Conditions · Metabolism: carbohydrates and glycogen
Lactose intolerance !
In lactose intolerance the gut makes little lactase: milk causes gas, bloating and diarrhoea, but it does not damage the gut.
Updated 2026-11-01
Structures involved
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In biochemistry
- Galactose metabolism: Step 1 · Lactase (intestinal β-galactosidase)
- Galactose
- Carbohydrate chemistry
- Membrane transport
In histology
In physiology
- Carbohydrate digestion and absorption
- Water and electrolyte absorption and how diarrhea happens
- Patterns of malabsorption
What it is
Digestive symptoms from poor lactose digestion due to low lactase at the brush border of the small intestine. The commonest cause is the normal decline of lactase after infancy.
Who gets it
About 65 % of the world's population loses some lactase after infancy. It is very common in Indigenous, Afro-descendant and mestizo populations of Latin America, including Colombia; the congenital form is very rare.
Causes
- Primary: lactase non-persistence, set by regulatory variants near the LCT gene (in MCM6)
- Secondary: mucosal damage from gastroenteritis, coeliac disease, Crohn disease or chemotherapy
- Congenital: LCT gene variants (very rare)
Risk factors
- Asian, African, Indigenous American or Mediterranean ancestry
- Recent gut infection
- Prematurity (lactase still immature)
Pathophysiology
Lactase splits lactose into glucose and galactose. If it is lacking, lactose reaches the colon: it draws in water by osmosis (diarrhoea) and bacteria ferment it to short-chain fatty acids, hydrogen, methane and CO₂ (gas, bloating, pain). The gut lining is not damaged in the primary form.
Classification
- Primary (adult-type)
- Secondary (transient)
- Congenital
Symptoms and signs
- Bloating, gas and cramping pain 30 minutes to 2 hours after dairy
- Watery diarrhoea
- Bowel rumbling and nausea
Diagnosis
- Clinical
- Symptoms that appear with lactose and improve after removing it for 2–4 weeks.
- Tests
- Hydrogen breath test after a lactose load; acidic stool pH and stool reducing sugars in children; genetic testing for lactase non-persistence. Rule out coeliac disease if there are warning signs.
- Imaging
- Imaging is not needed for diagnosis.
Treatment
- Lifestyle
- Reduce lactose to the tolerated amount; many people tolerate small portions, yogurt and aged cheeses
- Lifestyle
- Lactose-free products
- Medical
- Lactase supplements with dairy-containing meals
- Lifestyle
- Ensure calcium and vitamin D from other foods if dairy is reduced
- Medical
- In the secondary form, treat the cause: lactase usually recovers
Complications
- Low calcium intake and lower bone density if dairy is removed without replacement
Prognosis
Excellent with dietary adjustment; the secondary form usually resolves.
Test yourself
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Why does undigested lactose cause diarrhoea?
It draws water into the colon by osmosis and bacteria ferment it into gases and acids.
Which test is most used to confirm lactose maldigestion?
The hydrogen breath test after a lactose load.
Is lactose intolerance a milk allergy?
No: it is an enzyme deficiency; cow's milk protein allergy is an immune reaction.
Related conditions
- Irritable bowel syndrome
- Coeliac disease
- Acute (viral) gastroenteritis
- Crohn disease
- Classic galactosaemia
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Lactose intolerance: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Lactose intolerance: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); LCT gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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