What enzyme digests lactose in the body?

December 10, 2025 •

3 min read

Over 65% of the global population has a reduced ability to digest lactose after infancy. The enzyme that digests lactose in the body is called lactase, and its insufficient production leads to uncomfortable symptoms associated with lactose intolerance.

Quick Summary

The small intestine produces lactase, which breaks down lactose into absorbable simple sugars. A deficiency in this enzyme, known as lactose intolerance, causes digestive discomfort from undigested milk sugar.

Key Points

Lactase is the enzyme: The specific enzyme responsible for digesting lactose is called lactase.
Produced in the small intestine: Lactase is manufactured in the brush border of the small intestine.
Breaks down lactose: Its primary job is to break down the disaccharide lactose into the simple sugars glucose and galactose.
Lactase deficiency causes intolerance: A reduced production of lactase results in lactose intolerance, leading to digestive issues.
Undigested lactose ferments: In the absence of enough lactase, unabsorbed lactose travels to the large intestine, where bacteria ferment it and cause symptoms like bloating and gas.
Supplements can help: Over-the-counter lactase supplements are available to aid in the digestion of lactose for those with a deficiency.
Not a food allergy: Lactose intolerance is not the same as a milk allergy, which is an immune response to milk proteins.

The Crucial Role of Lactase in Digestion

Lactase is a digestive enzyme primarily found on the brush border of the small intestine. Its essential function is to break down lactose, a complex sugar (disaccharide) found in milk and dairy products, into two simpler sugars: glucose and galactose. These simple sugars are then absorbed through the intestinal walls into the bloodstream for energy. Without sufficient lactase, this process is disrupted, leading to the condition known as lactose intolerance.

The Breakdown Process of Lactose

When you consume dairy products, the journey of lactose begins in the small intestine. Here, lactase acts as a catalyst for a hydrolysis reaction, splitting the lactose molecule ($$C{12}H{22}O_{11}$$) into its two constituent monosaccharides, glucose and galactose. This enzymatic action is crucial for proper absorption. If the lactase enzyme is not present in adequate amounts, the lactose remains intact and passes undigested into the large intestine.

What Happens During Lactase Deficiency?

In the large intestine, undigested lactose meets with resident bacteria. These bacteria ferment the sugar, a process that produces a range of byproducts, including:

Gases: Methane, hydrogen, and carbon dioxide.
Acids: Short-chain fatty acids.
Water Attraction: The unabsorbed lactose creates an osmotic load, drawing water into the colon.

This fermentation and water attraction are the direct causes of the classic symptoms associated with lactose intolerance, such as bloating, gas, cramps, and diarrhea. The severity of these symptoms can depend on various factors, including the amount of lactose ingested and the individual's remaining lactase activity.

Different Types of Lactase Deficiency

Lactase deficiency isn't a one-size-fits-all condition. There are several types, each with a different cause:

Primary Lactase Deficiency (Lactase Nonpersistence): The most common type, it is a genetically determined reduction in lactase production that occurs after infancy. This is considered the norm for the majority of the world's population.
Secondary Lactase Deficiency: This results from an injury or disease affecting the small intestine, such as gastroenteritis, celiac disease, or Crohn's disease. Healing the underlying condition can sometimes restore lactase levels.
Congenital Lactase Deficiency (Congenital Alactasia): An extremely rare inherited disorder where infants are born with little to no lactase production.

Lactase Supplements and Managing Symptoms

For individuals with a lactase deficiency, managing symptoms often involves dietary changes and, in many cases, the use of lactase supplements. These supplements provide the enzyme that the body lacks, assisting in the digestion of lactose and preventing the unpleasant symptoms that would otherwise occur. They can be taken in pill form just before consuming dairy or added as drops to milk before drinking it.

Lactase vs. Intolerance vs. Allergy

It is important to differentiate between lactase deficiency, lactose intolerance, and a milk allergy. They are distinct conditions with different mechanisms and effects on the body. A comparison table can help illustrate these differences:


Feature	Lactase Deficiency	Lactose Intolerance	Milk Allergy
Cause	Insufficient production of the lactase enzyme in the small intestine.	Symptoms caused by the inability to digest lactose, stemming from lactase deficiency.	An immune system reaction to the proteins in milk, not the sugar.
Mechanism	The sugar lactose is not properly hydrolyzed into glucose and galactose.	Undigested lactose ferments in the colon, causing gas and digestive issues.	The immune system releases histamines and other chemicals, leading to allergic symptoms.
Symptoms	No inherent symptoms; the deficiency itself is asymptomatic, but it leads to intolerance symptoms.	Bloating, gas, cramps, and diarrhea.	Hives, swelling, wheezing, vomiting, or anaphylaxis.
Management	Often lifelong, managed through dietary changes and enzyme supplements.	Symptom management via diet and lactase supplements.	Complete avoidance of all milk and milk products.

Conclusion: The Critical Role of Lactase

The enzyme that digests lactose in the body is lactase, and its function is vital for processing dairy products. For individuals with a deficiency, understanding the role of lactase is the first step toward effective management. The advent of lactase supplements offers a practical solution, allowing many to enjoy dairy without severe gastrointestinal distress. While most of the world naturally produces less lactase after infancy, it is a manageable condition with the right knowledge and dietary strategies. For persistent or severe symptoms, consulting a healthcare provider is the best course of action to ensure proper diagnosis and nutritional adequacy.

NCBI Bookshelf: Lactose Intolerance

Frequently Asked Questions

The primary function of lactase is to break down lactose, a complex sugar found in milk, into the simpler sugars glucose and galactose, which can then be absorbed into the bloodstream for energy.

Lactase is produced and located on the brush border, a specialized lining of cells, in the small intestine.

If a person doesn't produce enough lactase, undigested lactose passes into the large intestine, where it is fermented by bacteria, causing symptoms such as bloating, gas, cramps, and diarrhea.

No, lactase deficiency and lactose intolerance are not the same as a milk allergy. Lactose intolerance is a digestive issue, while a milk allergy is an immune system reaction to milk proteins.

Yes, lactase supplements containing the lactase enzyme can be taken before consuming dairy products to help break down the lactose and prevent or reduce symptoms of intolerance.

Common symptoms of lactose intolerance include abdominal cramps, bloating, gas, nausea, and diarrhea, typically occurring 30 minutes to 2 hours after consuming dairy.

No, the severity of lactose intolerance symptoms varies widely among individuals, depending on the amount of lactase their body produces and the quantity of lactose they consume.