Lactose Intolerance



  • Lactose intolerance is defined as the inability to digest the ingested disaccharide lactose, secondary to a deficiency of the intrinsic enzyme lactase, resulting in clinical symptoms.
  • Lactase deficiency is an intrinsic low level of lactase production which may or may not be associated with clinical symptoms.
  • Lactose is a disaccharide composed of glucose and galactose.
  • Lactose is important as a source of energy; it is the major carbohydrate in human and other mammalian milks; promotes the absorption of calcium, phosphorus, and iron; and has a probiotic effect on the gut flora.
  • Four types of lactase deficiency
    • Congenital lactase deficiency
      • Extremely rare
      • Presents during the newborn period
      • Will cause severe diarrhea and failure to thrive and risk the newborn’s life
    • Primary lactase deficiency (adult-type hypolactasia)
      • Due to relative or absolute absence of lactase
      • Develops during childhood at different ages in different racial groups
      • Most common cause of lactose intolerance
    • Secondary lactase deficiency
      • Results from small bowel injury (acute gastroenteritis, persistent diarrhea, small bowel bacterial overgrowth, chemotherapy)
      • Can present at any age, more common in infancy
    • Developmental lactase deficiency
      • A relative lactase deficiency observed in premature infants <34 weeks’ gestation



  • ~70% of the world’s population is prone to developing primary lactase deficiency by adulthood.
    • The prevalence of primary lactase deficiency in northern Europeans, who have a diet rich in dairy, is 2%.
    • In Hispanic populations, the prevalence of primary lactase deficiency is 50–80%.
    • In Ashkenazi Jewish and African American populations, the prevalence is 60–80%.
    • In Asian populations, the prevalence of primary lactase deficiency is nearly 100%.
  • Nearly 20% of children <5 years from Hispanic, Asian, or African American descent have lactase deficiency and lactose malabsorption.
  • Caucasian children usually do not develop symptoms until after 5 years of age.

Risk Factors


  • Posttranslational regulatory mechanisms in primary lactase deficiency or adult-type hypolactasia
  • Correlation between the genetic polymorphism of mRNA and persistence of lactase activity with early loss at 1 to 2 years in Thai children and late loss at 10 to 20 years in Finnish children


  • Symptoms depend on the amount of lactose ingested.
  • Malabsorbed lactose creates an osmotic load that draws fluid and electrolytes into the bowel lumen, leading to an osmotic diarrhea.
  • Nonabsorbed lactose acts as a substrate for intestinal bacteria.
  • In the colon, bacteria metabolize lactose, producing volatile fatty acids and gases leading to flatulence, bowel distension, pain, and low pH.

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