Lactose Intolerance

Lactose Intolerance
(Lactase Deficiency)

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Medical Author: Jay W. Marks, M.D.
Medical Editor: Dennis Lee, M.D.

• What is lactose intolerance?
• What causes lactose intolerance?
• What are the symptoms of lactose intolerance?
• How are lactase deficiency and lactose intolerance diagnosed?
• Elimination diet
• Milk challenge
• Breath test
• Blood glucose test
• Stool acidity test
• Intestinal biopsy
• What are the sources of lactose in the diet?
• How is lactose intolerance treated?
• Dietary changes
• Lactase enzyme
• Adaptation
• Calcium and vitamin D supplements
• What are the long-term consequences of lactose intolerance?
• What is new in lactose intolerance?
• Lactose Intolerance At A Glance
• Bacterial Vaginosis At A Glance
• Related lactose intolerance article:
  Lactose Intolerance - on eMedicineHealth
• Patient Discussions: Lactose Intolerance - Symptoms at Onset of Disease

What is lactose intolerance?

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

What causes lactose intolerance?

Lactose is a larger sugar 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 then are 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 that line 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 (absence 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 is genetically programmed, and the prevalence of this type of lactase deficiency among different ethnic groups is highly variable. Thus, among Asian populations it is almost 100%, among American Indians it is 80%, and among blacks it is 70%; however, among American Caucasians the prevalence of lactase deficiency is only 20%. In addition to variability in the prevalence of lactase deficiency, there also is variability in the age at which symptoms of lactose intolerance appear. Thus, among Asian populations, the symptoms of deficiency (intolerance) occur around the age of 5, among Blacks and Mexican-Americans by the age of 10, and among the Finnish by age 20.

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.

Lactose Intolerance - Symptoms at Onset of Disease

The MedicineNet physician editors ask:

The symptoms of lactose intolerance can vary greatly from patient to patient. What were your symptoms at the onset of your disease?

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