Lactose Intolerance

Lactose intolerance is a clinical syndrome of abdominal pain, bloating, flatulence and diarrhoea occurring after ingestion of lactose (the disaccharide in milk and dairy products), caused by lactose malabsorption from low activity of lactase (lactase phlorizin hydrolase) in the brush border of the small intestine. Lactose malabsorption (a physiological finding) does not always cause symptoms, whereas intolerance includes the symptoms.

• Types:
  • Primary lactase non-persistence (adult onset hypolactasia): the most common type, a genetically programmed decline in lactase after weaning (from age 2 to 5 years). It affects about 65 to 70% of the world's adults, with prevalence of over 90% in East Asia, 70 to 90% in Africa, the Middle East and the Mediterranean, 50 to 80% in South America and the Indian subcontinent, and 5 to 15% in northern Europeans, in whom lactase persistence is common.
  • Secondary lactase deficiency: reversible damage to the small bowel brush border from gastroenteritis, coeliac disease, Crohn disease, giardiasis, small intestinal bacterial overgrowth, chemotherapy, radiation, HIV enteropathy, short bowel and malnutrition (kwashiorkor).
  • Congenital lactase deficiency: a rare autosomal recessive condition (mutation of the lactase gene) presenting from the first feeds with severe watery diarrhoea, dehydration and failure to thrive.
  • Developmental lactase deficiency: in premature infants born before 34 weeks, which resolves with maturity.
• Mechanism: unabsorbed lactose has an osmotic effect, drawing water into the lumen, and is fermented by colonic bacteria into hydrogen, methane, carbon dioxide and short chain fatty acids, causing gas, bloating, cramps, and loose stools. The severity depends on the dose of lactose, residual lactase activity, gastric emptying, intestinal transit, gut microbiota, and visceral sensitivity.
• Dose response: most adults with lactase non-persistence tolerate 12 g of lactose (one cup, 250 mL, of milk) with little or no symptoms, and many tolerate up to 15 to 18 g in divided doses with meals. Symptoms rise with 25 to 50 g in a single dose.
• Lactose content: milk 12 g per 250 mL cup, yoghurt 4 to 12 g per serving (live cultures improve digestion), hard cheeses (cheddar, parmesan) below 1 g per serving, soft fresh cheese (cottage, ricotta) 2 to 4 g, ice cream 6 to 9 g per half cup, butter below 0.5 g per serving. Lactose is also hidden in processed foods (bread, cereals, soups, sauces, processed meats, instant mashed potato, biscuits), medicines and supplements (as an excipient).
• Overlap with irritable bowel syndrome and other food intolerances is common, and many patients who attribute symptoms to lactose have other causes.

• Symptoms occur 30 minutes to 2 hours after ingestion of dairy: abdominal cramps and pain, bloating, borborygmi, flatulence, nausea, loose or watery stools (osmotic diarrhoea, often explosive), and sometimes vomiting (more in children). Symptoms resolve within hours after the lactose clears. Constipation can occur in methane producers.
• Extra intestinal symptoms (headache, fatigue, poor concentration, muscle and joint pain) are reported but are poorly validated.
• Infants and children: bloating, colic, and watery acid stools with perianal excoriation (infants with congenital or secondary deficiency), and failure to thrive.
• Examination is usually normal. Check for weight loss, signs of malnutrition, and perianal skin changes.
• Differential diagnosis: cows' milk protein allergy (immune mediated, with urticaria, vomiting, anaphylaxis, bloody stools in infants), irritable bowel syndrome, coeliac disease, fructose malabsorption, other food intolerance, small intestinal bacterial overgrowth, pancreatic insufficiency, inflammatory bowel disease, giardiasis, bile acid diarrhoea, and functional bloating.
• Red flags: weight loss, blood in stool, nocturnal diarrhoea, anaemia, fever, persistent vomiting, failure to thrive, steatorrhoea, symptoms starting in adulthood in someone who previously tolerated dairy (look for secondary causes such as coeliac disease, Crohn disease, or small intestinal bacterial overgrowth), an anaphylactic reaction, and severe dehydration in infants.
• Complications: reduced calcium and vitamin D intake with a risk of low bone mineral density and fracture (when dairy is avoided without replacement), nutritional deficiency in children, and reduced quality of life.

• A clear temporal link between dairy ingestion and symptoms, with improvement on a lactose free diet and recurrence on challenge, is sufficient in most cases. Confirmation is helpful where the diagnosis is uncertain, or when a secondary cause is suspected.
• Dietary exclusion: exclude all lactose containing foods for 2 weeks (including hidden sources), then reintroduce lactose in a stepwise way (increasing portions) to determine the personal threshold, with a diary of symptoms.
• Hydrogen breath test (the preferred non-invasive test): after an overnight fast, with no smoking, exercise, or antibiotics in the last 4 weeks, and no laxatives or colonoscopy preparation in the last week, the patient takes 25 g lactose (in 250 to 500 mL water, or 2 g/kg for children up to 25 g), and breath hydrogen is measured at baseline and every 30 minutes for 3 hours. A rise of 20 parts per million or more over baseline is positive, and a rise in methane of 10 parts per million or more indicates methane production. Non-hydrogen producers (about 15 to 20%) give false negatives, so measure methane as well. False positives occur with small intestinal bacterial overgrowth, and false negatives follow recent antibiotics.
• Lactose tolerance test (blood glucose): after 50 g lactose, a rise in glucose of less than 1.1 mmol/L (20 mg/dL) at 20 to 60 minutes suggests malabsorption. Less accurate than the breath test, and reserved for when breath tests are not available.
• Genetic test for lactase persistence (the LCT promoter variant C/T at position 13910 upstream in northern Europeans, and other variants in other populations) identifies primary non-persistence, but does not by itself prove symptoms.
• Small bowel biopsy with disaccharidase assay (the reference test, with a lactase activity below 10 units/g protein, or a lactase to sucrase ratio below 0.3) is rarely needed, and is used for suspected congenital deficiency and secondary causes.
• Stool testing in infants: stool pH below 5.5 and reducing substances above 0.5% suggest carbohydrate malabsorption.
• Tests for secondary causes when the history is atypical or the onset is recent: coeliac serology (tissue transglutaminase IgA with total IgA), faecal calprotectin, Giardia antigen, full blood count, ferritin, C-reactive protein, and endoscopy with duodenal biopsies.
• Allergy tests (skin prick, specific IgE) for suspected cows' milk allergy, since this is an immune condition and the management differs.

