You can determine lactose intolerance through a combination of symptom tracking, at-home elimination testing, and formal diagnostic tests like the hydrogen breath test. Most people start by noticing a pattern of bloating, gas, or diarrhea after consuming dairy, then confirm it either by cutting out lactose temporarily or through a clinical test ordered by their doctor.
What Happens in Your Body
Lactose intolerance comes down to a missing or reduced enzyme called lactase, which normally breaks down lactose (the sugar in milk) in your small intestine. Without enough lactase, lactose passes through undigested into your colon, where gut bacteria ferment it. That fermentation produces hydrogen, carbon dioxide, methane, and short-chain fatty acids.
The effects are twofold. First, the gas production directly causes bloating, cramping, and flatulence. Second, the undigested lactose pulls water into your intestines through osmosis, and fermentation amplifies that water-trapping effect roughly eightfold. If the amount of lactose overwhelms your colon’s ability to absorb those byproducts, the result is diarrhea. This is why a small amount of dairy might not bother you, but a large glass of milk does: it’s dose-dependent.
Symptoms to Watch For
The hallmark symptoms are bloating, gas, stomach cramps, nausea, and diarrhea. They typically appear within a few hours of eating or drinking something containing lactose. Vomiting can happen but is less common.
The timing matters when you’re trying to identify a pattern. If your symptoms consistently show up one to three hours after dairy and resolve on their own within several hours, that’s a strong signal. Keeping a simple food diary for a week or two, noting what you ate and when symptoms hit, can make the connection much clearer than relying on memory alone.
Lactose Intolerance vs. Milk Allergy
Before testing yourself, it helps to rule out something different: a milk allergy. These two conditions are often confused, but they involve completely different body systems. Lactose intolerance is an enzyme deficiency. Your digestive system can’t break down a sugar. A milk allergy is an immune reaction to a protein in milk, and it can produce symptoms that lactose intolerance never does, including hives, rashes, itching, swelling, wheezing, and in severe cases, life-threatening anaphylaxis.
If your symptoms after dairy include any skin reactions, throat tightening, or difficulty breathing, that points toward an allergy rather than an intolerance. A milk allergy requires strict avoidance of all milk proteins, not just lactose, and is a more serious medical concern.
The Elimination Diet Approach
The simplest way to test yourself at home is an elimination diet. Remove all sources of lactose from your diet for four to eight weeks. That means milk, cheese, ice cream, cream-based sauces, and also less obvious sources like bread, cereal, salad dressings, and processed snacks that list milk, whey, or curds in their ingredients.
If your symptoms improve or disappear during that period, the next step is reintroduction. Add dairy back slowly, one food at a time, and watch for symptoms. This part requires patience. If you reintroduce multiple dairy products at once, you won’t be able to pinpoint what’s causing the problem. Start with a small portion of one dairy food, wait a day or two to observe your reaction, and then try the next one. Many people find they can tolerate hard cheeses or yogurt (which contain less lactose) but not a glass of milk.
The Hydrogen Breath Test
The hydrogen breath test is the standard clinical test for lactose intolerance. It works by measuring the gas your gut bacteria produce when they ferment undigested lactose. You drink a solution containing a measured dose of lactose, then breathe into a collection device at regular intervals over two to three hours. A rise of more than 20 parts per million of hydrogen above your baseline reading is considered a positive result.
Preparation matters for accuracy. About a month before the test, you’ll need to stop taking antibiotics and probiotics, since both change your gut bacteria in ways that can skew the results. A week before, you’ll stop smoking and discontinue any laxatives, fiber supplements, or antacids. The day before, you’ll eat only low-fiber, easily digested foods. On the morning of the test, you’ll fast for 12 hours (no food or water) and avoid exercise or sleeping in the hours leading up to it. The test itself is noninvasive and painless, though drinking the lactose solution may trigger your usual symptoms if you are intolerant.
Blood Glucose Test
A less commonly used option is the lactose tolerance blood test. The logic is straightforward: if your body properly digests lactose, it gets broken down into glucose, which enters your bloodstream. After drinking a lactose solution, blood samples are taken over two hours. If your blood glucose rises more than 30 mg/dL, your body is digesting lactose normally. A smaller rise suggests your body isn’t breaking the sugar down, pointing to intolerance.
This test is used less often than the breath test because it’s slightly more invasive (requiring blood draws) and can be affected by other conditions that influence blood sugar, like diabetes.
Genetic Testing
Genetic tests can identify whether you carry variants associated with lactose persistence, the ability to keep producing lactase into adulthood. These tests look at a regulatory region near the gene responsible for lactase production. Specific single-nucleotide changes in this region determine whether your body continues making lactase or gradually shuts it down after childhood.
A genetic test can tell you whether you’re predisposed to losing lactase production, but it can’t tell you how severe your symptoms are or how much dairy you can tolerate in practice. Two people with the same genetic profile can have very different experiences depending on their gut bacteria, how much dairy they consume, and other digestive factors. Genetic testing is most useful as a complementary piece of information rather than a standalone diagnosis.
Testing in Infants and Young Children
The hydrogen breath test isn’t practical for babies and very young children, so doctors use a stool acidity test instead. When lactose goes undigested, the fermentation byproducts make the stool more acidic. A stool pH below 6 suggests sugar malabsorption and points toward lactose intolerance. This test is simple and noninvasive, making it well suited for infants who can’t follow the preparation and breathing protocols required for the breath test.
Who Is Most Likely to Be Lactose Intolerant
Lactose intolerance isn’t evenly distributed across populations. Most humans naturally lose some lactase production after weaning, which is actually the biological default. Populations with a long history of dairy farming, particularly those of Northern European descent, are more likely to carry the genetic variants that keep lactase production going into adulthood.
In the United States, self-reported lactose intolerance runs about 7.8% among white adults, 20.1% among Black adults, and 8.8% among Hispanic adults. Globally, rates are far higher in East Asian, West African, and Indigenous populations, where up to 90% or more of adults have reduced lactase activity. If you belong to a higher-prevalence group and have symptoms after dairy, the odds that lactose is the cause go up accordingly.
Putting the Pieces Together
For most people, the practical path looks like this: notice a pattern of GI symptoms after dairy, try a four-to-eight-week elimination diet to see if symptoms resolve, then reintroduce dairy in controlled amounts. If the picture is still unclear, or if you want a definitive answer, the hydrogen breath test provides the most reliable confirmation. Genetic testing and blood glucose testing are available but play supporting roles. The key insight is that lactose intolerance exists on a spectrum. Your goal isn’t just a yes-or-no diagnosis but figuring out how much lactose your body can handle, so you can adjust your diet without unnecessarily cutting out foods you enjoy.