Can lactose intolerance cause constipation?

Can Lactose Intolerance Cause Constipation?

Yes, lactose intolerance can cause constipation in approximately 30% of cases, though diarrhea remains the more typical presentation. 1

Understanding the Clinical Presentation

Lactose intolerance typically manifests with abdominal pain, bloating, flatulence, and diarrhea due to osmotic effects of undigested lactose reaching the colon. 2, 3 However, constipation is a recognized but less common symptom that occurs in roughly 30% of lactose intolerant patients. 1

The mechanism involves:

• Undigested lactose creating osmotic effects in the colon 2
• Bacterial fermentation producing gas and other metabolites 3
• Variable individual responses based on intestinal microbiota composition 4
• Visceral hypersensitivity affecting symptom expression 2

Diagnostic Considerations

The hydrogen breath test is the most widely used and recommended diagnostic method, involving ingestion of 25-50g lactose with breath samples collected at 15-30 minute intervals for three hours. 5 A rise in hydrogen concentration >20 ppm above baseline indicates lactose malabsorption. 2, 5

Important Diagnostic Pitfalls:

• False negative rates reach 25%, so a negative test does not exclude the diagnosis 5
• Patient self-reports of lactose intolerance correlate poorly with objective evidence and cannot be relied upon 2, 5
• A simpler approach: implement a 2-week dietary restriction trial with symptom resolution as the diagnostic indicator 2, 5

Who Should Be Tested

Testing is most appropriate for patients who:

• Consume >0.5 pint (280 ml) of milk or equivalent dairy products daily 2, 5
• Come from racial groups with high lactose malabsorption prevalence (60% in Asians, 90% in Chinese populations) 2
• Have symptoms suggestive of carbohydrate intolerance 2

Management Strategy

Primary treatment involves dietary modification with temporary reduction or removal of lactose from the diet. 5

Practical dietary guidance:

• Most lactose intolerant individuals can tolerate small amounts of milk 6
• Hard and semi-hard cheeses contain no lactose and are well-tolerated 6
• Yogurt with live cultures is typically well-tolerated despite lactose content due to bacterial lactase activity 6
• Only patients consuming substantial lactose (>0.5 pint milk daily) benefit from restriction 2

Critical Clinical Caveat:

Lactose intolerance may be part of broader FODMAP intolerance, present in at least half of IBS patients. 3 If symptoms persist despite lactose restriction alone, consider that the patient may require a comprehensive low FODMAP diet rather than isolated lactose avoidance. 3

When Constipation is the Primary Symptom

If constipation predominates rather than diarrhea:

• Consider lactose intolerance as a potential contributor, particularly if accompanied by bloating and abdominal pain 1
• Rule out other causes of constipation first (Rome IV criteria for IBS-C, pelvic floor dysfunction) 2
• Implement a trial of lactose restriction for 2 weeks 5
• If no improvement, the constipation is likely unrelated to lactose intolerance

Secondary Causes to Consider

Lactose intolerance can be secondary to:

• Celiac disease 5
• Gastroenteritis 5
• NSAID use 5
• Small bowel disease 5
• Chemotherapy-induced mucosal injury 2

In these contexts, the lactose intolerance is typically reversible once the underlying condition is treated. 2