• Dietary management (the mainstay), using the personal tolerance level instead of strict exclusion:
  • Reduce lactose intake to below the symptom threshold (often 12 g per day or less taken in divided doses, with meals, which improves tolerance), then reintroduce gradually.
  • Prefer foods that are naturally low in lactose or better tolerated: hard and aged cheeses, yoghurt with live cultures, kefir, butter, and lactose free milk and dairy products (milk treated with lactase), and fermented products.
  • Choose fortified plant based alternatives (soya, oat, almond, rice) for those who avoid dairy, check that they are fortified with calcium (at least 120 mg per 100 mL) and vitamin D, and note that protein content varies.
  • Read labels and watch for hidden lactose (milk solids, whey, curds, milk sugar).
• Lactase enzyme supplements: lactase 3000 to 9000 FCC units orally taken with the first bite or sip of dairy containing meals (one to two capsules or tablets), adjusting the dose to the lactose load. Results vary, and cost is a factor.
• Probiotics (Bifidobacterium and Lactobacillus species, and live culture yoghurt) may improve tolerance by increasing lactose digesting bacteria. Adaptation to regular small doses of lactose (colonic adaptation) can increase tolerance over 2 to 4 weeks, and galacto oligosaccharides may support it.
• Ensure adequate calcium and vitamin D when dairy is reduced:
  • Calcium 1000 mg per day for adults (1200 mg for women over 50 years and men over 70 years), from non-dairy sources (calcium fortified plant milks, tinned fish with bones, tofu set with calcium, almonds, green leafy vegetables), and calcium carbonate 500 mg orally twice daily with meals (or calcium citrate 500 mg twice daily) when diet is inadequate.
  • Vitamin D3 800 to 1000 international units (20 to 25 micrograms) orally once daily, with higher doses for deficiency (50,000 units weekly for 8 weeks, or 2000 to 4000 units daily).
  • Dietitian review for children, pregnant women, the elderly, and those with a restricted diet.
• Check medicines and supplements for lactose excipients, which usually contain small amounts (below 1 g) and rarely cause symptoms, but may matter in severe intolerance.
• Treat secondary lactose intolerance by treating the cause: gluten free diet for coeliac disease, treatment of giardiasis (tinidazole 2 g once or metronidazole 400 mg 3 times daily for 5 days), treatment of small intestinal bacterial overgrowth (rifaximin 550 mg 3 times daily for 14 days), and management of inflammatory bowel disease. Lactase activity often recovers in weeks to months after the cause is treated, so a temporary reduction in lactose is enough.
• Infants and children:
  • Secondary lactose intolerance after gastroenteritis is usually transient. Continue breastfeeding and normal milk feeds, and restrict lactose only in persistent diarrhoea (use a lactose free formula for 2 to 4 weeks, then reintroduce).
  • Congenital lactase deficiency: lifelong lactose free formula from the first days of life, with specialist paediatric and dietetic care.
  • Primary lactase non-persistence rarely presents before the age of 5 years, so persistent symptoms in toddlers should prompt a search for other diagnoses (cows' milk protein allergy, coeliac disease, giardiasis).
  • Ensure adequate calcium (700 mg for ages 1 to 3 years, 1000 mg for ages 4 to 8 years, 1300 mg for ages 9 to 18 years) and vitamin D (400 units daily in infants and 600 units daily in older children).
  • Do not use a lactose free diet for colic without evidence of lactose intolerance.
• Special groups:
  • Pregnancy and breastfeeding: continue to take enough calcium (1000 mg daily) and vitamin D (600 to 1000 units daily), use lactose free dairy or fortified alternatives, and use lactase supplements freely (not absorbed).
  • Elderly: monitor for osteoporosis, ensure a protein and calcium intake, and avoid unnecessary restriction. Check bone density when risk factors exist.
  • Irritable bowel syndrome: a lactose reduction is part of a low fermentable carbohydrate diet, and should be time limited.
  • Medicines: lactase may be added with metformin. Review drugs causing diarrhoea.
  • Renal impairment: avoid excess calcium and phosphate, and use fortified alternatives with caution.
• Follow up: review in 4 to 8 weeks to assess symptom control, tolerance threshold, calcium and vitamin D intake, and nutritional adequacy. Refer to a dietitian for complex cases, and to a gastroenterologist or paediatrician for atypical symptoms, weight loss, failure to thrive, blood in stool, or suspected secondary causes.