What is the difference between lactase and sucrase?

November 18, 2025 •

3 min read

Affecting up to two-thirds of the world's population, lactose malabsorption highlights the critical role of digestive enzymes. To understand digestion at a deeper level, it's helpful to explore the fundamental distinction between lactase and sucrase, two specialized enzymes. While both help process sugars, their substrates and biological implications are distinct.

Quick Summary

This article explains the core distinctions between lactase and sucrase, including their functions, substrates (the sugars they digest), and the specific products they yield in the body. It covers how a deficiency in each enzyme can lead to different digestive disorders.

Key Points

Substrate Difference: Lactase breaks down lactose (milk sugar), while sucrase breaks down sucrose (table sugar).
Product Variation: Lactase produces glucose and galactose, whereas sucrase produces glucose and fructose.
Age-Related Decline: Lactase activity often decreases significantly after infancy, a trait known as lactase nonpersistence, while sucrase activity generally remains stable.
Different Deficiencies: A lack of lactase causes lactose intolerance, whereas a deficiency in sucrase results in Congenital Sucrase-Isomaltase Deficiency (CSID).
Symptom Triggers: The symptoms of lactase deficiency are triggered by dairy, while sucrase deficiency symptoms are triggered by table sugar and certain starches.
Digestive Location: Both enzymes perform their hydrolytic function at the brush border lining of the small intestine.

What Are Digestive Enzymes?

Digestive enzymes are biological catalysts, typically proteins, that speed up chemical reactions in the body. In the context of digestion, they break down large, complex food molecules into smaller, simpler ones that can be absorbed by the body. Without these enzymes, nutrients would pass through the digestive system unused, leading to malabsorption and other gastrointestinal issues. Lactase and sucrase are two specific examples of these enzymes, both operating in the small intestine to break down specific types of disaccharides, which are sugars made of two bonded units.

Lactase: The Dairy Disaccharide Digestor

Lactase is an enzyme produced in the brush border of the small intestine. Its primary function is the hydrolysis of lactose, a disaccharide often referred to as 'milk sugar'. Lactose is composed of one glucose molecule and one galactose molecule joined by a beta-1,4 glycosidic bond. During digestion, lactase breaks this bond, separating the lactose into its two constituent monosaccharides. These simple sugars are then easily absorbed into the bloodstream from the small intestine.

Lactose Intolerance and Lactase Deficiency

A reduced ability to produce the lactase enzyme leads to lactose intolerance, a condition affecting many adults. When undigested lactose reaches the large intestine, gut bacteria ferment it, producing gas and acids that cause common symptoms such as:

Bloating
Gas
Abdominal pain and cramps
Diarrhea

For many populations of non-European descent, this decrease in lactase production is a normal developmental process that occurs after weaning. People with lactose intolerance can manage their symptoms by limiting dairy intake, choosing lactose-free products, or using lactase enzyme supplements.

Sucrase: The Table Sugar Specialist

Like lactase, sucrase is also a digestive enzyme located on the brush border of the small intestine. Its specific role is to break down sucrose, more commonly known as table sugar. Sucrose is a disaccharide made of one glucose molecule and one fructose molecule connected by an alpha-1,2 glycosidic bond. Sucrase performs hydrolysis to cleave this bond, releasing glucose and fructose for absorption into the bloodstream. It's important to note that sucrase activity typically remains stable throughout an individual's life, unlike the age-related decline often seen with lactase.

Sucrase Deficiency and Carbohydrate Digestion

When sucrase is deficient, it results in a condition known as Congenital Sucrase-Isomaltase Deficiency (CSID). Without sufficient sucrase, sucrose and certain starches (like those in breads and potatoes) cannot be properly digested. The undigested sugars move into the large intestine, causing similar fermentation symptoms as lactose intolerance, but potentially more severe.

Symptoms of CSID can include:

Chronic, watery diarrhea
Abdominal bloating and pain
Failure to thrive in infants
Malnutrition in severe cases

This condition can be managed through a sucrose-restricted diet or enzyme replacement therapy using sacrosidase.

Side-by-Side Comparison: Lactase vs. Sucrase


Feature	Lactase	Sucrase
Substrate	Lactose (Milk Sugar)	Sucrose (Table Sugar)
Products	Glucose + Galactose	Glucose + Fructose
Function	Breaks down milk sugar	Breaks down table sugar
Glycosidic Bond	Beta-1,4 glycosidic bond	Alpha-1,2 glycosidic bond
Lifespan Activity	Often decreases after infancy	Typically remains stable throughout life
Associated Deficiency	Lactose Intolerance	Sucrase-Isomaltase Deficiency (CSID)
Dietary Sources	Milk and dairy products	Sugar cane, sugar beets, many fruits, and vegetables

The Implications of Enzyme Specificity

One of the most profound differences between lactase and sucrase is their strict substrate specificity. Lactase cannot digest sucrose, and sucrase cannot digest lactose. This is due to the precise three-dimensional structure of each enzyme's active site, which is designed to fit and act on only one specific type of molecule. This specificity explains why a person with lactose intolerance can still digest table sugar without problems, and vice versa.

Furthermore, the decline of lactase activity with age is a well-documented phenomenon, a genetic trait that persists in some populations but is lost in others. This evolutionary adaptation allowed for milk consumption into adulthood. No such widespread decline is observed for sucrase, reinforcing the differences in their biological roles and regulation.

Conclusion

Understanding the distinction between lactase and sucrase reveals the intricate specificity of the human digestive system. While both are critical enzymes for breaking down carbohydrates in the small intestine, they target different sugar molecules and produce different results. Lactase is essential for digesting lactose from dairy, while sucrase tackles sucrose from plants. Deficiencies in each enzyme lead to distinct malabsorption disorders—lactose intolerance and CSID, respectively—that require different dietary management strategies. Recognizing this specificity empowers individuals to better understand their own digestive health. For more information on enzymes and digestion, you can consult resources like the National Institutes of Health.

Frequently Asked Questions

The primary function of lactase is to break down the milk sugar lactose into the simpler sugars, glucose and galactose, which can then be absorbed by the body.

Sucrase is responsible for breaking down sucrose, or table sugar, into its constituent monosaccharides, glucose and fructose, for energy absorption.

Both lactase and sucrase are located on the brush border, a specialized surface of the intestinal cells in the small intestine, where they perform their digestive actions.

Lactose intolerance is caused by a deficiency of lactase and is triggered by dairy, while sucrose intolerance (CSID) is caused by a deficiency of sucrase and is triggered by table sugar and starches.

It is possible, though rare, to have a deficiency in both enzymes. Congenital Sucrase-Isomaltase Deficiency (CSID) specifically involves a deficiency in both sucrase and isomaltase, and some individuals may also have reduced lactase activity.

Lactase activity typically decreases in many individuals after infancy, a trait referred to as lactase nonpersistence. Sucrase activity, however, tends to remain stable throughout a person's life.

Yes, lactase deficiency can be managed with a low-lactose diet and over-the-counter lactase supplements. Sucrase deficiency (CSID) is treated with a sucrose-restricted diet and can be managed with prescription enzyme replacement therapy.

Enzymes are highly specific. The active site of the lactase enzyme has a unique shape that fits only the lactose molecule, not the sucrose molecule, which has a different structure and chemical bond.