Lactose Intolerance

Lactose intolerance is the inability to properly digest lactose, a sugar found in milk and other dairy products such as yogurt, ice cream and soft cheeses. It is caused by lack of an enzyme called lactase.

Normally, lactase is produced in the small intestine, where it breaks lactose down into a form that can be absorbed by the intestine and deliver glucose into the blood. Without adequate lactase, the body can't digest lactose. As a result, lactose stays in the intestines and causes gas, bloating, stomach cramps and diarrhea.

Many parents confuse lactose intolerance with milk allergy. While they have similar symptoms, they are completely different conditions. Lactose intolerance is a digestive problem, while milk allergy involves the immune system. Your child can be tested for milk allergy or lactose intolerance.

It's very rare for babies to be born without the ability to produce lactase. For most people, lactase deficiency develops naturally over time. In some populations — such as Latinos and African Americans — the body begins to produce less lactase after about age 2. In other groups, such as Northern Europeans, this occurs around 3 to 4 years of age. However, many children, and even adults, may not experience symptoms until they are much older.

In addition, certain digestive diseases and injuries to the small intestine can reduce the amount of enzymes produced.

Symptoms of lactose intolerance differ for each person. The severity of symptoms may depend on how much lactose is consumed and the amount each person can tolerate. Common symptoms, which usually begin 30 minutes to two hours after consuming foods with lactose, include:

  • Nausea
  • Stomach cramps
  • Bloating and distention
  • Noisy bowel sounds (called "borborygmi")
  • Gas
  • Diarrhea

Other digestive disorders can cause similar symptoms. If your child frequently experiences the symptoms listed above, it's important to visit a qualified pediatric gastroenterologist for a proper diagnosis.

The tests most commonly used to diagnose lactose intolerance are:

  • Lactose Hydrogen Breath Test — This relatively easy test measures hydrogen produced by bacteria in the large intestine that is fermenting the unabsorbed and undigested lactose.
  • Intestinal Biopsy — The biopsy measures the amount of lactase enzyme in the intestinal lining.

A stool acidity test also may be performed. Undigested lactose fermented by bacteria in the colon creates lactic acid and other short-chain fatty acids that can be detected in a stool sample. Sugars, including lactose, also may be detected in the stool by a simple test.

Although lactose intolerance can't be cured, symptoms can be controlled with a modified diet that reduces dairy intake or, if necessary, avoids dairy altogether. Most children don't require a completely lactose-free diet. Our team's doctors and dietitians will work closely with you and your child to develop a diet that best meets your child's nutritional needs.

Studies show that the following strategies can help control symptoms:

  • Drink low-fat or fat-free milk in servings of 1 cup or less.
  • Consume low-fat or fat-free milk with other food, such as breakfast cereal.
  • Consume dairy products other than milk, such as low-fat or fat-free hard cheeses, cottage cheese, ice cream or yogurt. These foods contain less lactose per serving compared with milk and may cause fewer symptoms.
  • Show More

UCSF Research & Clinical Trials


Reviewed by health care specialists at UCSF Benioff Children's Hospital.

Related Information

UCSF Clinics & Centers

Gastroenterology & Nutritional Disorders

Gastroenterology & Liver Practice
1825 Fourth St., Sixth Floor
San Francisco, CA 94158
Phone: (415) 353-2813
Fax: (415) 476-1343
Appointment information