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.
- 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.
Primary (lactase nonpersistence)
- Most common type of lactose intolerance
- A decrease in lactase activity is primarily observed during childhood or adolescence.
- Genetic polymorphism of the lactase‑coding gene with subsequent down‑regulation (absence of lactase-persisting allele)
- Lactase levels decline with age and are further influenced by genetic factors.
Due to underlying disorders of the small intestine that result in mucosal damage, e.g.:
- Autoimmune disorders (e.g., gluten-sensitive enteropathy)
- (Viral) gastroenteritis (e.g., rotavirus)
- Due to underlying disorders of the small intestine that result in mucosal damage, e.g.:
- Developmental: occurs in children born prematurely, as lactase activity develops late during pregnancy
- Congenital: autosomal recessive gene defect (extremely rare)
- Lactase is a brush‑border enzyme that cleaves lactose, a disaccharide, into absorbable monosaccharides (galactose and glucose).
In the case of lactase deficiency, an almost complete absence of lactose digestion is observed, resulting in decreased absorption in the small intestine (particularly the jejunum). 
- The transfer of osmotically active amounts of lactose into the large intestine leads to the osmotic binding of water → diarrhea
- Increased peristalsis due to increased intestinal filling → abdominal pain
Metabolism of lactose via the physiological bacterial flora of the colon
- Formation of short‑chain fatty acids that exacerbate diarrhea
- Increased gas formation and flatulence
Lactase deficiency is a malabsorption disorder.
Symptoms occur about an hour to several hours following consumption of milk products. The intensity of symptoms correlates with the amount of lactose consumed. 
- Diarrhea (often watery, bulky, and frothy)
- Cramping abdominal pain (often periumbilical or in the lower abdomen)
- Abdominal bloating, flatulence
Symptoms vary widely as most patients have residual amounts of lactase.
- 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.
- 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 osmotic gap
- ↓ Stool pH (< 6): due to lactose fermentation by colonic bacterial flora
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).
- Primary lactose intolerance: normal intestinal architecture
Secondary lactose intolerance
- Mucosal damage (especially injured tips of intestinal villi)
- Specific histopathological findings corresponding to the underlying condition (see “Etiology”)
- Genetic test (if primary lactose malabsorption is suspected)
- Food allergy 
- Food intolerance: fructose intolerance, intolerance of short-chain fermentable carbohydrates
- Irritable bowel syndrome
- Inflammatory bowel disease
- Gastrointestinal infections (e.g., giardiasis, bacterial GE, viral GE)
- Small bowel bacterial overgrowth
- Cystic fibrosis
- Bowel malignancy
The differential diagnoses listed here are not exhaustive.
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
- Treatment of the underlying condition in patients with secondary lactose intolerance