What enzyme is used to break down milk? A guide to lactase and lactose intolerance

December 26, 2025 •

4 min read

Lactase production naturally declines in most humans after infancy, leading to a common inability to digest the lactose in milk. The enzyme used to break down milk, called lactase, is crucial for separating the milk sugar into absorbable components, preventing digestive issues. This vital process is the focus of understanding both healthy digestion and the root cause of lactose intolerance.

Quick Summary

Lactase, also known as beta-galactosidase, is the enzyme that breaks down lactose, the sugar found in milk, into glucose and galactose. A deficiency in lactase is the primary cause of lactose intolerance, leading to gastrointestinal symptoms after consuming dairy products. Commercial lactase supplements can assist digestion for those affected.

Key Points

Lactase is the Key Enzyme: The primary enzyme responsible for breaking down milk's sugar, lactose, is lactase.
Lactose and Lactase are Different: Lactose is the milk sugar, while lactase is the enzyme that digests it.
Lactase Deficiency Causes Intolerance: A lack of sufficient lactase enzyme leads to lactose intolerance and its associated gastrointestinal symptoms.
Lactase Declines with Age: Most people naturally produce less lactase after infancy, a condition known as lactase non-persistence.
Commercial Sources are Available: Lactase can be taken as a supplement or found in pre-treated, lactose-free dairy products.
Bacteria Ferment Undigested Lactose: In the absence of lactase, lactose reaches the large intestine where bacteria ferment it, causing gas, bloating, and other discomfort.
Multiple Enzymes Digest Milk: While lactase handles the sugar, other enzymes like pepsin, proteases, and lipases break down milk's proteins and fats.

The role of lactase in digesting lactose

Lactase is a brush border enzyme, meaning it is produced by and located on the surface of the cells lining the small intestine, specifically the intestinal epithelial cells. Its primary function is to break down the disaccharide lactose into its two constituent monosaccharides: glucose and galactose. These simpler sugars can then be easily absorbed into the bloodstream for energy. In most human populations, the production of lactase peaks during infancy when milk is the primary food source and then naturally declines after the weaning phase. This decline is known as lactase non-persistence, and it is the most common form of lactase deficiency.

What happens when lactase is deficient?

When the small intestine does not produce sufficient amounts of lactase, undigested lactose continues its journey to the large intestine. Here, it is fermented by the resident gut bacteria. This fermentation process produces gases, such as hydrogen and carbon dioxide, and short-chain fatty acids. The resulting symptoms of this process can include abdominal bloating, cramps, gas, and diarrhea, which collectively characterize lactose intolerance. The severity of these symptoms depends on the amount of lactose consumed and the individual's remaining lactase activity.

Natural and commercial sources of lactase

While our bodies are the natural producers of lactase, deficiencies are common. Thankfully, there are several ways to supplement this enzyme, including through food and supplements.

Natural and commercial lactase sources include:

Fermented dairy products: Certain dairy products like yogurt and kefir contain live, active bacterial cultures (e.g., Lactobacillus acidophilus and Bifidobacterium) that produce lactase as a byproduct, aiding in the digestion of lactose within the product itself.
Lactase enzyme supplements: Available in tablets, capsules, and drops, these supplements provide a dose of the lactase enzyme derived from microorganisms like yeasts (Kluyveromyces lactis) or molds (Aspergillus oryzae). Taking them just before consuming dairy can significantly reduce symptoms.
Lactose-free dairy products: These products, such as lactose-free milk, have had commercial lactase enzyme added during production to pre-hydrolyze the lactose into glucose and galactose. The resulting product is slightly sweeter but digestible for everyone.

