Lactose intolerance is a condition in which the body is unable to fully digest lactose, a type of sugar found in milk and dairy products. This is due to a deficiency of the enzyme lactase, which is needed to break down lactose into simpler sugars that can be absorbed into the bloodstream.

Symptoms of lactose intolerance may include bloating, gas, abdominal pain, diarrhea, and nausea after consuming dairy products. These symptoms can range from mild to severe, and may occur immediately or several hours after consuming lactose.

Lactose intolerance is not the same as a milk allergy, which is an immune response to the proteins in milk rather than the lactose sugar. Milk allergy can cause a range of symptoms such as hives, wheezing, and anaphylaxis.

Lactose intolerance is common in adults, and is more prevalent in certain populations, such as those of Asian, African, and Native American descent. It can also occur as a result of certain medical conditions, such as celiac disease or inflammatory bowel disease.

Treatment for lactose intolerance involves managing the symptoms by avoiding or limiting dairy products in the diet. Lactose-free dairy products are available, as well as lactase supplements that can be taken before consuming dairy products to help break down lactose. It is important to maintain a balanced and nutritious diet, and to speak with a healthcare provider or registered dietitian to ensure that nutrient needs are being met.

Lactose intolerance occurs when your small intestine doesn't produce enough of an enzyme (lactase) to digest milk sugar (lactose).

Normally, lactase turns milk sugar into two simple sugars — glucose and galactose — which are absorbed into the bloodstream through the intestinal lining.

If you're lactase deficient, lactose in your food moves into the colon instead of being processed and absorbed. In the colon, normal bacteria interact with undigested lactose, causing the signs and symptoms of lactose intolerance.

There are three types of lactose intolerance. Different factors cause the lactase deficiency underlying each type.

Primary lactose intolerance

People who develop primary lactose intolerance — the most common type — start life producing enough lactase. Infants, who get all their nutrition from milk, need lactase.

As children replace milk with other foods, the amount of lactase they produce normally drops, but usually remains high enough to digest the amount of dairy in a typical adult diet. In primary lactose intolerance, lactase production falls off sharply by adulthood, making milk products difficult to digest.

Secondary lactose intolerance

This form of lactose intolerance occurs when your small intestine decreases lactase production after an illness, injury or surgery involving your small intestine. Diseases associated with secondary lactose intolerance include intestinal infection, celiac disease, bacterial overgrowth and Crohn's disease.

Treatment of the underlying disorder might restore lactase levels and improve signs and symptoms, though it can take time.

Congenital or developmental lactose intolerance

It's possible, but rare, for babies to be born with lactose intolerance caused by a lack of lactase. This disorder is passed from generation to generation in a pattern of inheritance called autosomal recessive, meaning that both the mother and the father must pass on the same gene variant for a child to be affected. Premature infants can also have lactose intolerance because of an insufficient lactase level.

In people with lactose intolerance caused by an underlying condition, treating the condition might restore the body's ability to digest lactose, although that process can take months. For other causes, you might avoid the discomfort of lactose intolerance by following a low-lactose diet.

To lower the amount of lactose in your diet:

• Limit milk and other dairy products
• Include small servings of dairy products in your regular meals
• Eat and drink lactose-reduced ice cream and milk
• Add a liquid or powder lactase enzyme to milk to break down the lactose