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. thus in asian populations it is almost 100% among american indians it is 80% and in blacks it is 70%; however in 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 in asian populations the symptoms of lactase deficiency (intolerance) occur around the age of 5 among blacks and mexican-americans by the age of 10 and in 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.
what causes lactose intolerance?