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Lactose Intolerance

Lactose Intolerance is a topic covered in the 5-Minute Clinical Consult.

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  • Inability to digest lactose into constituent components (glucose and galactose) due to low levels of lactase in the brush border of the small intestinal mucosa
  • Causes bloating, borborygmi (audible stomach “rumblings”), abdominal pain, and diarrhea
    • 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)
  • Lactase activity peaks at birth then decreases after the first few months of life, declining continuously throughout life. 75% of adults’ worldwide exhibit decline in lactase activity. Only 50% of lactase activity is needed to digest lactose without causing symptoms of lactose intolerance.
  • Lactose malabsorption also results from reduction of lactase activity; asymptomatic and equally common in healthy patients and 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 common with giardiasis, ascariasis, irritable bowel syndrome (IBS), tropical and nontropical sprue, and AIDS malabsorptive 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 Hispanic Americans, 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: Normal decline in the lactase activity in the intestinal mucosa is genetically controlled and permanent after weaning from breast milk.
  • Secondary lactose intolerance: associated with gastroenteritis in children and with nontropical and tropical sprue, regional enteritis, abetalipoproteinemia, cystic fibrosis, inflammatory bowel disease, celiac disease, and immunoglobulin deficiencies in both adults and children

  • In Caucasians, lactase deficiency has been associated with a single nucleotide polymorphism (SNP) consisting of a nucleotide switch of T for C 13910bp 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) have been linked to lactase persistence in Africans.

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
  • Inflammatory bowel disease
  • Immunoglobulin deficiencies
  • Celiac disease
  • Cystic fibrosis

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Stephens, Mark B., et al., editors. "Lactose Intolerance." 5-Minute Clinical Consult, 27th ed., Wolters Kluwer, 2019. Medicine Central, im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116337/all/Lactose_Intolerance.
Lactose Intolerance. In: Stephens MB, Golding J, Baldor RA, et al, eds. 5-Minute Clinical Consult. 27th ed. Wolters Kluwer; 2019. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116337/all/Lactose_Intolerance. Accessed March 22, 2019.
Lactose Intolerance. (2019). In Stephens, M. B., Golding, J., Baldor, R. A., & Domino, F. J. (Eds.), 5-Minute Clinical Consult. Available from https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116337/all/Lactose_Intolerance
Lactose Intolerance [Internet]. In: Stephens MB, Golding J, Baldor RA, Domino FJ, editors. 5-Minute Clinical Consult. Wolters Kluwer; 2019. [cited 2019 March 22]. Available from: https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116337/all/Lactose_Intolerance.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - Lactose Intolerance ID - 116337 ED - Stephens,Mark B, ED - Golding,Jeremy, ED - Baldor,Robert A, ED - Domino,Frank J, BT - 5-Minute Clinical Consult, Updating UR - https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116337/all/Lactose_Intolerance PB - Wolters Kluwer ET - 27 DB - Medicine Central DP - Unbound Medicine ER -