Nutrition Diet: What Protein Digests Lactose?

October 21, 2025 •

4 min read

An estimated 65% of the global population has a reduced ability to digest lactose after infancy, relying on the specific protein—an enzyme called lactase—to break down this milk sugar. Understanding what protein digests lactose is key to navigating diets for those with intolerance and ensuring proper nutrition.

Quick Summary

The lactase enzyme, a type of protein, is responsible for digesting lactose in the small intestine by breaking it down into simple sugars. Its deficiency leads to lactose intolerance, causing digestive discomfort. Dietary modifications, supplements, and alternative protein sources can effectively manage symptoms.

Key Points

Lactase is the protein: The specific protein that digests lactose is an enzyme called lactase, produced in the small intestine.
Lactose intolerance is common: A majority of the global population experiences a natural decline in lactase production after infancy, leading to lactose intolerance.
Lactose is a milk sugar: Lactase's role is to break down lactose into the simple sugars glucose and galactose, which the body can absorb.
Dietary management is effective: Many individuals can manage symptoms by controlling portion sizes, choosing lower-lactose dairy products like aged cheese, or pairing dairy with other foods.
Lactase supplements offer an alternative: Over-the-counter lactase supplements can be taken to provide the body with the necessary enzyme to digest lactose.
Non-dairy protein options are plentiful: Excellent protein sources that are naturally lactose-free include whey protein isolate and plant-based proteins like soy, pea, and rice protein.
Symptoms result from fermentation: Without enough lactase, undigested lactose ferments in the large intestine, causing gas, bloating, and diarrhea.

Understanding the Lactase Enzyme

While the term "protein" is used in the query, the specific protein responsible for digesting lactose is an enzyme known as lactase. Enzymes are a type of protein that catalyze, or speed up, biochemical reactions in the body. The lactase enzyme is produced in the brush border of the small intestine, the area lined with finger-like projections called microvilli. Its primary function is to break down lactose, a complex sugar found in milk and other dairy products. Without sufficient lactase, lactose remains undigested and causes a variety of uncomfortable symptoms.

When lactose is consumed, lactase splits the disaccharide into two simpler, more easily absorbable monosaccharides: glucose and galactose. These simple sugars are then absorbed into the bloodstream from the small intestine and used for energy or other bodily functions. For most humans, the body's natural production of lactase decreases significantly after infancy, a condition known as lactase nonpersistence, leading to lactose intolerance in adulthood.

The Spectrum of Lactose Intolerance

Lactose intolerance is not a one-size-fits-all condition. It can vary in severity depending on the level of lactase deficiency and the amount of lactose consumed. When undigested lactose reaches the large intestine, gut bacteria ferment it, producing gases and fatty acids that cause common symptoms.

There are three main types of lactase deficiency:

Primary Lactase Deficiency (Lactase Nonpersistence): The most common type, where lactase production decreases naturally over time, typically after childhood. It is genetically determined and varies in prevalence among different ethnicities.
Secondary Lactase Deficiency: This occurs when the small intestine is damaged by an illness, injury, or surgery, leading to a temporary or sometimes long-term reduction in lactase production. Conditions like celiac disease or Crohn's disease are common causes.
Congenital Lactase Deficiency: A rare genetic disorder where infants are born with little or no ability to produce lactase. This requires special lactose-free infant formulas from birth.

Dietary Strategies for Managing Lactose

Navigating a diet with lactose intolerance involves more than just cutting out dairy completely. Many individuals can still consume small amounts of lactose without experiencing symptoms. Here are some effective strategies:

Gradual Introduction: Start by adding small amounts of dairy back into your diet to test your tolerance levels. Many can handle up to 12 grams of lactose per day, which is about the amount in one cup of milk.
Pair with Other Foods: Eating dairy with other foods slows down the digestive process, which can give the small intestine more time to process the lactose.
Choose Cultured Dairy: Products like yogurt with active cultures and aged cheeses (e.g., cheddar, Swiss) are often better tolerated. The bacteria used in their production break down much of the lactose.
Use Lactose-Free Products: Most supermarkets offer lactose-free milk, ice cream, and other dairy alternatives that have the lactase enzyme added to them during processing.
Try Probiotics: Some studies suggest that certain probiotic strains can help improve symptoms by supporting the gut microbiome's ability to process lactose.

