If you have trouble digesting milk and other dairy products, you might be lactose intolerant. Lactose intolerance happens because your body can't break down lactose, which is the sugar in milk. It’s a fairly common occurrence in adults, especially among certain populations and ethnicities. Lactose intolerance isn't usually dangerous, and you can often manage it with smart diet choices. AncestryDNA^® + Traits can tell you if people with DNA like yours tend to be lactose intolerant.

Lactose Intolerance Symptoms

People's lactose intolerance symptoms can vary, depending on how much lactose their body can handle. Symptoms may include nausea, cramps, diarrhea, bloating, and feeling gassy. These reactions can happen as soon as 30 minutes after eating, but they may also take a few hours to show.

One approach to understanding and managing your body’s response is to cut out lactose for two weeks to see if your symptoms go away. If they do, start adding dairy foods back into your diet and see how your body tolerates it. For example, try small amounts of milk—half a cup or less—and drink it with a meal.

Some people have an easier time digesting yogurt than other milk products. Not all cheese contains lactose at the same level, either. Aged cheeses—like blue, cheddar, Parmesan, and Swiss—have a smaller amount of lactose than soft cheeses—like ricotta and cottage—or cheese spreads.

Milk Allergy Symptoms

Being lactose intolerant and having a milk allergy aren't the same thing. Also, lactose intolerance isn't caused by, nor does it cause, a milk allergy. But they do often occur together.

• A food intolerance, like lactose intolerance, is when the digestive system doesn't quite work right and causes digestive problems.
• A food allergy, such as a milk allergy, involves the immune system. Milk allergies are more common in children than in adults, and most children outgrow them.

Milk allergy symptoms are more serious, and include hives, a tingling or itchy feeling in the lips, trouble breathing, and swelling of the lips, throat, or tongue. It can also cause vomiting. The most serious reaction is anaphylaxis, which is a medical emergency. This is when airways swell so much that someone can't breathe,needs epinephrine to open airways, and should be taken to the emergency room. Someone experiencing anaphylaxis should be seen by a medical professional even if epinephrine has been administered.

Why People Are Lactose Intolerant

Lactose intolerant people don't make enough lactase, the enzyme that breaks down lactose. Primary lactose intolerance, which occurs from birth, often runs in families—it’s mainly passed down through the LCT gene. In the cases where lactose intolerance starts in adulthood, this is usually caused as the body gradually stops making enough lactase. Whether the body continues to produce lactase later in life is largely determined by differences in the gene MCM6, which controls the activity of the LCT gene.

People may experience secondary lactose intolerance due to an injury, surgery, or illness, unrelated to the LCT gene. Injuries and illnesses affecting your small intestine (which makes lactase) can also cause lactose intolerance, as can cancer treatment. Premature babies are sometimes lactose intolerant, but this often corrects itself. It's possible for a newborn to be unable to make lactase (due to a congenital lactase deficiency) because the mother and father both passed on versions of the LCT gene that don’t make functioning lactase.

Interesting Facts About Lactose Intolerance

Congenital lactase deficiency is very rare, but most common in Finland where it affects 1 in 60,000 newborns. However, about 65% of adults worldwide have some degree of lactose intolerance. As for milk allergies, about 5% of people are allergic to cow's milk.

There is a test that may help to diagnose lactose intolerance. This lactose tolerance test works by having someone drink a dairy beverage and then measuring hydrogen levels in their breath. Too much hydrogen means your body isn't absorbing lactose properly. It's not a perfect test, though: About 20% of people who have trouble absorbing lactose will not test positive for lactose intolerance.

References (all accessed May 24, 2023)

“Definition & Facts for Lactose Intolerance." National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/definition-facts.

“Lactose Intolerance." Cedars Sinai. https://www.cedars-sinai.org/health-library/diseases-and-conditions/l/lactose-intolerance.html.

“Lactose Intolerance." Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232.

“Lactose Intolerance." MedlinePlus. https://medlineplus.gov/genetics/condition/lactose-intolerance/.

“Lactose Intolerance: Symptoms, Diagnosis & Treatments." Cleveland Clinic. https://my.clevelandclinic.org/health/diseases/7317-lactose-intolerance.

Mandl, Elise. “5 Signs and Symptoms of Lactose Intolerance." Healthline. https://www.healthline.com/nutrition/lactose-intolerance-symptoms.

“Milk Allergy." Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/milk-allergy/symptoms-causes/syc-20375101.