Lactose Intolerance Biochemistry
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Direct answer
Undigested lactose retained in the intestinal lumen explains every feature of lactose intolerance. Brush-border lactase (a beta-galactosidase complexed with glycosylceramide in the mature enterocyte) normally splits lactose into glucose and galactose; when activity falls, the disaccharide acts as an osmotic load drawing water into the lumen, and colonic bacteria ferment it to lactic acid, hydrogen and carbon dioxide. The result is watery, acidic diarrhoea with bloating, borborygmi and flatulence after milk. Primary lactase non-persistence is the norm in most of the world's adults — genetically programmed decline after weaning governed by regulatory variants upstream of the lactase gene on chromosome 2 — while secondary deficiency follows mucosal injury, and congenital alactasia is rare.
What you must remember
- Genetics: the LCT gene on chromosome 2 with MCM6 control elements; the persistence trait (classically the C/T minus 13910 variant in Europeans, distinct variants in Indians and Africans) is a regulatory change, not a structural one — the enzyme made is identical.
- Lactase biology: outermost brush-border enzyme, least abundant disaccharidase, highest at birth; decline begins after about 3-5 years in non-persistent individuals.
- Stool findings: pH below 5.5, positive reducing substances; an oral lactose challenge followed by a blood glucose rise under 20 milligrams per decilitre indicates malabsorption (flat tolerance curve).
- Hydrogen breath test: a rise of more than 20 parts per million of expired hydrogen after a 25-gram lactose load is diagnostic — bacterial fermentation, not human enzymes, makes the hydrogen.
- Secondary causes: viral gastroenteritis (rotavirus destroys villous tips), coeliac disease, tropical sprue, giardiasis and protein-energy malnutrition — all damage the lactase-bearing mature enterocyte.
- Distinguish from cow-milk protein allergy: intolerance is a dose-related enzymatic problem without immune involvement; allergy is IgE-mediated with urticaria, vomiting or anaphylaxis and can occur with traces.
- Management: reduce or hydrolyse the lactose, not the calcium — lactose-free milk, curd and yoghurt tolerated because starter-culture bacteria carry their own beta-galactosidase, with calcium supplementation where intake falls.
How to work through a dairy-symptom case
A 25-year-old reports abdominal cramps and loose stools within two hours of milk, but tolerates tea and curd. Reason in steps. First, the pattern — dose-related bloating and diarrhoea without urticaria or blood — favours intolerance over allergy. Second, localise the enzyme: symptoms require lactose to reach the colon intact, so the defect is brush-border, and the persistence question is ancestral; lactase persistence is common in northern and pastoral populations, while a majority of southern Indians, Africans and East Asians are non-persistent — a population-genetics point examiners enjoy. Third, confirm cheaply: the hydrogen breath test after a lactose load, or a two-week trial of lactose-free milk with symptom diary, does in an outpatient clinic what genetic testing would over-confirm.
Extend to the paediatric version: an infant with persistent diarrhoea after rotavirus gastroenteritis has secondary deficiency from villous tip loss; management is temporary lactose-free or soya formula until the mucosa regenerates over weeks, with zinc supplementation as per Indian acute-diarrhoea guidelines. Congenital lactase deficiency, by contrast, presents from the first feed with severe watery diarrhoea and dehydration and needs lifelong lactose exclusion — autosomal recessive, reported most often from Finland.
Where students slip
Writing "lactose intolerance is lactase gene mutation" is the classic error — non-persistence is a regulatory polymorphism deciding whether the wild-type gene stays switched on; the enzyme itself is normal. Second, students order a stool pH and stop; a low pH supports malabsorption but says nothing about cause, and the treatable secondary causes (coeliac disease, giardiasis — both common in Indian practice) must be excluded before labelling primary non-persistence. Third, the glucose tolerance curve in lactose malabsorption is flat after lactose but normal after a glucose-galactose load, proving absorption machinery is intact; confusing the two is a viva trap.
Frequently asked questions
Why do patients with lactose intolerance tolerate curd better than milk?
Fermentation by lactic-acid bacteria partly hydrolyses lactose and their beta-galactosidase continues working in the gut, reducing the lactose load reaching the colon.
Which test confirms lactose malabsorption?
The hydrogen breath test after a standard lactose load, with a rise over 20 parts per million in expired hydrogen indicating colonic bacterial fermentation.
How does primary lactase non-persistence differ from congenital alactasia?
Non-persistence is common, genetically programmed post-weaning decline of a normal enzyme, while congenital alactasia is a rare autosomal-recessive absence presenting with severe neonatal diarrhoea.
Why does lactose intolerance cause diarrhoea?
Unsplit lactose is osmotically active, retaining fluid in the lumen, and bacterial fermentation adds short-chain acids and gas, driving watery acidic stools.
What stool findings support lactose malabsorption?
Stool pH below 5.5 with positive reducing substances, typically with a sour smell and flatulence history after dairy intake.