Some people have low levels of lactase, an enzyme in the intestine that is needed to digest the form of sugar that is found in milk.

People who can't completely digest lactose have lactose intolerance. Children under six years old rarely have lactose intolerance. People who do not completely digest milk sugar in the intestine may have abdominal cramps, gas or diarrhea shortly after drinking milk or eating other dairy foods.

Some people experience brief episodes of lactose intolerance when they have the flu or other intestinal illness. In most other cases, however, lactose intolerance is a permanent condition that does not go away or get better as time passes.

If you have a mild case of lactose intolerance, you can still consume small portions of milk or other dairy foods. Eating these foods with a meal is helpful, because the lactose enters the intestine more slowly when there is other food in your stomach. Some dairy products like yogurt and aged cheese are easier to tolerate than milk, possibly because the fermenting or aging process breaks down some of the lactose.

If you have a severe lactose intolerance, you should seek a doctor's or dietitian's advice. Severe lactose intolerance calls for careful management including limiting yourself to very small servings of milk and dairy foods. Avoid foods with lactose between meals and try some of the newer low-lactose milk products. You can also try using special tablets or liquids that predigest the lactose in dairy foods. If your intake of milk and dairy foods is low, you should also ask for medical advice on whether you need to use calcium-fortified foods or calcium supplements.