Lactose intolerance is a condition in which a person cannot digest enough lactose, the sugar found in milk and milk products. The individual has a deficiency of an enzyme known as lactase.

What is going on in the body?

People with lactose intolerance don't have enough lactase to break down the lactose they eat or drink. They are unable to break the lactose down into glucose, which is the form of sugar used by body cells. When lactase is missing from the intestine, the condition is called lactase deficiency. There are three types of lactase deficiency:

Congenital lactase deficiency is a rare disorder that appears to run in families. Infants begin to have symptoms of bloating and watery diarrhea shortly after starting on breast milk or formula feeding.

Acquired lactase deficiency comes on gradually over time. It also seems to run in families. It affects certain ethnic groups more than others. The enzyme lactase is present in the baby's intestine from about the middle of pregnancy onward. The amount of lactase in the intestine begins to drop after weaning. By 5 to 7 years of age, a child's lactase activity is about 10% of what it was at birth.

Temporary lactase deficiency sometimes follows gastroenteritis in children. When the child has diarrhea, his or her intestines are stripped of the enzyme lactase. The child then has trouble digesting lactose when he or she drinks milk products.

When lactose is not digested, water is retained in the bowel. This results in bloating and watery diarrhea. Lactose that passes into the large intestine is fermented by bacteria. This produces carbon dioxide, hydrogen, and methane. This in turn leads to bloating, cramping, and flatulence, or passing gas.

What are the causes and risks of the condition?

Having no lactase causes lactose intolerance. In young children, temporary lactose intolerance may follow a stomach virus. About 75% of African Americans and Native Americans have lactose disorder. As many as 90% of Asian Americans are lactose-intolerant.

Milk and milk products are the only natural sources of lactose. However, dry milk powder, whey, curds, and milk solids are common ingredients in processed foods. Some foods that often contain lactose include:

Lactose may also be found in both over-the-counter and prescription medicines. Common examples include antacids and oral contraceptives.