccharides.
Active and Passive Transport: Glucose and galactose are absorbed via the active transport protein SGLT1, while fructose is absorbed via facilitated diffusion using GLUT5. All three can use the GLUT2 transporter to exit the cell into the bloodstream.
Dietary Factors are Crucial: The type of carbohydrates consumed, especially the presence of fiber, significantly impacts the efficiency of digestion and absorption, influencing metabolic responses like blood sugar levels.

The Journey of a Carbohydrate: From Digestion to Absorption

To understand carbohydrate absorption, it is crucial to first clarify a common misconception: no single enzyme directly absorbs carbohydrates. Instead, a series of digestive enzymes first breaks down complex carbohydrates (like starches) into simple sugars (monosaccharides). These tiny molecules are then absorbed into the bloodstream by specialized transport proteins. This journey is a finely tuned process that begins the moment food enters your mouth.

The Digestion Journey Begins: From Mouth to Stomach

Chemical digestion of carbohydrates starts in the mouth with the action of salivary amylase. When you chew starchy foods like bread or crackers, the salivary glands release this enzyme, which begins to hydrolyze, or break down, the complex starch molecules into smaller sugar chains. This is why chewing a starchy food for a while can make it taste slightly sweet. However, this initial digestive phase is limited due to the brief amount of time food stays in the mouth. Once swallowed, the food—now a partially digested bolus—travels to the stomach, where the acidic environment deactivates the salivary amylase, halting its function. No significant carbohydrate digestion occurs in the stomach; the main action awaits in the small intestine.

The Small Intestine: The Central Hub for Digestion

The small intestine is where the bulk of carbohydrate digestion and all nutrient absorption occurs. As the partially digested food, now called chyme, moves from the stomach into the small intestine, it is met with a new arsenal of enzymes.

A Team of Enzymes: Pancreatic Amylase and the Brush Border

First, the pancreas releases a powerful digestive enzyme called pancreatic amylase into the small intestine. This enzyme continues the work of breaking down starches and glycogen into smaller carbohydrate fragments, primarily disaccharides (two sugar units) like maltose. The final stage of digestion takes place at the brush border—the dense, microvilli-lined surface of the intestinal walls. This surface is equipped with several crucial enzymes, each responsible for breaking down a specific type of disaccharide into its simplest, absorbable form.

Maltase: This enzyme breaks down maltose into two molecules of glucose, which is the body's primary energy source.
Sucrase-isomaltase: This complex enzyme breaks down sucrose (table sugar) into glucose and fructose, while also acting on other starches to produce glucose.
Lactase: For many people, this enzyme breaks down lactose (milk sugar) into glucose and galactose. However, in most adults, lactase production declines, leading to lactose intolerance.

From Digestion to Absorption: The Role of Transport Proteins

After these brush border enzymes have completed their work, the simple sugars—glucose, fructose, and galactose—are ready to be absorbed. This is where the absorption stage, distinct from enzymatic digestion, occurs. The monosaccharides are transported from the intestinal lumen, across the intestinal cell wall, and into the bloodstream. This is facilitated by specific transport proteins rather than enzymes.

The Mechanism of Carbohydrate Absorption

Absorption relies on different types of protein carriers:

Glucose and Galactose: These monosaccharides are actively transported into intestinal cells by the sodium-glucose co-transporter, or SGLT1. This process requires energy and involves the movement of sodium ions along with the sugar molecule. Once inside the intestinal cell, they exit into the bloodstream via a protein called GLUT2.
Fructose: This sugar uses a separate, facilitated diffusion transporter known as GLUT5 to enter the intestinal cells. It then uses GLUT2 to exit into the bloodstream.

This precise transport system ensures that only the most basic building blocks of carbohydrates enter the bloodstream, where they can be transported to the liver for processing and then distributed to the body's cells for energy.

The Importance of a Balanced Diet

A balanced diet rich in whole foods, which contain fiber and other nutrients, supports the efficient functioning of this entire digestive process. Dietary fiber, a type of carbohydrate that cannot be broken down by human digestive enzymes, passes through the small intestine largely undigested. It promotes gut health and aids in the regulation of blood sugar levels. In contrast, a diet high in processed, refined sugars can overwhelm the system, leading to rapid blood sugar spikes. For individuals with specific enzyme deficiencies, such as lactose intolerance or congenital sucrase-isomaltase deficiency (CSID), dietary adjustments or enzyme supplements may be necessary.

Conclusion

In summary, while the question of what enzyme absorbs carbs is a common one, the process is far more complex. The final absorption of simple sugars into the bloodstream is not performed by digestive enzymes but by specialized transport proteins like SGLT1 and GLUT2. This absorption is the culmination of a multi-stage digestive process that begins in the mouth and is completed by a team of enzymes in the small intestine, including amylase, maltase, sucrase, and lactase. By understanding this intricate system, we can better appreciate the vital role of nutrition in supporting overall health and bodily functions.


Enzyme	Location	Primary Function	Substrate	Final Product
Salivary Amylase	Mouth	Initial breakdown of starches into smaller glucose chains	Starches	Maltose, dextrins, maltotriose
Pancreatic Amylase	Small Intestine	Major digestion of starches into disaccharides	Starches, Glycogen	Maltose, maltotriose, dextrins
Maltase	Small Intestine (Brush Border)	Breaks down maltose into two glucose molecules	Maltose	Glucose
Lactase	Small Intestine (Brush Border)	Breaks down lactose into glucose and galactose	Lactose	Glucose, Galactose
Sucrase-Isomaltase	Small Intestine (Brush Border)	Breaks down sucrose, some starches	Sucrose, Maltose	Glucose, Fructose
SGLT1 (Transport Protein)	Small Intestine (Cell Membrane)	Absorbs Glucose and Galactose into cells	Glucose, Galactose	N/A (Carrier)
GLUT2 (Transport Protein)	Small Intestine (Cell Membrane)	Absorbs all simple sugars into bloodstream	Glucose, Galactose, Fructose	N/A (Carrier)

(For further reading on the mechanisms of carbohydrate absorption, see: The Role of SGLT1 and GLUT2 in Intestinal Glucose Transport and Sensing)

Frequently Asked Questions

Amylase is the primary enzyme that breaks down starches. It is released in two forms: salivary amylase in the mouth and pancreatic amylase in the small intestine.

The majority of carbohydrate digestion occurs in the small intestine, where pancreatic amylase and specific brush border enzymes work to break down carbohydrates into simple sugars.

Salivary amylase, which starts digestion in the mouth, is inactivated by the acidic environment of the stomach. Therefore, no significant carbohydrate digestion occurs in the stomach itself.

Simple sugars are absorbed by protein transporters in the walls of the small intestine. Glucose and galactose use the SGLT1 and GLUT2 transporters, while fructose uses the GLUT5 and GLUT2 transporters.

Digestion is the chemical process of breaking down large food molecules into smaller ones using enzymes. Absorption is the process of transporting these smaller, simple nutrients from the intestinal lumen across the intestinal wall into the bloodstream.

Fiber is a complex carbohydrate that human digestive enzymes cannot break down. It passes through the digestive tract largely undigested and is eventually fermented by bacteria in the colon.

People with lactose intolerance have a deficiency of the lactase enzyme, which breaks down milk sugar. Without enough lactase, the lactose is not properly digested and can cause gastrointestinal issues.