What is lactose intolerance?
Lactose intolerance is an inability to digest and absorb lactose (the sugar in milk) that results in gastrointestinal symptoms when milk or products containing milk are drunk or eaten.

What causes lactose intolerance?
Lactose is a larger sugar that is made up of two smaller sugars, glucose and galactose. In order for lactose to be absorbed from the intestine and into the body, it must first be split into glucose and galactose. The glucose and galactose then are absorbed by the cells lining the small intestine. The enzyme that splits lactose into glucose and galactose is called lactase, and it is located on the surface of the cells that line the small intestine.

Lactose intolerance is caused by reduced or absent activity of lactase that prevents the splitting of lactose (lactase deficiency). Lactase deficiency may occur for one of three reasons, congenital, secondary or developmental.

How is lactose intolerance treated?

Dietary changes
The most obvious means of treating lactose intolerance is by reducing the amount of lactose in the diet. Fortunately, most people who are lactose intolerant can tolerate small or even moderate amounts of lactose. It often takes only elimination of the major milk-containing products to obtain sufficient relief from their symptoms. Thus, it may be necessary to eliminate only milk, yogurt, cottage cheese, and ice cream. Though yogurt contains large amounts of lactose, it often is well-tolerated by lactose intolerant people. This may be so because the bacteria used to make yogurt contain lactase, and the lactase is able to split the lactose during storage of the yogurt as well as after the yogurt is eaten (in the stomach and intestine). Yogurt also has been shown to empty more slowly from the stomach than an equivalent amount of milk. This allows more time for intestinal lactase to split the lactose in yogurt, and, at least theoretically, would result in less lactose reaching the colon.

Lactase enzyme
Caplets or tablets of lactase are available to take with milk-containing foods.

Adaptation
Some people find that by slowly increasing the amount of milk or milk-containing products in their diets they are able to tolerate larger amounts of lactose without developing symptoms. This adaptation to increasing amounts of milk is not due to increases in lactase in the intestine. Adaptation probably results from alterations to the bacteria in the colon. Increasing amounts of lactose entering the colon change the colonic environment, for example, by increasing the acidity of the colon. These changes alter the way in which the colonic bacteria handle lactose. For example, the bacteria produce less gas. There also may be a reduction in the secretion of water and, therefore, less diarrhea