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What is lactose intolerance? Causes and symptoms, explained

If you’ve ever had an upset stomach after eating cheese or ice cream, you may be intolerant to lactose. It’s is a sugar found in milk and other dairy products. The FDA estimates that 30 to 50 million people in the United States can’t properly digest it. Here’s what that means for you.

What is lactose?

Lactose is a large milk sugar molecule found in dairy products. It makes up 2% to 8% of milk—and is even found in some medications. Lactose is a disaccharide (double sugar) that the body breaks down into the simple sugars glucose and galactose. The body can use the energy from these sugars for many things like repairing cells, building muscles, and fueling everyday activities. 

What is lactose intolerance?

Lactose intolerance (also called lactose malabsorption) is the inability to digest lactose. People with lactose intolerance don’t have enough lactase enzyme in their body, which is what’s needed to digest lactose. Without lactase, lactose can’t break down into its smaller units, which means the body can’t access those important sugar molecules.   

This digestive disorder affects about 36% of the U.S. population. Risk factors for developing lactose intolerance include being of African American, American Indian, Asian, or Hispanic descent; being older; or being born prematurely.  

It’s a chronic condition that currently has no cure. It’s possible to become lactose intolerant all of a sudden if another medical condition—such as gastroenteritis—or prolonged abstinence from dairy triggers the body. It is normal to lose tolerance for lactose as you age. 

Causes of lactose intolerance

There are two types of lactose intolerance that scientists recognize: primary and secondary lactose intolerance. Primary lactose intolerance is caused by either a deficiency of lactase or decreased lactase production that becomes more prevalent with age. 

Problems in the small intestine, resulting in decreased production of lactase, cause secondary lactose intolerance. Illness, injury, infection, or celiac disease can cause these problems. 

Both types of intolerance have to do with an inability to digest lactose due to low lactase levels. Primary lactose intolerance is much more common than secondary lactose intolerance. In North America, 79% of Native Americans, 75% of African Americans, 51% of Hispanics, and 21% of Caucasians have primary lactose intolerance.   

Acquired lactase deficiency is also possible. In these cases, individuals acquire lactose intolerance as they age.

Symptoms of lactose intolerance

Lactose intolerance causes some easily recognizable symptoms. If you’ve just eaten dairy products and have any of the following symptoms within 30 minutes to two hours after eating, you may be lactose intolerant.    

  1. Bloating
  2. Flatulence
  3. Diarrhea 
  4. Nausea
  5. Vomiting  
  6. Abdominal cramping
  7. Indigestion
  8. Belching 

These symptoms all happen because the small intestine can’t properly digest the sugar in dairy products. As a result, bacteria in the colon ferment the undigested lactose, causing a buildup of gas and water. Adults and children will experience many of the same symptoms if they’re lactose intolerant. It’s very uncommon but still possible for infants to have lactose intolerance. 

For infants and children, both breast milk and milk-based formulas contain lactose. If parents believe an infant might have a lactose intolerance, they should consult their pediatrician and consider eliminating dairy from diet (if breastfeeding) or switching to non-dairy infant formula. Parents should discuss their concerns with their pediatrician before eliminating foods from their children’s diet to ensure adequate nutrition and growth.

Sometimes lactose intolerance is confused for a milk allergy in young children, but being allergic to milk is a very different thing. Children with milk allergies may develop hives, wheezing, a runny nose, diarrhea, or abdominal cramping.  

How is lactose intolerance diagnosed?

Lactose intolerance is usually self-diagnosable, but many of the symptoms of lactose intolerance are the same as irritable bowel syndrome and a milk allergy. So if you suspect you are intolerant, it is important to discuss it with your primary care provider to make sure there are no other medical or nutritional concerns. 

Some medical tests can help accurately diagnose the condition so that people can treat their symptoms appropriately. A hydrogen breath test, which is administered by a gastroenterology specialist, measures how much hydrogen is in the breath after consuming dairy products. It tests for hydrogen because the body turns undigested lactose into hydrogen gas.  

Blood tests are another type of laboratory test that can help diagnose lactose intolerance. A blood test looks for elevated blood glucose levels after the patient consumes a standard amount of lactose. If blood glucose levels don’t go up, this means the body isn’t breaking lactose down into glucose. 

If someone has genetic lactose intolerance, they’ll continue to have symptoms unless they stay away from dairy products. Secondary lactose intolerance may go away after the intestinal tract heals and begins to function normally again, which could take weeks or months.  However, once lactose is eliminated from a diet, the body’s ability to produce the lactase enzyme decreases, resulting in less ability to digest lactose.

Lactose intolerance treatments

Managing this intolerance is usually a matter of making diet changes, but some medications may be helpful.

Diet changes

Many doctors agree that the best way to treat an intolerance is to avoid consuming lactose to begin with. Lactose is in dairy products and non-dairy products, so reading food and medication labels is important.

Foods that are high in lactose include:

  • Cow’s milk
  • Goat’s milk
  • Breast milk and milk-based formula
  • Ice cream
  • Half and half 
  • Some yogurt (Greek yogurt has less lactose)
  • Dry milk powder, milk solids, and milk by-products
  • Cheese, especially soft cheeses (Parmesan, Swiss, and cheddar have less lactose)
  • Cream cheese
  • Cottage cheese
  • Heavy cream
  • Buttermilk 
  • Condensed milk
  • Sherbert
  • Coffee creamers 
  • Butter
  • Ghee
  • Whey 

Non-dairy sources of lactose:

  • Medications
  • Instant foods
  • Margarine
  • Salad dressings
  • Processed grains

Checking food labels is the best way to see whether or not a packaged food item or medication has lactose in it—the label will read “dairy-free” or “lactose-free.” Even small amounts can be difficult to digest, and some foods might cause more symptoms than others.  

Barry Sears, Ph.D., author of The Zone Diet series says some foods have less lactose in them than others. For people who can’t tolerate any lactose in their diet, Dr. Sears recommends lactose-free milk products as a source of high-quality protein. Health food stores will typically carry these types of foods, and regular grocery stores are starting to stock up on things like lactose-free milk as consumer demands go up. Substitutes have become quite trendy. In the milk aisle, you might find soy, rice, almond, coconut, macadamia, and oat milk alternatives.

If you’re concerned that taking dairy products out of your diet will mean you’re not getting enough vitamin D or calcium, you can try adding other foods into your diet. “Milk” isn’t necessary outside of infancy, so it’s very possible to supplement with other products. Fatty fishes, eggs, mushrooms, green leafy vegetables, and nuts are all great sources of calcium and vitamin D. 

“For some, eating yogurt is low enough in lactose not to cause problems,” Sears says. “Hard cheese is much lower in lactose, and lactose-free dairy products have no lactose at all.” The best way to determine which foods cause the most trouble for you is to eliminate all sources of lactose for a week or two, and then add them back in one at a time.  


Some medicines help the digestive system process lactose. Over-the-counter drops and tablets that contain the lactase can help with digestion. Adding drops of lactase to milk before drinking it, or taking a tablet before eating dairy products can make a big difference.  

Lactase is the active ingredient in products like Lactaid and Lac-Dose and their generics. It’s an enzyme supplement a patient with an intolerance should take before eating anything with lactose in it. This type of medication works well for some people, but it isn’t a cure.   

RELATED: What is Lactase? | What is Lactaid? 

Lactose intolerance will never completely go away for someone genetically predisposed to it. It’s possible to manage symptoms, and many people find that their symptoms go away within a couple of days after eliminating dairy products from their diet. The best way to learn more about lactose intolerance and how to treat it is to talk with your dietitian or healthcare provider.   

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