How Lactose Is Digested to Glucose and Galactose Before It Is Absorbed into the Blood

December 25, 2025 •

3 min read

Over 65% of the global population has a reduced ability to digest lactose after infancy. For those who can, the process is a well-coordinated sequence where lactose is digested to glucose and galactose before it is absorbed into the blood.

Quick Summary

The disaccharide lactose must first be broken down by the enzyme lactase into the simple sugars glucose and galactose in the small intestine before these monosaccharides can be absorbed.

Key Points

Enzymatic Digestion: The enzyme lactase breaks down the disaccharide lactose into the monosaccharides glucose and galactose in the small intestine.
Small Intestine Action: The small intestine is the crucial site where lactose is digested into absorbable forms and then transported into the bloodstream.
Monosaccharides are Absorbed: Only simple sugar units like glucose and galactose can be absorbed through the intestinal walls, not the larger lactose molecule.
Lactose Intolerance Cause: A deficiency of the lactase enzyme prevents the proper digestion of lactose, leading to its fermentation by bacteria in the large intestine.
Transport to Liver: Once absorbed into the bloodstream, glucose and galactose are transported to the liver for further metabolic processing.
Distinct Processes: Digestion (breaking down) and absorption (uptake) are two distinct and sequential stages in the overall process of nutrient utilization.

The Journey of Carbohydrates: From Mouth to Small Intestine

Carbohydrate digestion is a multi-stage process that begins the moment food enters your mouth. Chewing, or mastication, physically breaks down food into smaller pieces, while salivary glands release the enzyme salivary amylase. This enzyme starts the chemical breakdown of complex carbohydrates, like starches, into smaller chains and sugars such as maltose. This initial digestion is limited, however, as the process is halted once the food reaches the acidic environment of the stomach, which inactivates salivary amylase.

Stomach's Role in Digestion

While the stomach's primary role is to churn and mix food with stomach acid, it does not chemically digest carbohydrates. The carbohydrates pass through the stomach largely unaltered until they reach the next major stage of digestion in the small intestine.

Lactose Digestion in the Small Intestine

The small intestine is the primary site for the digestion and absorption of most nutrients. Upon entering, carbohydrates are met with pancreatic amylase, which further breaks down starches. Most importantly for lactose, the intestinal cells lining the small intestine, called enterocytes, have a 'brush border' rich with specific enzymes.

The Critical Role of the Lactase Enzyme

Among these enzymes is lactase, the key player responsible for breaking down lactose. Lactose is a disaccharide, meaning it is composed of two sugar units bonded together. The lactase enzyme acts on the lactose molecule, hydrolyzing the bond to separate it into its two constituent monosaccharides: glucose and galactose. This breakdown is a prerequisite for absorption, as the body can only absorb these simple, single sugar units directly into the bloodstream.

If an individual has a lactase deficiency, as is the case with lactose intolerance, the undigested lactose travels to the large intestine. There, gut bacteria ferment the lactose, producing gases and other byproducts that cause common symptoms like bloating, pain, and diarrhea.

The Absorption of Monosaccharides

After the disaccharides and starches have been broken down into their simplest forms, the monosaccharides are ready for absorption into the bloodstream. The enterocytes of the small intestine have specific transport proteins for this purpose.

Active Transport: Glucose and galactose are absorbed against their concentration gradient via a sodium-glucose cotransporter, known as SGLT1. This process requires energy and ensures efficient uptake of these crucial fuel sources.
Facilitated Diffusion: Fructose, another monosaccharide resulting from sucrose digestion, is absorbed through facilitated diffusion via the GLUT5 transporter. This process does not require energy.

Once inside the enterocytes, all three monosaccharides—glucose, galactose, and fructose—pass through another set of transporters (like GLUT2) on the basolateral membrane to enter the bloodstream. They are then transported to the liver via the portal vein for further processing. In the liver, galactose and fructose are largely converted into glucose, ensuring that glucose becomes the primary sugar circulating in the blood.

Digestion vs. Absorption: A Comparison


Aspect	Digestion	Absorption
Definition	The chemical and mechanical breakdown of complex food molecules into smaller, simpler ones.	The uptake of these simple molecules from the digestive tract into the bloodstream.
Location for Lactose	Primarily the small intestine's brush border, via the lactase enzyme.	Primarily the cells lining the small intestine, using specific transport proteins.
Final Product	The disaccharide lactose is broken down into monosaccharides (glucose and galactose).	Monosaccharides (glucose, galactose, fructose) enter the bloodstream.
Mechanism	Enzymatic hydrolysis breaks chemical bonds.	Transport proteins move molecules across cell membranes.
Requirement	Sufficient lactase enzyme activity is required for lactose digestion.	Functional transport proteins in the intestinal lining are essential for absorption.

The Fate of Indigestible Carbohydrates

Not all carbohydrates are digested and absorbed in the small intestine. Fiber, for instance, cannot be broken down by human digestive enzymes. It passes into the large intestine, where it is fermented by gut bacteria. This process produces beneficial short-chain fatty acids and some gas, contributing to overall gut health.

Conclusion

In summary, the statement that lactose is digested to glucose and galactose before it is absorbed into the blood is accurate. This essential process, driven by the enzyme lactase in the small intestine, ensures that the body receives the simple sugar fuel it needs from dairy products. Without this critical digestive step, the disaccharide lactose would remain indigestible, leading to the discomfort associated with lactose intolerance. The final destination for these absorbed monosaccharides is the liver, where they are further managed to regulate blood sugar levels and provide energy throughout the body. For more detailed information on carbohydrate digestion, consult reputable sources like Healthline on Carbohydrate Digestion.

Frequently Asked Questions

The primary enzyme responsible for digesting lactose is lactase. It is produced by the cells lining the small intestine and breaks lactose down into glucose and galactose.

The absorption of glucose and galactose occurs in the small intestine. Specific transport proteins in the intestinal wall move these simple sugars into the bloodstream.

In individuals with lactose intolerance, undigested lactose travels to the large intestine. There, resident bacteria ferment it, which can cause symptoms such as bloating, gas, and diarrhea.

No. While glucose and galactose are the end products of lactose digestion, other carbohydrates like starch and sucrose are also broken down into other monosaccharides, including fructose, before absorption.

Lactase production is typically highest during infancy. In most people, lactase activity gradually decreases after childhood, leading to a reduced ability to digest lactose in adulthood.

After absorption, monosaccharides travel to the liver. The liver then converts galactose and fructose into glucose and regulates the amount of glucose that is released into the bloodstream.

No, the human body cannot absorb the disaccharide lactose directly. It is too large a molecule to pass through the intestinal wall and must first be broken down into its smaller monosaccharide components, glucose and galactose.