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. 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. 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. 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.
Mayo Clinic does not endorse companies or products. Advertising revenue supports our not-for-profit mission. This content does not have an English version. This content does not have an Arabic version. Most cases that develop in adults are inherited and tend to be lifelong, but cases in young children are often caused by an infection in the digestive system and may only last for a few weeks.
Lactose intolerance can develop at any age. Many cases first develop in people aged 20 to 40, although babies and young children can also be affected. Lactose intolerance is not the same as a milk or dairy allergy. Food allergies are caused by your immune system reacting to a certain type of food. This causes symptoms such as a rash, wheezing and itching. If you're allergic to something, even a tiny particle can be enough to trigger a reaction, while most people with lactose intolerance can still consume small amounts of lactose without experiencing any problems, although this varies from person to person.
There's no cure for lactose intolerance, but cutting down on food and drink containing lactose usually helps to control the symptoms. Your GP may also recommend calcium and vitamin D supplements. You may be advised to take lactase substitutes, which are drops or tablets you can take with your meals or drinks to improve your digestion of lactose.
Milk and other dairy products contain calcium, protein and vitamins, such as A, B12 and D. Lactose also helps your body absorb a number of other minerals, such as magnesium and zinc. These vitamins and minerals are important for the development of strong, healthy bones.
If you're lactose intolerant, getting the right amount of important vitamins and minerals can prove difficult. The symptoms of lactose intolerance typically occur between 30 minutes and two hours after eating or drinking a milk or dairy product. Symptoms may include:. The symptoms can range from mild to severe. The severity depends on how much lactose was consumed and how much lactase the person has actually made.
Confirmatory tests measure lactase activity in the body. These tests include:. A hydrogen breath test measures the amount of hydrogen in your breath after consuming a drink high in lactose. If your body is unable to digest the lactose, the bacteria in your intestine will break it down instead. The process by which bacteria break down sugars like lactose is called fermentation.
Fermentation releases hydrogen and other gases. These gases are absorbed and eventually exhaled. This test is more often done in infants and children. It measures the amount of lactic acid in a stool sample. Lactic acid accumulates when bacteria in the intestine ferment the undigested lactose. Treatment for lactose intolerance involves decreasing or completely removing milk products from the diet. Lactose-free milk products can also be found at most supermarkets.
And not all dairy products contain a lot of lactose. You may still be able to eat some hard cheeses, such as cheddar, Swiss, and Parmesan, or cultured milk products like yogurt. Low-fat or nonfat milk products typically have less lactose as well. An over-the-counter lactase enzyme is available in capsule, pill, drops, or chewable form to take before consuming dairy products. The drops can also be added to a carton of milk. People who are lactose intolerant and not consuming milk or dairy products may become deficient in:.
Taking calcium supplements or eating foods that are either naturally high in calcium or are calcium-fortified is recommended. Symptoms will go away if milk and milk products are removed from the diet. Read food labels carefully to detect ingredients that may contain lactose. Aside from milk and cream, look out for ingredients derived from milk, such as:.
Many foods that you would not expect to contain milk may actually contain milk and lactose. Examples include:. Milk and milk products are often added to processed foods. Even some nondairy creamers and medications may contain milk products and lactose. Lactose intolerance cannot be prevented.
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