Lactase: What Splits Lactose into Glucose and Galactose?

January 11, 2026 •

2 min read

An estimated 65% of the global adult population has a reduced ability to digest lactose after infancy, a condition known as lactase nonpersistence. This happens because the enzyme lactase, which splits lactose into glucose and galactose, declines in production over time.

Quick Summary

Lactase is the enzyme responsible for splitting the milk sugar lactose into the simple sugars glucose and galactose through a hydrolysis reaction in the small intestine.

Key Points

Lactase is the enzyme: The enzyme lactase, or beta-galactosidase, is solely responsible for splitting lactose into glucose and galactose.
It works through hydrolysis: Lactase breaks the bond between glucose and galactose by adding a water molecule in a process called hydrolysis.
Digestion occurs in the small intestine: In humans, lactase is produced at the brush border of the small intestine, where it breaks down lactose so the resulting sugars can be absorbed.
Lactose intolerance is caused by deficiency: When there's not enough lactase, undigested lactose ferments in the large intestine, causing gas, bloating, and diarrhea.
Production typically declines with age: Most humans experience a genetically determined decline in lactase production after infancy, known as lactase nonpersistence.
Industrial application is common: Microbial beta-galactosidases are widely used in the food industry to produce lactose-free dairy products.

The Role of Lactase in Digestion

Lactose, a disaccharide in dairy products, is composed of glucose and galactose. For the body to absorb it, lactose must be broken down by the enzyme lactase, also called beta-galactosidase. Produced by the cells in the small intestine's brush border, lactase catalyzes the hydrolysis of lactose. This process uses water to break the bond between glucose and galactose. The resulting monosaccharides are then absorbed into the bloodstream.

Lactase Deficiency and Lactose Intolerance

A deficiency in lactase leads to lactose intolerance. Undigested lactose reaches the large intestine, where bacteria ferment it, producing gas and fatty acids. This fermentation causes symptoms like bloating, pain, gas, and diarrhea.

Types of Lactase Deficiency

Primary: The most common type, where lactase production decreases after infancy. This decline is often genetic.
Secondary: A temporary condition caused by illness or injury to the small intestine.
Congenital: A rare genetic disorder resulting in little to no lactase production from birth.
Developmental: Temporary in premature infants due to an underdeveloped small intestine.

Factors Influencing Lactase Activity

Lactase activity is affected by environmental factors, including:

Temperature: Optimal activity for human lactase is around 37°C. High temperatures can cause denaturation.
pH Level: Human lactase works best in the small intestine's neutral environment (pH 6-7). Industrial versions can be more pH resilient.
Substrate Concentration: Activity increases with lactose concentration until the enzyme is saturated.
Enzyme Concentration: Higher lactase concentration increases the reaction rate.
Inhibitors: Substances like galactose can reduce lactase activity.

Human vs. Industrial Lactase


Feature	Human Lactase (LCT)	Industrial Beta-Galactosidase
Source	Small intestine brush border cells.	Microorganisms (yeast, fungi).
Optimal pH	Neutral to slightly acidic (approx. 6-7).	Varies by source (acidic for fungi, neutral for yeast).
Optimal Temperature	Human body temperature (approx. 37°C).	Varies by source, often higher for efficiency.
Application	Digests lactose for absorption.	Creates lactose-free dairy products.
Persistence	Production declines in most adults.	Stable and reusable.

Conclusion

Lactase is the enzyme that hydrolyzes lactose into glucose and galactose, enabling absorption. A lack of lactase causes lactose intolerance. Beyond human digestion, microbial beta-galactosidase is used industrially to make lactose-free dairy products. Enzyme activity is influenced by factors like temperature and pH, and the natural decline in human lactase production highlights a gene-nutrition interaction.

For more detailed information on the function of lactase and its role in digestion, see the MedlinePlus resource on the LCT gene.

Frequently Asked Questions

If a person doesn't produce enough lactase, they are lactose intolerant. The undigested lactose travels to the large intestine where bacteria ferment it, producing gases and causing symptoms like bloating, cramps, and diarrhea.

The body produces lactase in the small intestine, specifically at the tips of the small, finger-like projections called microvilli that form the brush border of the intestinal lining.

Yes, lactase is a type of beta-galactosidase. While lactase specifically refers to the enzyme that hydrolyzes lactose, beta-galactosidase is a broader term for a family of enzymes that break down beta-galactosides.

Yes, there are several types, including primary (age-related decline), secondary (due to intestinal injury), congenital (genetic absence from birth), and developmental (temporary in premature babies).

Industrially, beta-galactosidase is used to hydrolyze lactose in dairy products like milk and yogurt to create lactose-free versions. The enzyme is typically derived from yeast or fungal sources.

Lactase is highly sensitive to its environment. Optimal human lactase activity occurs at body temperature and a neutral pH. Extreme temperatures or pH levels can reduce its activity or even cause it to denature.

Yes, for many people, taking lactase enzyme supplements before consuming dairy products can help break down the lactose, thus preventing the uncomfortable symptoms of lactose intolerance.