Comparison of Protein Sources for Lactose Intolerance


Protein Source	Lactose Content	Suitability for Lactose Intolerance	Notes
Whey Protein Concentrate	High	Generally not suitable	Contains significant lactose, may cause symptoms.
Whey Protein Isolate	Very Low	Highly suitable	Most lactose is removed during processing, with less than 0.1g per serving.
Casein Protein	Varies	Depends on processing	Some products contain more lactose than others; check the label carefully.
Soy Protein	None	Highly suitable	Complete plant-based protein, naturally lactose-free.
Pea Protein	None	Highly suitable	Hypoallergenic, plant-based alternative.
Brown Rice Protein	None	Highly suitable	Plant-based, often a blend with other proteins.
Goat Protein	Low	May be suitable for mild cases	Contains less lactose than cow's milk and may be easier to digest for some.
Lactose-Free Dairy	None	Highly suitable	Lactase enzyme is added during production to break down lactose.

Supplements for Lactose Digestion

For those who wish to consume dairy products, lactase enzyme supplements are a popular solution. These are available in various forms, including tablets, drops, and capsules.

Tablets/Capsules: Taken orally just before or with the first bite of a meal containing lactose, these supplements provide the missing lactase enzyme to aid digestion in the small intestine.
Drops: These can be added directly to milk or other liquid dairy products and left to sit for a period (e.g., 24 hours in the refrigerator), allowing the enzyme to break down the lactose before consumption.

It's important to note that these supplements do not work for everyone, and their effectiveness can vary. For some, dietary changes are the most effective method for managing symptoms. Always consult a healthcare professional to determine the best strategy for your specific needs.

The Final Word on Lactose Digestion

Contrary to the simple phrasing of "what protein digests lactose," the process is specific to the lactase enzyme. This digestive powerhouse is essential for breaking down milk sugar into manageable components for the body. For the vast majority of the global population, the natural decline of lactase production necessitates dietary management or supplementation to avoid uncomfortable digestive symptoms. By understanding the specific role of the lactase enzyme and the various dietary strategies available, individuals can better control their intake of dairy and ensure they are meeting their nutritional needs without unnecessary discomfort. Whether through targeted supplements, choosing lower-lactose dairy, or opting for plant-based alternatives, managing lactose intolerance is highly achievable for most people.

For further reading on the science of digestion, you can consult reputable sources such as the National Institutes of Health.

Frequently Asked Questions

The most common cause of lactose intolerance is a natural decline in the body's production of the lactase enzyme after childhood, a condition known as primary lactase deficiency or lactase nonpersistence.

No, not all dairy products have the same amount of lactose. Aged, hard cheeses like cheddar and Swiss, as well as cultured products like yogurt with active cultures, contain significantly less lactose and are often better tolerated.

Lactase supplements do not cure lactose intolerance but help manage its symptoms by providing the enzyme needed to digest lactose in the meal being consumed. They must be taken with the first bite of a dairy food to be effective.

Excellent lactose-free protein sources include whey protein isolate (which has most lactose removed), plant-based protein powders (like soy, pea, and rice), as well as whole foods like meat, eggs, and legumes.

No, they are different conditions. A milk allergy is an immune system reaction to milk proteins, while lactose intolerance is a digestive issue caused by a deficiency of the lactase enzyme.

Yes, some probiotics, particularly those from the Lactobacillus family, can aid in processing lactose in the gut and may help reduce symptoms for some individuals.

No, many people with lactose intolerance can handle small amounts of lactose without experiencing symptoms. Dietary management involves finding your personal tolerance level, which may allow for moderate consumption of certain dairy products.