Lactose Intolerance
(Lactase Deficiency)

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Lactose intolerance facts

  • Lactose intolerance is an inability to digest lactose, the main sugar in milk, that gives rise to gastrointestinal symptoms.
  • Lactose intolerance is caused by a deficiency of the intestinal enzyme lactase that splits lactose into two smaller sugars, glucose and galactose, and allows lactose to be absorbed from the intestine.
  • The primary symptoms of lactose intolerance are diarrhea, flatulence (passing gas), and abdominal pain. Abdominal bloating, abdominal distention, and nausea also may occur.
  • Lactose intolerance can be diagnosed by eliminating lactose from the diet, milk challenge, breath test, blood glucose test, stool acidity test, and intestinal biopsy.
  • Lactose intolerance is treated with dietary changes, supplements of lactase enzyme, and adaptation to increasing amounts of milk.
  • Avoidance of milk and milk-containing products can lead to a dietary deficiency of calcium and vitamin D that, in turn, can lead to bone disease (osteoporosis).

What is lactose intolerance?

Lactose intolerance is the inability to digest and absorb lactose (the sugar in milk) that results in gastrointestinal symptoms when milk or food products containing milk are consumed.

What causes lactose intolerance?

Lactose is a large sugar molecule 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 are then 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 lining 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.

Congenital causes of lactose intolerance

Lactase deficiency may occur because of a congenital absence (absent from birth) of lactase due to a mutation in the gene that is responsible for producing lactase. This is a very rare cause of lactase deficiency, and the symptoms of this type of lactase deficiency begin shortly after birth.

Secondary causes of lactose intolerance

Another cause of lactase deficiency is secondary lactase deficiency. This type of deficiency is due to diseases that destroy the lining of the small intestine along with the lactase. An example of such a disease is celiac sprue.

Developmental causes of lactose intolerance

The most common cause of lactase deficiency is a decrease in the amount of lactase that occurs after childhood and persists into adulthood, referred to as adult-type hypolactasia. This decrease in lactase is genetically programmed, and the prevalence of this type of lactase deficiency in different ethnic groups is highly variable. Lactose intolerance is most common among Asians, affecting more than 90 % of adults in some communities. People with ancestry from Northern Europe, on the other hand, have a 5% rate of lactose intolerance. In addition to variability in the prevalence of lactase deficiency, there also is variability in the age at which symptoms of lactose intolerance appear.

It is important to emphasize that lactase deficiency is not the same as lactose intolerance. Persons with milder deficiencies of lactase often have no symptoms after the ingestion of milk. For unclear reasons, even persons with moderate deficiencies of lactase may not have symptoms. A diagnosis of lactase deficiency is made when the amount of lactase in the intestine is reduced, but a diagnosis of lactose intolerance is made only when the reduced amount of lactase causes symptoms.

Medically Reviewed by a Doctor on 12/12/2013

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Lactose Intolerance - Symptoms Question: The symptoms of lactose intolerance can vary greatly from patient to patient. What were your symptoms at the onset of your disease?
Lactose Intolerance - Causes Question: When did you develop lactose intolerance?
Lactose Intolerance - Diet Question: What's it like to try the elimination diet? What were the results?
Lactose Intolerance - Treatment Question: What forms of treatment, including diet and supplements, have been effective for your lactose intolerance?
Is it food allergies or food intolerance?

Food Allergies vs. Food Intolerance

Sometimes people become sick from eating a particular food, because they cannot properly process or digest the food, or because they have a true allergic (immune) reaction to the food. Food allergies and food intolerance are sometimes confused with each other, but they are quite different in terms of their origin, symptoms and treatment.


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