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Lactose Intolerance

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Lactose intolerance definition and facts

  • Lactose intolerance is an inability to digest lactose, the main sugar in milk, which 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.
  • Virtually all individuals are born with lactase and the ability to digest lactose. The disappearance of lactase is either genetically programmed to occur after childhood or is due to diseases of the lining of the intestine that destroys lactase.
  • Lactose intolerance that occurs after age 21 (genetically-determined lactase deficiency usually occurs between ages 5-21) is rarely due to genetic lactase deficiency; it suggests another process is interfering with lactose digestion.
  • The primary signs and symptoms of lactose intolerance are
  • Other signs and symptoms of lactose intolerance that may occur are:
    • abdominal bloating,
    • abdominal distention, and
    • nausea
  • Constipation is not a symptom of lactose intolerance.
  • The symptoms are similar in adults, toddlers, and infants,
  • Among individuals, the severity of the signs and symptoms of lactose intolerance vary and may be provoked by greater or lesser quantities of lactose.  Most people are able to tolerate small amounts of lactose even if they are lactase deficient, for example lactose in yogurt. Some people develop severe symptoms with minimal intake of lactose.
  • Lactose intolerance can be diagnosed by eliminating lactose from the diet and observing for a disappearance of symptoms or provocation of symptoms with a milk challenge,
  • Tests that are useful for diagnosing lactose intolerance or lactase deficiency, include a lactose breath test, blood glucose test, stool acidity test, intestinal biopsy and genetic testing looking for the gene that controls the production of lactase,
  • Lactose intolerance is treated with dietary changes, supplements of lactase enzyme, correction of underlying conditions in the small intestine, and, possibly, by adaptation to increasing amounts of milk.
  • Lactose intolerance is rarely due to milk allergy in adults. Children with milk allergy, where it is more common, usually outgrow the allergy by adulthood.
  • 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).
  • There is no "cure" for genetically-programmed lactase deficiency with lactose intolerance.

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. It is due either to a genetically-programmed loss of lactase, the intestinal enzyme responsible for the digestion of lactose, or diseases affecting the small intestine that destroy lactase. Genetically programmed loss of lactase occurs between early childhood and age 21. The timing of this loss is determined primarily by ethnicity.

Because diseases of the intestine can occur at any age, lactose intolerance can begin at any age; however, that does not mean that an individual has become genetically lactase deficient.

What causes lactose intolerance?

Lactose is a 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 disease (sprue).

Genetically programmed 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. Lactase deficiency (and 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 lactase deficiency. In addition to variability in the prevalence of lactase deficiency among ethnic groups, there also is variability in the age at which symptoms of lactase deficiency (and lactose intolerance) appear.

As people age they may develop lactose intolerance; however, the extent of the intolerance appears to be mild and not associated with clinical symptoms.  Therefore, the development of lactose intolerance in the elderly should not be made lightly.

Medically Reviewed by a Doctor on 10/28/2016


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