Which Carbohydrates Can Be Absorbed from the Small Intestine?

January 13, 2026 •

3 min read

According to the Canadian Sugar Institute, dietary carbohydrates must be broken down into their single-unit form, monosaccharides, before they can be absorbed into the bloodstream. This crucial step in the digestive process determines which carbohydrates can be absorbed from the small intestine and utilized for energy.

Quick Summary

The small intestine absorbs only simple sugars, specifically glucose, fructose, and galactose, into the bloodstream. Complex carbohydrates must first be broken down by enzymes during digestion to become these absorbable monosaccharides.

Key Points

Only Monosaccharides Can Be Absorbed: The small intestine can only absorb single-unit sugars like glucose, fructose, and galactose into the bloodstream.
Enzymes Break Down Complex Carbs: Polysaccharides (like starch) and disaccharides (like lactose, sucrose) must first be digested into monosaccharides by enzymes like amylase, lactase, and sucrase.
Digestion Occurs in Stages: Carbohydrate digestion starts in the mouth and continues in the small intestine, where the majority of enzymatic breakdown and absorption happens.
Transport Proteins Are Key: Specialized protein transporters, including SGLT1 for glucose/galactose and GLUT5 for fructose, facilitate the movement of monosaccharides across the intestinal wall.
Fiber is Not Absorbed: Indigestible carbohydrates like dietary fiber pass through the small intestine and are not absorbed into the bloodstream. They are fermented by bacteria in the large intestine instead.
The Liver Processes Absorbed Sugars: Once absorbed, monosaccharides travel to the liver, where fructose and galactose are converted into glucose for use or storage.
Active and Passive Transport Used: The absorption of monosaccharides involves both active transport (for glucose and galactose) and facilitated diffusion (for fructose).

The Essentials of Carbohydrate Absorption

Carbohydrate absorption is a complex and highly efficient process that occurs primarily in the small intestine. It is a critical function for providing the body with energy. For a carbohydrate to cross the intestinal wall and enter the bloodstream, it must be in its simplest form, a monosaccharide, or single sugar unit. The body's digestive system, starting in the mouth and continuing through the small intestine, employs various enzymes to break down more complex carbohydrates into these tiny, absorbable units.

The Three Absorbable Monosaccharides

The digestive process ultimately yields three primary monosaccharides that are ready for absorption into the bloodstream. These include:

Glucose: The most abundant monosaccharide in the body and a primary source of energy. It is derived from the breakdown of starches, sucrose, and maltose.
Fructose: A simple sugar found in fruits, vegetables, and honey. It is absorbed into the enterocytes of the small intestine via facilitated diffusion.
Galactose: A monosaccharide that is a component of lactose, or milk sugar. It is actively transported into the enterocytes alongside glucose.

How Complex Carbohydrates Become Absorbable

Polysaccharides (like starch) and disaccharides (like lactose and sucrose) are too large to be absorbed directly through the intestinal wall. They undergo a series of enzymatic breakdown steps:

Starch: Digestion begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase. This breaks down starch into smaller disaccharides like maltose. Maltase, an enzyme on the surface of the intestinal cells (the brush border), then breaks down maltose into two molecules of glucose.
Lactose: The enzyme lactase, also found on the brush border, breaks lactose down into glucose and galactose. A deficiency of this enzyme is what causes lactose intolerance.
Sucrose: Sucrase, another brush border enzyme, splits sucrose into its two monosaccharide components, glucose and fructose.

Comparing Different Carbohydrate Absorption Pathways


Carbohydrate Type	Form in Food	How It's Digested	Absorbed Form(s)	Absorption Mechanism
Monosaccharide	Glucose, Fructose, Galactose	None (already in absorbable form)	Glucose, Fructose, Galactose	Facilitated Diffusion & Active Transport
Disaccharide	Sucrose, Lactose, Maltose	Digested by brush border enzymes (sucrase, lactase, maltase)	Glucose, Fructose, Galactose	Facilitated Diffusion & Active Transport
Polysaccharide	Starch, Glycogen	Digested by amylase (salivary and pancreatic), then maltase	Glucose	Facilitated Diffusion & Active Transport
Dietary Fiber	Cellulose, Pectin	Not digested by human enzymes	N/A (fermented in large intestine)	N/A

The Role of Specialized Transport Proteins

Once digestion is complete, the resulting monosaccharides must cross the membranes of the small intestine's absorptive cells (enterocytes). This is done by specialized transport proteins. Glucose and galactose are transported using a sodium-dependent active transport system (SGLT1), which moves them against their concentration gradient. Fructose, on the other hand, is absorbed via facilitated diffusion, a passive process that does not require energy (GLUT5). All three monosaccharides exit the enterocyte into the bloodstream through the GLUT2 transporter.

Indigestible Carbohydrates and Their Fate

Not all carbohydrates are created equal in the digestive process. Dietary fiber, a type of complex carbohydrate found in plant foods, cannot be broken down by human digestive enzymes. This is because humans lack the necessary enzymes to digest the specific bonds within fiber. Instead of being absorbed, fiber continues its journey to the large intestine. Here, it is fermented by gut microbes, a process that can produce short-chain fatty acids and other substances. This fermentation process plays an important role in gut health, though it does not contribute to the body's primary energy supply in the same way as monosaccharides.

Conclusion: The Final Word on Absorption

The fundamental principle of carbohydrate absorption is that only monosaccharides are small enough to be absorbed into the bloodstream from the small intestine. Glucose, fructose, and galactose are the end products of carbohydrate digestion, regardless of whether the original source was a simple sugar or a complex starch. This intricate and highly regulated process, involving multiple enzymes and specialized transport proteins, ensures that the body receives the necessary fuel for energy. Understanding this mechanism is key to comprehending how nutrition is assimilated and used by the body. For a more detailed look into this topic, you can explore resources on carbohydrate digestion and absorption via biology education websites like King of the Curve, or official health and nutrition organization websites.

Frequently Asked Questions

No, starch is a complex carbohydrate (polysaccharide) and is too large to be absorbed directly. It must first be broken down by digestive enzymes, like amylase, into smaller units and eventually into glucose before it can be absorbed.

Monosaccharides (single-unit sugars) like glucose are the final absorbable form of carbohydrates. Disaccharides (two-unit sugars) like sucrose and lactose are too large and must be broken down by specific enzymes into monosaccharides at the brush border of the small intestine before absorption can occur.

Dietary fiber is not absorbed because the human digestive system lacks the necessary enzymes to break it down into monosaccharides. It passes through the small intestine and is eventually fermented by bacteria in the large intestine.

After absorption into the bloodstream, monosaccharides travel to the liver. The liver converts fructose and galactose into glucose, which can then be used for energy, stored as glycogen in muscles and the liver, or distributed throughout the body.

No, their absorption mechanisms differ slightly. Glucose and galactose are absorbed via active transport (SGLT1 transporter), which requires energy, while fructose is absorbed through facilitated diffusion (GLUT5 transporter), a passive process that doesn't use energy.

Individuals with lactose intolerance do not produce enough of the lactase enzyme, which is needed to break down lactose. As a result, lactose passes undigested into the large intestine, where it is fermented by bacteria, causing symptoms like bloating and diarrhea.

All three monosaccharides—glucose, fructose, and galactose—exit the intestinal cells and enter the bloodstream through a specific protein transporter called GLUT2, which is located on the basolateral membrane of the cells.