Medically reviewed by Amelia MacIntyre, DO
:max_bytes(150000):strip_icc():format(jpeg)/Health-GettyImages-LactoseIntolerance-e7d59f52886b4da09a86cf74c893d282.jpg "A person pouring milk into a glass mug")
- Lactose intolerance is caused by a lack of the enzyme lactase, leading to digestive symptoms when the body cannot break down milk sugar.
- Milk protein intolerance involves an immune response to proteins like casein and whey and can cause symptoms that range from digestive issues to allergic reactions.
- The two conditions differ in cause, symptom timing, and triggers, so proper testing is important to determine the correct diagnosis.
If you're a cheese lover, but it frequently upsets your stomach, you might think you have lactose intolerance. But if taking digestive enzymes or eating lactose-free dairy doesn't help your diarrhea and gas, it might be something else: milk protein intolerance. Experts share the differences here.
What Is Lactose Intolerance?
Lactose intolerance is the inability to digest lactose, or the sugar in milk. It happens when the small intestine doesn't have enough lactase—an enzyme that helps your body break lactose down. When you eat dairy, "undigested lactose passes into the colon where bacteria ferment it, causing gas bloating, diarrhea, and abdominal pain," Sarah Robbins, MD, MSc, FRCPC, a gastroenterologist, told Health.
Most people are born with enough lactase in their gut, but levels can decrease over time. This is why it's common for adults to be lactose intolerant. Certain conditions, like GI infections and celiac disease, can also cause lactose intolerance, said Sarah Glinski, RD, a registered dietitian in Canada who specializes in gut health.
What Is Milk Protein Intolerance or Allergy?
Milk protein intolerance is when your body negatively reacts to the protein in cow's milk, primarily casein and whey. Instead of occurring from a lack of enzymes in the gut, it affects the immune system and triggers a response similar to an allergy. Essentially, your body perceives the protein as a threat, said Glinski.
There are two types of milk protein intolerance: IgE or non-IgE-mediated. The first involves immunoglobulin antibodies (IgE) that trigger immediate symptoms like wheezing, trouble breathing, swelling, abdominal pain, and diarrhea. The second does not involve IgE antibodies and causes symptoms that are delayed, such as diarrhea, gas, pain, and bloody stool. This is why it's often confused with lactose intolerance.
"Milk protein intolerance is much less common, affecting around 2 to 3 percent of infants and fewer than 1 percent of children 6 and older," said Glinski. Research shows that adults can have it, too, but symptoms are typically mild. People with relatives (like a sibling or parent) who have milk allergies, other food allergies, eczema, or asthma are at greater risk of milk protein intolerance or allergy.
Key Differences Between Lactose Intolerance and Milk Protein Allergy
The biggest difference between lactose intolerance and milk protein intolerance lies in where they start. Lactose intolerance starts (and only impacts) the gut, while milk protein intolerance starts with the immune system. Another key difference is the irritant itself: Lactose is the sugar in milk, whereas casein and whey are proteins.
Symptom timing and onset also vary. "Lactose intolerance produces exclusively gastrointestinal symptoms—abdominal cramping, bloating, flatulence, and watery diarrhea—typically within 30 minutes to two hours of consuming dairy," said Robbins. In contrast, IgE-mediated milk protein allergies cause symptoms such as wheezing, swelling, itching, or pain within minutes of consuming dairy. Non-IgE-mediated reactions, which can cause bloody stool and diarrhea, take hours to days to develop, she added.
Lastly, lactose intolerance is often dose-dependent, said Robbins. That means symptom severity may depend on how much dairy you eat. If you're only mildly lactose intolerant, you may eat yogurt, cheese, or milk with little issue. But if you're severely intolerant, the smallest portion can wreak havoc on your gut. Milk protein symptoms tend to be more consistent, no matter the dose.
One possible way to narrow things down is to drink lactose-free milk, like Lactaid, and see how you feel. Ultimately, "a person with milk protein intolerance will develop symptoms regardless of whether the milk is lactose-free," said Glinski.
Diagnosing Each Condition
Most people self-diagnose lactose intolerance after cutting dairy from their diet. If they feel better, then they know dairy is the culprit. But elimination diets and symptom tracking won't tell you if protein or lactose is the issue. Only diagnostic tests can do that.
"For lactose intolerance, the hydrogen breath test is the most widely used tool," said Robbins. It involves drinking a lactose solution and breathing into a machine to measure hydrogen in your breath. The higher the number, the higher the likelihood of lactose intolerance. Doctors also use blood glucose tests, which measure your levels before and after drinking a lactose solution. "If you're lactose intolerant, you won't typically see an increase in blood glucose," said Glinski.
Skin prick tests performed by an allergist can help diagnose a milk protein allergy. There is no validated lab test for non-IgE-mediated milk protein intolerance, so a two- to four-week supervised elimination diet is often used, said Robbins. If all other tests come back negative, a non-IgE milk protein intolerance might be your issue.
Accurate testing matters to avoid unnecessarily cutting dairy (a nutrient-rich food group) from your diet. Sometimes, self-diagnosed lactose intolerance is inaccurate, said Robbins, so seeing a medical professional is advised.
Managing and Living With Each Condition
The best way to manage milk intolerance is by avoiding dairy. If you have a true milk protein allergy, it might be necessary to prevent life-threatening complications like anaphylaxis, according to Adam Pont, MD, a gastroenterologist with Medical Offices of Manhattan.
There are a few other options you can try for both lactose and milk protein intolerance, according to the experts:
- For lactose intolerance: Try eating lactose-free milk, cheese, or other related dairy products. Take lactase pills (a digestive enzyme commonly sold as Lactaid) before meals to prevent GI upset.
- For non-IgE-mediated milk protein intolerance: Try A2 milk. It's a brand that removes the protein A1 beta-casein from the final product, making it easier for some people with casein sensitivity to digest, said Glinski.
- For both: Try eating goat or sheep milk. They're lower in lactose and don't contain the A1 beta-casein protein, said Glinski. You can also try non-dairy products, like plant-based milk instead, said Pont.
