Lactose Intolerance: Causes, Symptoms, and Treatment

Lactose intolerance is extremely common. Learn more about its causes, symptoms, and treatment.

Many delicious foods contain dairy in some way, shape, or form—and those dairy-filled dishes may lead to symptoms including pain or gas for most people. Many people have been reducing or eliminating dairy from their diet because of this discomfort, among other health benefits.

If you think you may have lactose intolerance, learn more about this condition and its associated symptoms below.

What Is Lactose Intolerance?

Lactose intolerance is when your inability to digest lactose causes symptoms. This condition is caused by not having enough lactase, a substance that breaks down lactose.

Lactase works in your small intestine to break down sugars from dairy. However, if there is not enough lactase to break down the lactose in your food and drink, those sugars move through your system to your colon. According to the National Institute of Diabetes and Digestive Kidney Diseases (NIDDK), bacteria in your colon break down lactose and cause the symptoms of lactose intolerance.


Lactase can be produced by cells in your small intestine. However, if these cells are injured or have genetic changes, they may create little to no lactase and cause lactose intolerance.

According to the National Library of Medicine (NLM), lactase production is controlled by a gene named LCT. As such, mutated LCT genes create lactase that cannot digest lactose. In rare cases, people can be born with LCT gene mutations and have lactose intolerance from birth. More commonly, people develop lactose intolerance as they age because their body stops using the LCT gene to make lactase. Lactose intolerance can often run in families because of these genetic causes.

Injuries to the small intestine can also cause lactose intolerance. According to the NIDDK, some examples of injuries include gastrointestinal conditions like Crohn's disease and celiac disease, medications, surgery, or radiation. After your small intestine heals, you may regain your ability to digest lactose.

Lactose Intolerance Symptoms

Symptoms of lactose intolerance typically start 30 minutes to two hours after ingesting dairy products and include:

  • abdominal pain
  • diarrhea
  • gas
  • nausea
  • bloating
  • stomach "growling" or rumbling sounds
  • vomiting

The first three—diarrhea, gas, and abdominal pain—are the most common symptoms, Rabia de Latour, MD, gastroenterologist and assistant professor of medicine at NYU Langone Health, told Health. These symptoms are caused by bacteria in your colon breaking down undigested lactose, creating extra fluid and gas that your body then expels.

How Long Lactose Intolerance Symptoms Last

Lactose intolerance symptoms vary depending on the person and their digestive speed. Symptoms typically start 30 minutes to two hours after consuming a dairy product. However, it can take 2-5 days to digest food. If you have diarrhea, the dairy product will pass through your system faster, but you can expect symptoms to last several hours.

Diagnosis and Testing

Healthcare providers can diagnose lactose intolerance using a few different techniques. "Most of the time, it's a clinical diagnosis," Dr. de Latour said. When patients say something like, "Every time I have milk in my coffee, I get these symptoms," the statement signals to healthcare providers that their patient could have lactose intolerance.

During the diagnosis process, you may also be asked to keep a food diary. Writing down what you eat might make you more aware of what causes your stomach to hurt. Other methods of diagnosis include using a scope to examine your intestines or lactose tolerance tests.

According to the NLM, some lactose intolerance tests include the hydrogen breath test and the glucose blood (blood sugar) test. For both tests, avoid drinking 8-12 hours before your appointment. Then, after drinking a liquid with lactose, the hydrogen in your breath or your blood sugar is measured for several hours. High levels of hydrogen and no increases in blood sugar indicate that you are lactose intolerant.


The good news is that even if you have lactose intolerance, the condition is easily manageable. One option is to limit or eliminate lactose-containing foods from your diet. You can also try lactase products that can help you digest dairy products. If your lactose intolerance is due to a small intestine injury, you can work with your healthcare provider to treat that injury.

According to Dr. de Latour, lactose intolerance is "not a super dangerous disease to have—just inconvenient."

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