Can a Vegan Diet Make You Lactose Intolerant? The Truth About Lactase and Dairy

December 10, 2025 •

5 min read

According to the National Institutes of Health, approximately 65% of the human population has a reduced ability to digest lactose after infancy. While this common genetic trait, known as lactase nonpersistence, determines your lifelong digestive capacity, a prolonged vegan diet can temporarily mimic symptoms of lactose intolerance when dairy is reintroduced. This happens because the body's production of the lactase enzyme adjusts to what you consume, but this change is often reversible with careful reintroduction.

Quick Summary

Long-term dairy avoidance on a vegan diet can lead to reduced lactase production and changes in gut bacteria. These temporary shifts can cause digestive issues if dairy is suddenly reintroduced, creating the perception of newly developed lactose intolerance. The ability to digest dairy can be regained gradually.

Key Points

Genetic vs. Adaptive: A vegan diet does not alter the underlying genetic condition of lactose intolerance, but rather affects the body's adaptive enzyme production.
Reduced Lactase Production: Abstaining from dairy reduces the body's need to produce the lactase enzyme, leading to lower levels over time.
Gut Microbiome Shift: A plant-based diet causes the gut microbiome to adapt to fermenting fiber, reducing the population of lactose-digesting bacteria.
Symptoms of Reintroduction: The initial reintroduction of dairy can cause temporary symptoms like gas and bloating due to low lactase levels and an unprepared microbiome.
Reversibility: Tolerance to lactose can often be rebuilt by gradually reintroducing small amounts of dairy, allowing the digestive system to readapt.
Start Slow: Reintroducing dairy slowly with low-lactose products like hard cheeses or yogurt is the best method to minimize discomfort.

The Core Science: Lactase Production and Genetics

Lactose intolerance is not an illness but a naturally occurring condition for the majority of the world's population. The condition is caused by a deficiency of the lactase enzyme, which is produced in the small intestine to break down lactose, the sugar found in milk.

There are several types of lactase deficiency:

Primary Lactase Nonpersistence: The most common type, where lactase production naturally decreases after infancy. This is genetically predetermined, and it is what most people mean when they refer to being lactose intolerant.
Secondary Lactase Deficiency: Occurs due to injury, illness, or surgery to the small intestine. Damage to the intestinal lining can temporarily disrupt lactase production. This form is often reversible once the underlying condition is treated.
Congenital Lactase Deficiency: A rare genetic disorder where infants are born unable to produce lactase.

A vegan diet, by definition, excludes dairy and thus lactose, but it does not change your genetic predisposition for lactase production. For someone with primary lactase nonpersistence, the underlying condition remains whether they consume dairy or not. However, for those with lactase persistence (the genetic trait allowing them to digest lactose throughout life), ceasing dairy intake can affect their digestive system's efficiency.

The “Use It or Lose It” Effect on Your Digestive System

Think of your body's enzyme production like a muscle. If you stop exercising a muscle, it atrophies. Similarly, when you eliminate dairy, your small intestine recognizes it no longer needs to produce as much lactase. It’s an energy-saving mechanism. For individuals with lactase persistence, this means that while their underlying genetic capacity to produce lactase remains, the actual amount of active enzyme can decrease over time.

This is why someone who has been vegan for years might experience symptoms like bloating, gas, and diarrhea upon having a milkshake. Their body hasn't permanently lost the ability to produce lactase, but it has become less efficient at it due to disuse. This is a reversible state, not a permanent intolerance.

The Gut Microbiome's Role

Beyond enzyme levels, the bacterial population in your gut plays a significant role in lactose digestion. On a vegan diet, the gut microbiome adapts to fermenting plant-based fibers rather than the lactose found in dairy.

Before Veganism: The gut contains specific bacteria that thrive on lactose. When these bacteria are fed lactose regularly, they produce enzymes that help with digestion.
On a Vegan Diet: With no lactose entering the system, these specific bacteria may diminish or be replaced by other microbes more suited to a high-fiber plant-based diet.
Reintroducing Dairy: The sudden reintroduction of lactose encounters a gut biome unprepared to process it efficiently. This leads to the fermentation process in the colon that causes unpleasant symptoms.

Reintroducing Dairy: A Gradual Process for Tolerance

If you decide to reintroduce dairy after a period of being vegan, a slow and cautious approach is best to help your body and gut bacteria adapt. Purdue University research indicates that gradually reintroducing dairy can help retrain your intestinal bacteria to aid in lactose digestion.

