Lactose Intolerance: Diagnosis and Management
Diagnostic Approach
The hydrogen breath test is the most widely used and recommended method for diagnosing lactose intolerance, though a 2-week dietary restriction trial with symptom resolution offers a simple and economical alternative. 1
When to Test
- Reserve testing for patients consuming more than 280 ml (0.5 pint) of milk or dairy equivalent daily, as those consuming less typically tolerate moderate quantities even if lactose intolerant 1, 2
- Patient self-reports of lactose intolerance correlate poorly with objective evidence and cannot be relied upon for diagnosis 1
Hydrogen Breath Test Protocol
- Administer 25-50g lactose dissolved in 200-500ml water after overnight fast 1
- Collect end-expiratory breath samples at 15-30 minute intervals for three hours 1
- A rise in hydrogen concentration >20 parts per million from baseline indicates positive diagnosis 1
- Be aware that false negative rates can reach 25%, often due to hydrogen non-excretion occurring in approximately 18% of subjects 1
Alternative Diagnostic Strategy
- A 2-week lactose elimination trial with symptom resolution provides practical confirmation when testing is unavailable or negative despite suggestive symptoms 1
Critical Distinction: Lactose Intolerance vs. Milk Allergy
Lactose intolerance is a non-immunologic food intolerance caused by inability to digest lactose, resulting in excess gastrointestinal fluid production, abdominal pain, and diarrhea—not an allergic reaction. 3 This distinguishes it from milk protein allergy, which involves an immunologic response to milk proteins and requires completely different management 3.
Management Strategy
Primary Treatment: Dietary Modification
If lactose intolerance is confirmed, the primary management approach should be dietary modification with temporary reduction or removal of lactose from the diet. 1
Practical Dietary Guidelines
- Most adults with lactose intolerance can tolerate 12-15g of lactose (approximately 1 cup of milk) daily without symptoms 4, 5
- Yogurt and hard cheeses are generally well-tolerated even in lactose-intolerant individuals and should not be eliminated unnecessarily 3, 1, 2
- Hard cheeses contain negligible or no lactose, making them universally tolerable 2
- During acute chemotherapy-induced diarrhea, avoid milk and dairy products except yogurt and firm cheeses to reduce symptom intensity and duration 3, 2
Lactase Enzyme Supplementation
- Lactase enzyme supplements may be helpful for symptom reduction 6
- Consider trial use when patients wish to consume lactose-containing foods 7
Medication Considerations
Lactose-containing medications rarely need to be avoided, as most people with lactose malabsorption can tolerate lactose amounts <12g, and pharmaceutical lactose content is typically well below this threshold. 3, 2
Secondary Causes to Consider
Always evaluate for reversible secondary lactose intolerance before assuming primary lactase deficiency. 1, 2
Common Secondary Causes
- Celiac disease 1
- Gastroenteritis 1
- Non-steroidal anti-inflammatory drug use 1
- Small bowel disease 1, 2
- Chemotherapy-induced bowel mucosal injury (affects up to 35% of patients, though only 11% become symptomatic; fully reversible after therapy discontinuation) 3, 2
Special Populations
- Lactase deficiency should be regarded as normal in most non-Caucasian populations after age two, with the exception of northern European populations where lactase persistence occurs as an autosomal dominant trait 1
- Prevalence varies: 15% in northern Europeans, up to 80% in blacks and Latinos, and up to 100% in American Indians and Asians 6
Critical Clinical Pitfalls
FODMAP Overlap
At least 50% of patients with presumed lactose intolerance actually have broader intolerance to FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols). 1, 8 If lactose restriction alone fails to resolve symptoms, consider a low FODMAP diet trial 1, 8.
Visceral Hypersensitivity
Patients with irritable bowel syndrome or visceral hypersensitivity associated with anxiety may experience symptoms independent of lactose malabsorption. 5, 8 Symptoms alone (bloating, abdominal pain, diarrhea) are not specific to lactose intolerance and overlap significantly with IBS 1.
Diagnostic Limitations
- A negative breath test does not exclude the diagnosis due to high false-negative rates; a trial of lactose-free diet should still be considered if symptoms are suggestive 1
- The association between self-reported lactose intolerance, objective findings, and clinical outcome of dietary intervention is variable 5
Absolute Contraindication
Classic galactosemia is the only absolute contraindication to lactose ingestion. 2 This rare genetic disorder requires complete lactose avoidance, unlike lactose intolerance which allows for graduated tolerance.
Nutritional Considerations
Lactose-intolerant patients must ensure adequate calcium intake when restricting dairy products. 6 Consider calcium supplementation or increased consumption of calcium-fortified non-dairy alternatives to prevent long-term bone health consequences.