How Do You Know You're Lactose Intolerant

If you feel bloated or have stomach cramps and diarrhea after drinking milk, you could be lactose intolerant, but these are also common symptoms of other digestive conditions. Find out how to determine if you're really lactose intolerant.

Lactose intolerance is not the same as an allergy to milk or dairy products. If you are allergic to milk, you might find that your mouth itches or you break out in hives after drinking it. But if you are lactose intolerant, you will start to have certain abdominal symptoms once your body starts trying to digest what you've eaten, usually within 30 minutes to 2 hours, says Susan Lucak, MD, a gastroenterologist in New York City and a special lecturer at the College of Physicians & Surgeons of Columbia University.

Lactose is a sugar found in milk, cheese, ice cream, and other dairy products. In some people, the small intestine isn't able to break down this sugar into simpler forms that can then be absorbed into the bloodstream. If you're one of the 30 to 50 million Americans who are lactose intolerant, eating dairy foods can give you digestive trouble and have you running to the bathroom.

Not everyone who is lactose intolerant reacts the same way or within the same amount of time. For instance, you may be able to handle more lactose than others can. "Some people have partial lactose intolerance," Dr. Lucak explains. "They can ingest a certain amount of milk and milk products, but if they exceed this amount, they will develop the symptoms."

Lactose intolerance also tends to run in families. "If someone in your family has the condition, it is possible that you have it, too," Lucak says. "And it is more common in Asians and African-Americans."

Signs of Lactose Intolerance

The most common signs of lactose intolerance are abdominal bloating, abdominal cramping, excessive gas, nausea, and diarrhea, Lucak says. Because these are common symptoms, they do not always mean you are lactose intolerant. A simple practice can help you determine the answer.

"Eliminate all dietary lactose for two weeks and see if your symptoms disappear," Lucak says. If your symptoms improve when you're on a dairy-free diet and the signs of lactose intolerance reappear as soon as you start eating dairy again, chances are you are lactose intolerant.

Your doctor can then confirm lactose intolerance with one of two tests available for older children and adults:

• Lactose tolerance test. After fasting, a blood test measures your fasting blood sugar (glucose) level. Next, you drink a large amount of a liquid that contains lactose. Then blood is drawn over a two-hour period to determine how well your body is able to digest the lactose.
• Hydrogen breath test. This test measures the amount of hydrogen in your breath. Normally, very little can be detected. However, when you are unable to digest lactose, the bacteria you have in your colon causes various gases, including hydrogen, to form. Your intestines absorb the hydrogen, which is carried through the bloodstream to your lungs. When you breathe out, the hydrogen is exhaled. For this test, you will be asked to drink a lactose-loaded beverage, and your breath will be analyzed at regular intervals. Before the test, you may need to avoid certain foods, medications, and smoking because they can affect the accuracy of the test.

Infants and very young children may be given a stool acidity test, rather than a lactose tolerance or hydrogen breath test. Measuring the amount of acid in the stool will show lactose intolerance.

Managing Lactose Intolerance

If testing confirms that you are lactose intolerant, your symptoms can usually be controlled simply by avoiding milk and milk products, Lucak says. And you may be able to enjoy dairy foods — and get the nutrients they provide — by taking lactase tablets when you eat them, since lactase is the enzyme lacking in people with lactose intolerance. Also consider milk and other dairy products specifically formulated for people who are lactose-intolerant.

Talk to your doctor if you suspect that you're lactose intolerant. If he makes this diagnosis, you can work with the doctor or a nutritionist on a plan to manage lactose intolerance while still getting the important nutrients that dairy foods provide.

By Beth W. Orenstein

Medically reviewed by Pat F. Bass III, MD, MPH