Lactose intolerance is the inability to digest lactose (a type of sugar found in milk and other dairy products).

Lactose intolerance happens when the small intestine does not make enough of the enzyme lactase. Babies' bodies make this enzyme so they can digest milk, including breast milk. Before humans became dairy farmers, most people did not continue to drink milk, so their bodies did not make lactase after early childhood.

Lactose intolerance is more common in Asian, African, African-American, Native American, and Mediterranean populations than it is among northern and western Europeans.

Lactose intolerance can begin at different times in life. In Caucasians, it usually starts to affect children older than age 5. In African-Americans, lactose intolerance often occurs as early as age 2.

Lactose intolerance is very common in adults and is not dangerous. Approximately 30 million American adults have some amount of lactose intolerance by age 20.

Lactose intolerance is sometimes seen in premature babies. Children who were born at full term generally do not show signs of lactose intolerance until they are at least 3 years old.

Not having enough lactase (lactase deficiency) may also occur as a result of intestinal diseases such as celiac sprue and gastroenteritis, or after bowel surgery. Temporary lactase deficiency can result from viral and bacterial infections, especially in children, when the cells lining the intestine are injured.

• Abdominal bloating
• Abdominal cramps
• Diarrhea
• Floating stools
• Foul-smelling stools
• Gas (flatulence)
• Malnutrition
• Nausea
• Slow growth
• Weight loss

Symptoms often occur after you eat or drink milk products, and are often relieved by not eating or drinking milk products. Large doses of milk products may cause worse symptoms.

• Enteroscopy
• Lactose-hydrogen breath test
• Lactose tolerance test
• Test for stool-reducing substances

Removing milk products from the diet usually improves the symptoms. However, not having milk in the diet can lead to a shortage of calcium, vitamin D, riboflavin, and protein. Add other sources of calcium to the diet if you remove milk products.

Most people with low lactase levels can tolerate 2 - 4 ounces of milk at one time (up to one-half cup). Larger (8 oz.) servings may cause problems for people with some amount of milk intolerance.

These milk products may be easier to digest:

• Buttermilk and cheeses (they have less lactose than milk)
• Fermented milk products, such as yogurt
• Goat's milk (but drink it with meals, and make sure it is supplemented with essential amino acids and vitamins if you give it to children)
• Ice cream, milkshakes, and aged or hard cheeses
• Lactose-free milk and milk products
• Lactase-treated cow's milk for older children and adults
• Soy formulas for infants younger than 2 years
• Soy or rice milk for toddlers

You can add lactase enzymes to regular milk or take them in capsule or chewable tablet form.

You may need to find new ways to get calcium into your diet (you need 1,200 - 1,500 mg of calcium each day):

• Take calcium supplements
• Eat foods that have more calcium (leafy greens, oysters, sardines, canned salmon, shrimp, and broccoli)
• Drink orange juice that contains added calcium

Read food labels. Lactose is also found in some non-milk products -- including some beers.

Symptoms usually go away when milk products are removed from the diet.

Weight loss and malnutrition are possible complications.

Call your health care provider if:

• You or your child has symptoms of lactose intolerance and you need information on food substitutes.
• Your symptoms get worse or do not improve with treatment, or you develop new symptoms.

There is no known way to prevent lactose intolerance.

If you have the condition, avoiding or restricting the amount of milk products in your diet can reduce or prevent symptoms.

Information from your family doctor. Lactose intolerance: what you should know. Am Fam Physician. 2006;74:1927-1928.

Hogenauer C, Hammer HF. Maldigestion and Malabsorption. In: Feldman M, Friedman LS, Sleisenger MH, eds. Sleisenger & Fordtran's Gastrointestinal and Liver Disease. 8th ed. Philadelphia, Pa: Saunders Elsevier; 2006: chap98.