Lactose intolerance and carbohydrate malabsorption

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Lactose is split by brush-border lactase into glucose and galactose. Genetically programmed lactase decline is common globally and is normal biology, not mucosal disease. Unabsorbed lactose draws water and is fermented to gas and short-chain fatty acids. The resulting symptoms vary widely, so an abnormal breath test without symptoms indicates malabsorption, while intolerance means clinically meaningful symptoms at ordinary exposure.

Other carbohydrate syndromes include excess fructose or polyol malabsorption, the broader fermentable carbohydrate burden used in FODMAP models, and rare congenital sucrase-isomaltase disorders. Breath testing may detect hydrogen or methane after a substrate but does not automatically prove the substrate causes daily symptoms. Diet history must capture serving size, combined foods, caffeine, alcohol and sweeteners because several triggers often travel together.

Management aims to find a tolerated pattern and treat secondary disease. Complete dairy exclusion risks low calcium, vitamin D, iodine, protein and energy intake, particularly in children, pregnancy and older adults. Lactose-free dairy, fortified alternatives, yoghurt, cheese and lactase enzyme can maintain nutrition. If symptoms persist despite credible lactose avoidance, reconsider IBS, coeliac disease, bile acid diarrhoea, pancreatic insufficiency and SIBO.

Key points

Lactase non-persistence causes reduced lactose absorption, but symptoms depend on dose, meal context, transit, colonic adaptation and visceral sensitivity; malabsorption is not identical to intolerance.
Bloating, cramp, flatus and watery diarrhoea after lactose reflect osmotic retention and bacterial fermentation, not intestinal inflammation.
Cow's-milk protein allergy is an immune reaction with a different history; urticaria, wheeze, angioedema or anaphylaxis needs urgent allergy assessment.
New lactose intolerance can be secondary to coeliac disease, gastroenteritis, giardiasis, Crohn's disease or other proximal mucosal injury and may improve when the cause heals.
A short, single-variable lactose reduction followed by reintroduction is often more useful than permanent dairy exclusion; breath testing helps when the history is unclear or restriction would be burdensome.
Hydrogen breath tests require protocol adherence and can be confounded by methane production, SIBO, antibiotics, smoking, exercise and rapid or slow transit.
Many people tolerate yoghurt, hard cheese or modest milk with meals; use individual thresholds rather than a zero-lactose rule.
Broad low-FODMAP or carbohydrate restriction should be time-limited and dietitian-led, with planned reintroduction to avoid nutritional and microbiome harm.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Lactase non-persistence

Genetically programmed reduction of brush-border lactase after childhood is common, with symptoms depending on dose and individual colonic handling.

Secondary mucosal injury

Coeliac disease, gastroenteritis, giardiasis and Crohn disease can reduce lactase at villous tips, sometimes reversibly after healing.

Dose and transit effects

Large lactose loads, rapid transit and altered microbiota increase malabsorption or symptoms even when some enzyme activity remains.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Incomplete carbohydrate hydrolysis
Reduced brush-border enzyme activity leaves lactose or another carbohydrate unabsorbed in the small-intestinal lumen after a dietary load.
2
Osmotic water retention
Unabsorbed solute draws water into the bowel and accelerates transit, producing loose stool or urgency after ingestion.
3
Colonic fermentation
Bacteria metabolise the carbohydrate into gas and short-chain acids, causing bloating, cramps and flatus according to visceral sensitivity.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Dose-related lactose intolerance

Bloating, borborygmi, cramp, flatus and loose stool follow a substantial lactose portion and improve with reduction, while small amounts or fermented dairy may be tolerated.

Secondary lactase deficiency

A new syndrome after infection or with weight loss, iron deficiency, inflammatory markers, travel or chronic diarrhoea suggests mucosal disease rather than primary lactase non-persistence.

Immediate milk allergyRed flag

Rapid hives, swelling, wheeze, vomiting, hypotension or collapse after milk protein is an immune emergency, not lactose intolerance.

