What Is Lactose Intolerance? Symptoms, Causes, Diagnosis, Treatment, and Prevention

When you were a kid, your mother may have reminded you to always drink your milk. But these days many adults find themselves experiencing some degree of what’s known as lactose intolerance.

Lactose is a sugar found in dairy products. When you consume food or drink containing lactose, an enzyme in the small intestine called lactase helps you digest the sugar. But when you’re lactose intolerant, you can’t digest these sugars, and this creates excess gas and other gastrointestinal symptoms.

The most common type of lactose intolerance is primary lactose intolerance. In primary lactose intolerance, you’re born with a normal amount of lactase. But by the time you reach adolescence or adulthood, your lactase production decreases sharply, and it becomes very difficult to digest foods that contain dairy. (You may hear this described as lactose nonpersistence.)

Secondary or acquired lactase deficiency arises when an infection or disease — such as celiac disease, gastroenteritis, or Crohn's disease — damages the small intestine.

Congenital lactase deficiency is a rare inherited disorder in which the small intestine produces little to no lactase from birth. (More on that below.)

Here’s everything you need to know about lactose intolerance — and what to do about it.

Many people with lactose intolerance have a deficiency of the enzyme lactase because their small intestine doesn’t produce enough lactase. (The medical term for this state is hypolactasia.)

This deficiency may lead to lactose malabsorption, in which undigested lactose makes its way into the large intestine and colon. There, bacteria break it down, resulting in increased gas and fluid in the colon (and unpleasant bloating, flatulence, and other tummy symptoms).

Risk factors for lactose intolerance include:

• Age (it tends to appear after adolescence or young adulthood)
• A person’s race or ethnicity
• Premature birth
• Chemotherapy or radiation treatments for cancer

There's no cure for lactose intolerance, but most people are able to control their symptoms by making changes to their diet.

The Academy of Nutrition and Dietetics’s Eat Right website offers a wealth of clear and helpful information on how and what to eat when you’re lactose intolerant.

Additional reporting by Joseph Bennington-Castro.