The complete milk digestion process

Digesting a glass of milk is a complex process involving multiple enzymes, not just lactase. Here is a step-by-step breakdown of how the body handles milk:

Stomach Phase: Milk enters the stomach where its proteins, primarily casein and whey, begin to coagulate due to the action of stomach acid and the enzyme pepsin. In infants, the enzyme rennin (or chymosin) is also present to curdle the milk protein, slowing its passage for better digestion. Lipases (lingual and gastric) begin the initial breakdown of milk fats.
Small Intestine - Carbohydrate Digestion: The partially digested milk (chyme) moves into the small intestine. It is here, at the brush border, that the lactase enzyme breaks down the lactose into glucose and galactose. This is the critical step for individuals without lactase deficiency.
Small Intestine - Protein and Fat Digestion: In addition to lactase, other enzymes secreted by the pancreas, such as proteases (trypsin, chymotrypsin) and lipases (pancreatic lipase), continue the breakdown of proteins and fats into amino acids and fatty acids. Bile, secreted by the liver, emulsifies the fats, making them more accessible to the lipases.
Absorption and Elimination: The simple sugars (glucose, galactose), amino acids, and fatty acids are absorbed through the intestinal lining into the bloodstream. In individuals with lactose intolerance, undigested lactose bypasses absorption and proceeds to the large intestine for bacterial fermentation, leading to symptoms.

Comparing native and commercial lactase

To further understand the nuance of milk digestion, it is useful to compare how native human lactase and commercially sourced lactase function. This comparison highlights why external supplementation is a viable option for many.


Feature	Native Human Lactase	Commercial Lactase (e.g., in supplements)
Source	Produced by cells lining the small intestine (enterocytes).	Extracted from microorganisms like yeast (Kluyveromyces lactis) or mold (Aspergillus oryzae).
Location of Action	At the brush border of the small intestine.	Acts in the stomach and small intestine, breaking down lactose before it causes issues.
Timing of Action	Acts continuously as long as milk is consumed, but activity declines with age in many.	Must be taken just before or with the first bite of dairy products for optimal effect.
Genetic Influence	Production levels are influenced by genetics (LCT gene), leading to different rates of decline and persistence.	Is externally supplied, bypassing the body's genetic ability to produce it.
Role in Digestion	A single component in the complex multi-enzyme process of digesting milk.	A targeted solution primarily focused on digesting lactose for those who are deficient.

Conclusion

The primary enzyme responsible for breaking down the milk sugar lactose is lactase, a crucial enzyme produced in the small intestine. A decline in lactase production, a genetically influenced trait known as lactase non-persistence, is the underlying cause of lactose intolerance. For individuals with this condition, external lactase supplements or pre-treated lactose-free dairy products offer an effective way to enjoy dairy without the uncomfortable side effects. Understanding the role of lactase provides clear insight into both the biological process of milk digestion and the symptoms experienced by many people worldwide. With the right knowledge and tools, managing lactose intolerance and enjoying dairy is very achievable.

For more information on digestive enzymes and overall gut health, consider consulting with a healthcare professional or visiting an authoritative resource like the National Institutes of Health.

Frequently Asked Questions

The primary enzyme that breaks down the sugar in milk, known as lactose, is lactase.

If a person does not have enough lactase, they cannot properly digest lactose. This leads to lactose passing to the large intestine, where fermentation by bacteria causes symptoms of lactose intolerance like gas, bloating, and diarrhea.

Yes, it is very common for adults to become lactose intolerant. Lactase production naturally declines after infancy in most human populations, a condition called lactase non-persistence.

Lactase is a specific type of beta-galactosidase, an enzyme that hydrolyzes beta-galactosides. In the context of human digestion, the two terms are often used interchangeably when referring to the enzyme that breaks down lactose.

Lactase is produced by the cells that line the walls of the small intestine, specifically located at the brush border.

Lactase supplements provide an external source of the enzyme. When taken just before consuming dairy, it helps break down the lactose in the food or drink, preventing it from reaching the large intestine undigested.

Yes, other enzymes are involved in digesting milk's other components. Pepsin in the stomach begins protein digestion, while pancreatic proteases and lipases continue to break down proteins and fats in the small intestine.

Symptoms of lactase deficiency, or lactose intolerance, include abdominal pain, bloating, excessive gas (flatulence), and diarrhea after consuming milk products.

There is no known way to permanently increase the body's production of lactase. However, some research suggests that gradual reintroduction of lactose may improve tolerance by promoting the growth of lactose-digesting bacteria in the colon.