Severe dehydrationRed flag

Profuse diarrhoea, postural symptoms, oliguria or electrolyte disturbance requires acute assessment regardless of the suspected carbohydrate trigger.

Restrictive-diet harm

Weight loss, low calcium or vitamin D, osteopenia, frailty or food anxiety can result from excessive dairy and FODMAP restriction and warrants dietetic intervention.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Structured lactose exclusion and reintroductionFirst step
Why
Change lactose exposure alone for a defined short period, record symptoms, then reintroduce a measured amount in a usual meal.
Interpretation and limitations
Reproducible dose-related symptoms support intolerance; absence on reintroduction argues against indefinite restriction. The method is invalid if several dietary components change together.
02
Hydrogen and methane breath test
Why
Measure fermentation gases after a standard lactose dose when diagnosis remains uncertain, symptoms are atypical or dietary consequences are substantial.
Interpretation and limitations
A gas rise supports malabsorption, but symptoms and protocol quality determine clinical relevance. Recent antibiotics, SIBO and non-hydrogen producers can distort results.
03
Coeliac serology with total IgA
Why
Investigate secondary lactase loss where chronic diarrhoea, deficiency, autoimmune risk or weight loss is present while the patient consumes gluten.
Interpretation and limitations
Dietary gluten restriction reduces sensitivity. A positive test enters the current specialist coeliac pathway rather than confirming lactose as the primary disorder.
04
Stool and inflammation assessment
Why
Use faecal calprotectin, stool pathogen or Giardia testing and blood inflammatory markers when persistent diarrhoea, travel or alarm features suggest inflammation or infection.
Interpretation and limitations
Normal results make selected organic causes less likely but do not create a positive lactose diagnosis.
05
Nutrition and bone profile
Why
Assess dietary calcium, vitamin D, iodine, protein and energy; measure vitamin D, calcium and other tests in prolonged or high-risk restriction.
Interpretation and limitations
Normal serum calcium does not prove adequate intake because homeostasis draws from bone. Fortified plant drinks vary in iodine and protein.
06
Alternative carbohydrate evaluation
Why
Use dietitian-led FODMAP assessment or specialist fructose and sucrase investigations only after common causes and red flags are addressed.
Interpretation and limitations
Breath test positivity alone can overmedicalise normal dose-dependent fermentation; rare enzyme disorders require expert interpretation.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Cow's-milk protein allergy

Urticaria, wheeze, angioedema, anaphylaxis or immune gastrointestinal features indicate protein allergy rather than enzyme-dependent lactose malabsorption.

Coeliac or other mucosal disease

Anaemia, weight loss, inflammatory features or positive coeliac testing suggests secondary malabsorption whose cause requires treatment.

Irritable bowel syndrome

Symptoms across multiple fermentable foods with a pain-stool pattern and no objective malabsorption may reflect IBS and visceral hypersensitivity.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01TrialLikely lactose intoleranceFirst stepMeal-linked symptoms occur without allergy or organic alarm features.

1Quantify lactose portions and associated foods, then reduce lactose alone for a short predefined interval while maintaining calcium-rich alternatives.
2Reintroduce a measured amount with food and record symptom threshold; use breath testing if the result remains unclear or major restriction is contemplated.
3Offer tolerated yoghurt, hard cheese, lactose-free dairy or lactase enzyme rather than blanket milk-protein avoidance.
4Reassess the diagnosis if symptoms continue despite verified lactose reduction.

02SecondaryLook for mucosal diseaseLactose symptoms are new, severe, persistent or accompanied by weight loss, anaemia, inflammation or travel risk.

1Screen for coeliac disease while gluten is eaten and obtain stool or inflammatory tests according to exposure and phenotype.
2Investigate Crohn's disease, giardiasis, pancreatic insufficiency or SIBO when supported by history and initial results.
3Treat the underlying disorder and revisit lactose tolerance after mucosal recovery rather than declaring permanent intolerance.
4Use nutrition support and refeeding precautions when weight or intake is significantly compromised.

