Lactose intolerance is the inability to digest a sugar called lactose that is found in milk and dairy products. Normally when a person eats something containing lactose, an enzyme in the small intestine called lactase breaks down lactose into simpler sugar forms called glucose and galactose. These simple sugars are then easily absorbed into the bloodstream and turned into energy fuel for our bodies. People with lactose intolerance do not produce enough of the lactase enzyme to break down lactose. Instead, undigested lactose sits in the gut, causing gas, bloating, and stomach cramps.

When the intestine cannot absorb the lactose-containing foods, it can lead to diarrhea. Certain digestive diseases and injuries to the small intestine can reduce the amount of enzymes produced. In rare cases, children are born without the ability to produce lactase. For most people, lactase deficiency is a condition that develops naturally and over time. After the age of two, the body begins to produce less lactase. The reasons for this are unclear and under study.

Common symptoms are nausea, cramps, bloating, gas and diarrhea, which can begin between about 30 minutes to two hours after eating or drinking foods containing lactose. Many people who have never been diagnosed as lactose intolerant, or "lactase deficient," may notice that milk and other dairy products cause problems that do not occur when eating other foods. The severity of symptoms varies, depending on the amount of lactose each individual can tolerate.

Dietary changes can treat lactose intolerance by reducing the amount of lactose in the diet. Thus, it may be necessary to eliminate only milk, yogurt, cottage cheese, and ice cream. Another means to reduce symptoms of lactose intolerance is to ingest any milk-containing foods during meals. Meals (particularly meals containing fat) reduce the rate at which the stomach empties into the small intestine. This reduces the rate at which lactose enters the small intestine and allows more time for the limited amount of lactase to split the lactose without being overwhelmed by the full load of lactose at once. Studies have shown that the absorption of lactose from whole milk, which contains fat, is greater than from non-fat milk, perhaps for this very reason.

Substitution of whole milk or yogurt for non-fat milk or yogurt does not seem to reduce the symptoms of lactose intolerance. Caplets or tablets of lactase are available to take with milk-containing foods. Adaptation in some people 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, 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, fewer diarrheas. Calcium and vitamin D supplements are recommended because calcium deficiency is common among lactose intolerant persons.