What Causes Lactose Intolerance?

If you often have bloating, gas, diarrhea, stomach pain, or cramps after drinking milk or eating foods that contain dairy products, you may be lactose intolerant. It’s is common in adults. Between 30 million to 50 million adults have it.

What Is It?

Lactose intolerance means that your body can’t digest lactose, a sugar found in milk and dairy products. This happens when your small intestine doesn’t make enough lactase, the enzyme that breaks down lactose.

When you don’t make enough lactase, the lactose you eat or drink doesn’t get digested in your stomach or small intestine. Instead, it’s broken down by bacteria in your colon and creates gas. This can lead to those unpleasant symptoms.

Types and Causes

There are four types of lactose intolerance, and they all have different causes.

  • Primary lactose intolerance is the most common form. Our bodies typically stop making lactase by about age 5 (as early as age 2 for African-Americans). As lactase levels decrease, dairy products become harder to digest. People with primary lactose intolerance make a lot less lactase. That makes dairy products hard to digest by adulthood. It’s caused by genes and is common among people of an African, Asian, Hispanic, Mediterranean and southern European background. It’s less common if your heritage is from northern or western Europe.
  • Secondary lactose intolerance happens because of an injury, illness or possibly a surgery. Any of these can affect your small intestine and cause you to make less lactase. Celiac disease and Crohn’s disease are two of the most common intestinal diseases linked to low lactase.
  • Developmental lactose intolerance happens in babies who are born prematurely. It usually goes away on its own, lasting for only a short time after birth.
  • Congenital lactose intolerance is very rare and happens when no lactase (or a very small amount of it) is produced by the small intestine from birth. It’s a genetic disorder, and both parents have to pass the gene on to their child.

Lactose Intolerance or Dairy Allergy?

Lactose intolerance is not the same as a dairy allergy. The two are often confused. If you have dairy allergy, you’re allergic to certain proteins in milk and dairy products. Dairy allergy reactions can be life-threatening.

The symptoms of lactose intolerance are less severe than those of a dairy allergy. People with dairy allergy need to avoid all foods and drinks that contain milk or other dairy products. If you’re lactose intolerant, you may be able to eat and drink small amounts of dairy products. How much varies from person to person. Lactose intolerance reactions aren’t life-threatening.


Can It Be Prevented?

It can’t be prevented and there isn’t a cure for it. But it’s easily treated. Limit the amount of food and drink you have that contains lactose. You can also take lactase enzyme supplements to help your body digest lactose.

If you limit milk and other dairy products, you may not get enough calcium and vitamin D. Talk to your doctor about taking calcium supplements with vitamin D and eating calcium-rich foods, such as leafy greens, broccoli, soybeans and some seafood like salmon.

WebMD Medical Reference Reviewed by Melinda Ratini, DO, MS on May 19, 2021



American Family Physician: “Lactose Intolerance.”

Mayo Clinic: “Lactose intolerance.”

MedlinePlus: “Lactose intolerance.”

National Institutes of Health, National Institute of Child Health and Human Development: “What are the symptoms of lactose intolerance?”

National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases: “Lactose Intolerance.”

National Health Services (U.K.) Choices: “Lactose intolerance.”

NICHD Information Resource Center: "Lactose Intolerance: Information for Health Care Providers."

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