How to find out if you're really lactose intolerant

In a well-known trial, 30 adults who described themselves as severely lactose intolerant drank a cup of milk with breakfast every day for two separate weeks. One week it was regular milk, the other week it was lactose-hydrolyzed milk, and neither they nor the researchers knew which was which. Symptoms were minimal in both weeks, and the severity of bloating, pain, diarrhea and gas didn't differ significantly (Suarez et al., 1995).

That doesn't mean lactose intolerance isn't real. It means the label is easy to get wrong. Here's how to find out what's actually going on, one step at a time.

First, know the difference between malabsorption and intolerance

Lactose malabsorption means your small intestine doesn't fully digest lactose, the natural sugar in milk. Lactose intolerance means you get symptoms, such as abdominal pain, bloating or diarrhea, after eating it. You can have the first without the second (Misselwitz et al., 2019).

Whether you get symptoms depends on the dose of lactose, how much lactase enzyme your gut still makes and your gut bacteria. People with a sensitive gut, including those with IBS or anxiety, are more likely to react to the same amount (Misselwitz et al., 2019).

Step 1: Track before you cut anything

For one week, eat as you normally do. Write down each dairy food, the rough amount and any symptoms over the following hours.

Note other suspects too, such as onions, garlic, beans, sugar-free sweets and large meals. Lactose is only one of several poorly absorbed carbohydrates that can cause the same kinds of symptoms (Misselwitz et al., 2019).

Step 2: Run a simple home trial

1. Remove the main sources of lactose for two weeks. Milk, ice cream, soft cheeses and creamy sauces are the big ones.
2. Keep everything else the same. Don't start a new supplement or diet during these weeks.
3. Rate your symptoms every day on a scale from 0 to 10.
4. Bring lactose back in a measured way. Have one cup of regular milk with a meal, and watch the next 24 hours.

This isn't a formal diagnosis, and expectations can color the results. The link between self-reported lactose intolerance and objective test findings is variable (Misselwitz et al., 2019). If your answer is still unclear, the next step is a test.

Step 3: Ask about a hydrogen breath test

The lactose hydrogen breath test is the most widely used formal test. You drink a set dose of lactose, and breath samples are collected over time. If lactose isn't absorbed, gut bacteria ferment it and release hydrogen, which shows up in your breath.

A North American expert consensus recommends a 25-gram lactose dose and considers a rise in hydrogen of at least 20 parts per million from baseline a positive result. The same group advises ruling out small intestinal bacterial overgrowth first, because it can cause false positive results (Rezaie et al., 2017).

Your clinic will give you preparation instructions, such as fasting beforehand and avoiding certain foods the day before. Follow them closely, since they affect accuracy.

Step 4: Understand the other tests

Genetic tests and tests on small intestine tissue can also assess lactase status (Misselwitz et al., 2019). A genetic result shows whether you carry a gene variant linked to keeping lactase activity into adulthood. It doesn't tell you whether dairy is the cause of your symptoms.

Lactose problems can also be secondary. That means something that affects the lining of the small intestine, such as an infection, reduces lactase (Misselwitz et al., 2019). If dairy suddenly becomes a problem in your 40s along with other new symptoms, ask your doctor whether another condition, such as celiac disease, should be checked.

Step 5: Find your personal threshold

A positive test rarely means you need to avoid dairy completely. A systematic review of randomized trials concluded that most people with presumed lactose intolerance or malabsorption can tolerate 12 to 15 grams of lactose, about one cup of milk (Shaukat et al., 2010).

The same review found the evidence for lactose-reduced milk, probiotics and several other approaches too limited to draw firm conclusions (Shaukat et al., 2010). Your own careful trial is often the best guide. Some people take a lactase enzyme product before a larger dairy serving, and Meal Enzymes is one that includes lactase alongside other digestive enzymes.

When to see a doctor instead of experimenting

• Diarrhea that lasts more than a few weeks or wakes you at night
• Weight loss you can't explain, blood in your stool or anemia
• Symptoms that continue even after you've removed dairy
• A family history of celiac disease or inflammatory bowel disease
• Concerns about calcium or bone health after avoiding dairy for a long time

Sources

1. Suarez FL, Savaiano DA, Levitt MD. A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance. N Engl J Med. 1995;333(1):1-4. PMID 7776987
2. Misselwitz B, Butter M, Verbeke K, Fox MR. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut. 2019;68(11):2080-2091. PMID 31427404
3. Rezaie A, Buresi M, Lembo A, et al. Hydrogen and methane-based breath testing in gastrointestinal disorders: the North American consensus. Am J Gastroenterol. 2017;112(5):775-784. PMID 28323273
4. Shaukat A, Levitt MD, Taylor BC, et al. Systematic review: effective management strategies for lactose intolerance. Ann Intern Med. 2010;152(12):797-803. PMID 20404262

This article is for general information and isn't medical advice. Talk to your healthcare provider before cutting out a food group long term, or if your symptoms don't improve.