Lactose intolerance

Last updated: March 14, 2023

Summarytoggle arrow icon

Lactose intolerance is caused by the malabsorption of lactose. It may be genetically determined or due to a functional deficiency of the lactase enzyme in the epithelium of the small intestine. After consuming food or beverages containing lactose, affected individuals develop abdominal symptoms, such as pain, diarrhea, and bloating. Lactose intolerance is diagnosed with a hydrogen breath test or lactose intolerance test. The condition may be managed well with lactase supplements or by avoiding lactose altogether.

Definitiontoggle arrow icon

Lactose intolerance is the inability to absorb lactose, caused by lactase deficiency.

Epidemiologytoggle arrow icon

  • Approximately 70% of the world's population is lactose intolerant.
  • Lactose intolerance is more common in certain regions, particularly Asia, parts of Africa, and South America, where up to 90% of the population is affected.
  • Prevalence increases with age.


Epidemiological data refers to the US, unless otherwise specified.

Etiologytoggle arrow icon


Pathophysiologytoggle arrow icon

Lactase deficiency is a malabsorption disorder.

Clinical featurestoggle arrow icon

Symptoms occur about an hour to several hours following consumption of milk products. The intensity of symptoms correlates with the amount of lactose consumed. [4]

  • Diarrhea (often watery, bulky, and frothy)
  • Cramping abdominal pain (often periumbilical or in the lower abdomen)
  • Abdominal bloating, flatulence
  • Nausea

Symptoms vary widely as most patients have residual amounts of lactase.

Diagnosticstoggle arrow icon

  • Trial lactose‑free diet: to see if symptoms resolve
  • Hydrogen breath test
    • The amount of hydrogen in the expired air increases after administering lactose in the fasting state.
    • Procedure
      • Fasting for 8–12 hours
      • Ingestion of lactose
      • Measurement of breath hydrogen levels at baseline and at 30‑minute intervals over 3 hours
      • Breath hydrogen levels > 20 ppm are considered diagnostic of lactose intolerance.
  • Lactose tolerance test: Following the administration of lactose, the normal rise in blood glucose levels is pathologically reduced (< 20 mg/dL over 2 hours) and symptoms appear (rarely used, as the test has low sensitivity and specificity)
  • Stool analysis
  • Biopsy of the small intestine: qualitative and quantitative assessment of lactase via endoscopic tissue biopsy (conclusive, but rarely used, as the test is more invasive than other tests).
  • Genetic test (if primary lactose malabsorption is suspected)


Differential diagnosestoggle arrow icon

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon

  • Avoid or reduce intake of milk products: lactose‑free or lactose‑reduced products have become more readily available
    • Many patients tolerate small amounts of milk (∼ 240 mL per day).
    • Use of alternative foods, such as soy‑based products
    • Awareness of lactose in processed foods or foods other than dairy products (e.g., bread, salad dressings)
  • Oral lactase supplements
    • Recommended when traveling or before consuming food or milk products containing lactose
    • A wide variety of nonstandardized over‑the‑counter lactase supplements are available
  • Treatment of the underlying condition in patients with secondary lactose intolerance


Referencestoggle arrow icon

  1. Lactose Intolerance. Updated: April 4, 2017. Accessed: April 5, 2017.
  2. Lactose intolerance: Overview. Updated: June 17, 2015. Accessed: April 5, 2017.
  3. Suchy FJ, Brannon PM, Carpenter TO et al. Lactose Intolerance and Health. Annals of Internal Medicine. 2010; 152 (12): p.792-796.
  4. Heyman MB. Lactose intolerance in infants, children, and adolescents. Pediatrics. 2006; 118 (3): p.1279-1286.doi: 10.1542/peds.2006-1721 . | Open in Read by QxMD
  5. Swagerty DL Jr, Walling AD, Klein RM. Lactose intolerance. Am Fam Physician. 2002; 65 (9): p.1845-1851.
  6. Boyce JA, Assa'ad A, Burks AW, et al. Guidelines for the diagnosis and management of food allergy in the United States: report of the NIAID-sponsored expert panel. J Allergy Clin Immunol. 2010; 126 (6 Suppl): p.S1-58.doi: 10.1016/j.jaci.2010.10.007 . | Open in Read by QxMD
  7. Nowak-Węgrzyn et al.. Non–IgE-mediated gastrointestinal food allergy. J Allergy Clin Immunol. 2015; 135 (5): p.1114-1124.doi: 10.1016/j.jaci.2015.03.025 . | Open in Read by QxMD
  8. Guandalini S. Pediatric Lactose Intolerance Clinical Presentation. Pediatric Lactose Intolerance Clinical Presentation. New York, NY: WebMD. Updated: July 17, 2015. Accessed: April 5, 2017.

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