03AllergyImmediate reaction to milkMilk exposure produces rapid skin, airway, cardiovascular or reproducible acute gastrointestinal symptoms.

1Treat anaphylaxis immediately with the current emergency protocol when airway, breathing or circulation is affected.
2Refer to an allergy service for an allergy-focused history, IgE testing and supervised challenge where indicated.
3Avoid unsupervised reintroduction after a severe immediate reaction and provide emergency planning through the specialist team.
4Distinguish milk-protein avoidance from lactose restriction in all written advice.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Hydrolyses dietary lactose and can permit a wider diet when endogenous lactase is insufficient.

Lactase enzyme

Use the product-specific oral dose with the first mouthful of a lactose-containing meal, adjusted to portion size and individual response.

Products vary and do not treat milk-protein allergy or another mucosal disease. Failure should prompt review of timing, dose and diagnosis.

Protects bone health when dairy reduction makes nutritional requirements difficult to meet.

Colecalciferol and calcium supplementation

Use dietitian and BNF-guided supplementation only when dietary intake or measured vitamin D is inadequate, accounting for age, pregnancy, renal function and bone risk.

Supplements do not replace a balanced diet. Check hypercalcaemia, renal stones and interactions; severe deficiency needs a formal loading plan.

Replaces water and sodium more effectively than plain water during acute high-output diarrhoea.

Oral rehydration solution

Use an appropriate glucose-electrolyte formulation in small frequent volumes for significant diarrhoeal loss, following current local or BNF advice.

Severe dehydration, persistent vomiting, renal or cardiac disease needs clinical assessment and sometimes intravenous fluid; very sugary drinks can worsen osmotic diarrhoea.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Calcium and vitamin D shortfall

Unsupervised exclusion of all dairy can reduce calcium, vitamin D and protein intake, increasing long-term bone risk.

Dietary restriction and weight loss

Progressively avoiding broad food groups can impair nutrition and social eating despite tolerance often depending on portion and meal context.

Missed secondary disease

Labelling new intolerance as primary may delay coeliac, infectious or inflammatory diagnosis and leave the underlying mucosal injury untreated.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Record symptom severity against lactose dose and food context, not simply whether any dairy was eaten.
Track weight and nutritional intake during restriction, with special attention to calcium, vitamin D, iodine, protein and energy.
Review breath-test preparation and symptom concordance before using a positive result to justify lifelong avoidance.
Repeat assessment after coeliac, infection or inflammatory disease treatment because secondary lactase deficiency may resolve.
Monitor children, pregnancy, older adults and osteoporotic patients more closely through the appropriate specialty pathway.
Reintroduce excluded FODMAP groups systematically after any dietitian-led low-FODMAP phase.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Malabsorption is not intolerance

A person can generate breath hydrogen without symptoms, or have symptoms at ordinary doses despite modest gas changes. Clinical relevance needs both exposure and phenotype.

Cheese is not milk

Hard cheese contains little lactose, and yoghurt cultures aid digestion. Many people can preserve these foods and their nutrients.

Milk sugar is not milk protein

Lactose-free cow's milk still contains milk proteins and is unsafe for a person with genuine milk-protein allergy.

Secondary intolerance may heal

Brush-border lactase is vulnerable to mucosal injury but can recover after coeliac disease, infection or inflammation is treated.

Restriction changes the microbiome

A prolonged broad low-FODMAP diet reduces substrate diversity; reintroduction is part of treatment, not optional closure.

11Common pitfallsFrequent interpretation and management errors.

01
Calling any symptom after dairy an allergy.
02
Interpreting an abnormal breath test without recorded symptoms.
03
Removing all dairy when only large lactose portions trigger symptoms.
04
Missing coeliac disease because a lactose-free diet partly improves symptoms.
05
Combining lactose-free, gluten-free and low-FODMAP trials simultaneously.
06
Ignoring calcium, vitamin D, iodine and protein during restriction.