Lactose Intolerance



  • Lactose intolerance is a syndrome of abdominal pain, bloating, and flatulence after the ingestion of lactose.
  • Lactose malabsorption results from a reduction in lactase activity in the brush border of the small intestinal mucosa.
  • Lactase activity peaks at birth then decreases after the first few months of life, declining continuously throughout life. 75% of adults worldwide exhibit a decline in lactase activity after birth. Only 50% of lactase activity is needed to digest lactose without causing symptoms of lactose intolerance.
    • Congenital lactose intolerance: very rare
    • Primary lactose intolerance: common in adults who develop low lactase levels after childhood
    • Secondary lactose intolerance: inability to digest lactose caused by any condition injuring the intestinal mucosa (e.g., infectious enteritis, celiac disease, eosinophilic gastroenteritis, or inflammatory bowel disease) or a reduction of available mucosal surface (e.g., resection)
  • Lactose malabsorption may be asymptomatic and is equally common in healthy patients and in those with functional bowel disorders.
  • System(s) affected: endocrine/metabolic, gastrointestinal

Pediatric Considerations

  • Primary lactose intolerance begins in late childhood.
  • No consensus on whether young children (<5 years of age) should avoid lactose following diarrheal illness
  • Lactose-free formulas are available.
  • Exclude milk protein allergy.



  • ≥50% of infants with acute or chronic diarrheal disease have lactose intolerance; particularly common with rotavirus infection
  • Lactose intolerance is also common with giardiasis, ascariasis, irritable bowel syndrome (IBS), tropical and nontropical sprue, and AIDS malabsorption syndrome.


  • In South America, Africa, and Asia, rates of lactose intolerance are >50%.
  • In the United States, the prevalence is 15% among whites, 53% among Hispanics, and 80% among African Americans.
  • In Europe, lactose intolerance varies from 15% in Scandinavian countries to 70% in Italy.
  • Predominant age:
    • Primary: teenage and adult
    • Secondary: depends on underlying condition
  • Predominant sex: male = female

Etiology and Pathophysiology

  • Primary lactose intolerance: The normal decline in lactase activity in the intestinal mucosa is genetically determined and permanent after weaning from breast milk.
  • Secondary lactose intolerance: associated with gastroenteritis in children; also associated with any gastrointestinal infection or inflammation of the small intestine with resultant lactose malabsorption in both adults and children


  • In whites, lactase deficiency is associated with a single nucleotide polymorphism (SNP) consisting of a nucleotide switch of T for C 13910 bp on chromosome 2. This results in variants of CC-13910 (lactase nonpersistence) OR CT-13910/TT-13910 (lactase persistence) (1). SNP (C/T-13910) is associated with lactase persistence in northern Europeans.
  • Other SNPs (G/C-14010, T/G-13915, and C/G-13907) linked to lactase persistence in some of African descent.

Risk Factors

  • Adult-onset lactase deficiency has wide geographic variation.
  • Age:
    • Signs and symptoms usually do not become apparent until after age 6 to 7 years.
    • Symptoms may not be apparent until adulthood, depending on dietary lactose intake and rate of decline of intestinal lactase activity.
    • Lactase activity correlates with age, regardless of symptoms.

General Prevention

Lactose avoidance relieves symptoms. Patients can learn what level of lactose is tolerable in their diet.

Commonly Associated Conditions

  • Tropical or nontropical sprue
  • Giardiasis
  • IBS or other functional bowel disorders
  • Small intestinal bacterial overgrowth (SIBO)
  • Celiac disease

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