Digestive Disorders Health Center

Lactose Intolerance

Lactose intolerance is the inability to digest lactose, the sugar primarily found in milk and dairy products. It is caused by a shortage in the body of lactase, an enzyme produced by the small intestine, which is needed to digest lactose. While lactose intolerance is not dangerous, its symptoms can be distressing.

What Are the Symptoms of Lactose Intolerance?

Symptoms of lactose intolerance include:

• Nausea
• Cramps
• Painful gas
• Bloating
• Diarrhea

Symptoms tend to develop 30 minutes to two hours after drinking milk or eating dairy products. The severity of symptoms varies, depending on the amount of lactose a person can tolerate. Some people may be sensitive to extremely small amounts of lactose-containing foods, while others can eat larger amounts before they notice symptoms.

What Foods Contain Lactose?

The most common foods that are high in lactose include dairy products such as milk, ice cream, and cheese. Lactose is also added to some foods, such as bread and baked goods, cereals, salad dressings, candies, and snacks.

Foods that contain whey, curds, milk by-products, dry milk solids, and nonfat dry milk also contain lactose.

Lactose is also present in about 20% of prescription medications, such as birth control pills (oral contraceptives), and about 6% of over-the-counter medications, such as some tablets for stomach acid and gas.

Who Gets Lactose Intolerance?

Lactose intolerance is extremely common. It is estimated that 30 to 50 million Americans have some degree of lactose intolerance. Certain racial and ethnic populations are more affected than others, including 75% of African Americans, Jews, Mexicans and Native Americans, and 90% of Asians.

What Causes Lactose Intolerance?

For most people, lactose intolerance develops naturally as they grow older. The small intestine begins to produce less lactase in everyone after age two. Certain digestive diseases such as Crohn's disease, Celiac disease (a digestive disease that damages the small intestine and interferes with absorption of nutrients from food), and injuries to the small intestine can also reduce the amount of lactase available to process lactose properly.

How Is Lactose Intolerance Diagnosed?

Usually lactose intolerance is diagnosed based on symptoms and relief of those symptoms when avoiding dairy products. However, certain tests for lactose intolerance may be used to help confirm the diagnosis. Many doctors will ask patients who suspect they have lactose intolerance to avoid milk and dairy products for one or two weeks to see if their symptoms subside. One of the following tests may be given to confirm the diagnosis.

• Milk challenge test: A milk challenge is a simple way of diagnosing lactose intolerance. A person fasts overnight and then drinks a glass of milk in the morning. Nothing further is eaten or drunk for three to five hours. If a person is lactose intolerant, the milk should produce symptoms within several hours of ingestion.
• Hydrogen breath test: The hydrogen breath test measures the amount of hydrogen in the breath after drinking a lactose-loaded beverage. Raised levels of hydrogen in the breath three to five hours after ingestion of lactose indicate improper digestion of lactose.
• Blood glucose test: During the lactose intolerance test your blood sugar is measured over a two-hour period after drinking a lactose-loaded beverage. You are required to fast before the test. By measuring the level of sugar in the blood, the test indicates how well the body is digesting lactose.
• Stool acidity test: The stool acidity test is a test for lactose intolerance in infants and young children. The child is given a small amount of lactose to drink. Lactic acid turns the stool acidic. Therefore, children with lactose intolerance will develop acidic stool after consuming lactose.
• Intestinal biopsy: The most direct test for lactose intolerance is biopsy of the intestinal lining to measure lactase levels in the lining. However, these biopsies are invasive and require specialized analysis that is not available at most doctor's offices. Thus, lactase levels are not usually measured through biopsy except for research purposes.

The blood glucose test and hydrogen breath test are not given to infants and very young children, because they may cause severe diarrhea. If an infant or young child is having symptoms of lactose intolerance, your child's health care provider will likely recommend changing from a cow's milk formula to a soy milk formula until the symptoms disappear. Milk and dairy products may be slowly reintroduced at a later time. If needed to confirm the diagnosis, a stool acidity test may be given to infants and young children.