Start with Small Quantities: Begin with a quarter cup of milk with a meal, rather than a full glass on an empty stomach.
Choose Lower-Lactose Dairy: Hard cheeses like aged cheddar and fermented products like yogurt often contain less lactose, making them easier to digest initially.
Increase Slowly: If a small amount is tolerated well for a few days, you can incrementally increase the quantity.
Be Patient: Building tolerance takes time, and symptoms may reappear if you increase your intake too quickly.

Symptoms to Watch For

When reintroducing dairy, be mindful of these common symptoms of lactose intolerance:

Abdominal pain and cramps
Bloating
Gas (flatulence)
Diarrhea
Nausea and, in some cases, vomiting

These symptoms typically appear between 30 minutes to 2 hours after consuming dairy and vary in severity depending on the amount consumed.

Vegan vs. Dairy-Reintroduction Digestive Changes


Aspect	After Long-Term Veganism	Upon Initial Dairy Reintroduction
Lactase Enzyme Production	Reduced due to lack of exposure.	Insufficient to digest lactose, leading to symptoms.
Gut Microbiome Composition	Adapted to fermenting plant fibers; specific dairy-digesting bacteria may be reduced or absent.	Encounters a 'novel' food it's not equipped to break down, causing fermentation in the colon.
Lactose Digestion	No lactose consumed, so no digestive issues related to lactose.	Incomplete, leading to digestive upset and classic intolerance symptoms.
Potential Symptoms	Healthy gut function, optimized for high fiber.	Bloating, gas, cramping, or diarrhea, depending on amount consumed.

The Verdict: Not an Intolerance, But an Adjustment

To be clear, a vegan diet cannot make you lactose intolerant in the primary, genetic sense. The experience of digestive distress after eating dairy post-veganism is not a permanent state but a physiological adjustment. Your body has simply downregulated lactase production in the absence of a need for it, and your gut bacteria have evolved to match your fiber-rich diet. The good news is that for most people who did not have lactose nonpersistence beforehand, this is a reversible adaptation. Careful, gradual reintroduction of dairy can help your body's systems, including your gut microbiome, remember how to process lactose effectively once again. It is always wise to consult a healthcare professional for personalized advice. For more detailed information on lactase digestion, refer to this scientific review: Lactose digestion in humans: intestinal lactase appears to be an adaptation, not a genetic trait.

Conclusion

The perception that a vegan diet causes lactose intolerance is a common misconception. The reality is a nuanced interplay of genetics, enzyme regulation, and the gut microbiome's adaptation. A prolonged dairy-free diet can lead to a temporary decrease in lactase production, resulting in symptoms when dairy is reintroduced. This is a reversible process, not a new or permanent intolerance. By understanding this biological adaptation, individuals can either continue their vegan lifestyle confidently or reintroduce dairy mindfully, using small quantities and lower-lactose products to allow their digestive system to readjust. The takeaway is that your body's ability to digest certain foods is dynamic, and a return to dairy requires patience and a gradual approach.

Frequently Asked Questions

For most people with primary lactase nonpersistence, the underlying genetic condition is permanent. However, for those with lactase persistence, the adaptive reduction in lactase from being vegan is not permanent and can often be reversed by slowly reintroducing dairy.

The timeframe varies by individual, but the decrease in lactase production is a gradual process. The longer dairy is excluded, the more significant the reduction in enzyme levels will be.

Lactose intolerance is a digestive issue caused by the inability to break down lactose, a milk sugar. A milk allergy is an immune system reaction to milk proteins, which can be more severe.

Yes, lactase supplements are available to help your body digest lactose more effectively. They can be taken with dairy-containing foods to reduce symptoms, though they do not retrain your body to produce more lactase naturally.

Yogurt and hard cheeses contain less lactose than fresh milk because the fermentation process or aging breaks down most of the lactose. Additionally, the bacteria in yogurt can provide lactase enzymes that aid digestion.

To determine if your symptoms are lactose-related, a healthcare provider may recommend a temporary dairy elimination diet. Symptoms that disappear and return with dairy reintroduction suggest lactose intolerance. Other tests, like a hydrogen breath test, can also confirm the diagnosis.

Yes. Through gradual reintroduction of dairy, starting with small amounts and lower-lactose products, an individual can often restore their body's ability to digest lactose with minimal symptoms.