What is lactose intolerance, including its symptoms, diagnostic methods, and management strategies?

Lactose Intolerance: Definition, Diagnosis, and Management

What is Lactose Intolerance?

Lactose intolerance is a non-immunologic condition where individuals cannot digest significant amounts of lactose due to inadequate lactase enzyme activity, resulting in abdominal pain, bloating, flatulence, and diarrhea after consuming lactose-containing foods. 1, 2

• This differs fundamentally from cow's milk allergy, which is an immune-mediated response to milk proteins involving IgE or other immune mechanisms 3
• Lactose intolerance is a metabolic disorder, not a food allergy 4, 3
• The condition affects up to 15% of northern Europeans, 80% of Blacks and Latinos, and nearly 100% of American Indians and Asians 2
• Lactase deficiency should be considered normal in most non-Caucasian populations after age 2 years, with the exception being northern European populations where lactase persistence occurs as an autosomal dominant trait 1

Clinical Presentation

Primary Symptoms

• Abdominal pain and distension 5
• Bloating and excessive flatus 2
• Watery diarrhea following lactose ingestion 2
• Borborygmi (intestinal rumbling sounds) 5

Important Clinical Distinctions

• Symptoms alone are not specific to lactose intolerance and may overlap with irritable bowel syndrome or other conditions 1
• Patient self-reports of lactose intolerance correlate poorly with objective evidence and cannot be relied upon for diagnosis 1
• At least 50% of patients with presumed lactose intolerance actually have broader intolerance to FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) 1

Diagnostic Methods

Hydrogen Breath Test (Primary Diagnostic Tool)

The hydrogen breath test is the most widely used and recommended method for diagnosing lactose intolerance, as it is non-invasive, relatively inexpensive, and has good sensitivity and specificity. 1

Test Protocol:

• Patient ingests 25-50g lactose dissolved in 200-500ml water after an overnight fast 1
• End-expiratory breath samples are collected at 15-30 minute intervals for three hours 1
• A rise in hydrogen concentration from baseline by more than 20 parts per million indicates positive diagnosis 1

Critical Limitations:

• False negative rates can be as high as 25%, often due to hydrogen non-excretion which occurs in approximately 18% of tested subjects 1
• A negative breath test does not exclude the diagnosis; a trial of lactose-free diet should still be considered if symptoms are suggestive 1

Genetic Testing

• Genetic testing can detect lactase gene polymorphisms (LCT-13910 C→T polymorphism) that predict lactase deficiency with high concordance to actual lactose malabsorption 1, 6
• This is particularly useful in populations where lactase deficiency is common, with 68% of the world's population carrying the lactase-deficient genotype 1
• Genetic testing can rule out primary lactase deficiency in individuals from northern European descent where lactase persistence is common 1

Clinical Dietary Trial

• A dietary restriction trial of 2 weeks with symptom resolution can be a simple and economical diagnostic approach 1
• This method is practical when objective testing is unavailable or when symptoms strongly suggest lactose intolerance 7

Testing Indications

• Testing should be reserved for patients who are regular consumers of more than 0.5 pint (280 ml) of milk or equivalent dairy products daily 1
• Patients consuming less than this amount can generally tolerate moderate quantities even if lactose intolerant 1

Secondary Causes to Exclude

Before accepting primary lactase deficiency, rule out secondary causes of lactose intolerance: 1

• Celiac disease - causes mucosal damage leading to secondary lactase deficiency 1
• Gastroenteritis - temporary lactase deficiency during acute illness 4, 1
• Non-steroidal anti-inflammatory drug (NSAID) use - can damage intestinal mucosa 1
• Small bowel disease - any condition affecting mucosal integrity 1
• Chemotherapy - can cause reversible lactose intolerance in up to 35% of patients, though only 11% become symptomatic 1

Management Strategies

Primary 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 lactose-intolerant patients can ingest up to 12 oz of milk daily without symptoms 2
• Yogurt and hard cheeses are generally well-tolerated even in lactose-intolerant individuals and should not be eliminated unnecessarily 1, 8
• Yogurt contains living lactic acid bacteria that survive stomach passage and produce lactase, making it better tolerated than milk 8
• Hard and semi-hard cheeses contain no lactose; only 10% of soft cheeses contain small amounts 8

Specific Clinical Scenarios

Acute Diarrhea in Children:

• Full-strength, lactose-free formulas can be safely introduced immediately after rehydration therapy 4
• This approach reduces both stool output and duration of diarrhea by approximately 50% compared to gradual formula reintroduction 4
• Breast-feeding should continue immediately after rehydration 4

Cancer Patients on Chemotherapy:

• In patients presenting with diarrhea during chemotherapy, avoidance of milk and dairy products (apart from yogurt and firm cheeses) may be a reasonable strategy to reduce symptom intensity and duration 4
• There is insufficient evidence to suggest a lactose-free diet in patients with radiation-induced diarrhea except when clear diagnosis of lactose intolerance has been made 4

Lactase Enzyme Supplementation

• Lactase enzyme supplements may be helpful for patients who wish to consume lactose-containing foods 2
• Treatment effectiveness is modest because lactose is just one of several poorly absorbed carbohydrates that can cause symptoms 6

FODMAP Considerations

If lactose restriction alone doesn't resolve symptoms, implement a low FODMAP diet trial, as this addresses the broader spectrum of fermentable carbohydrate intolerance affecting at least half of patients with presumed lactose intolerance. 1

• A low FODMAP diet may be necessary for patients with concurrent IBS or visceral hypersensitivity 6, 5
• Visceral hypersensitivity associated with anxiety or IBS increases risk of symptoms independent of lactose digestion 6

Critical Pitfalls to Avoid

• Don't rely on patient self-reports alone - objective testing or systematic dietary elimination is essential 1
• Don't ignore the possibility of IBS - visceral hypersensitivity can cause symptoms independent of lactose malabsorption 1
• Don't diagnose based solely on symptoms - symptoms overlap significantly with other gastrointestinal conditions 1
• Don't unnecessarily eliminate all dairy - yogurt and hard cheeses are important calcium sources and are typically well-tolerated 1, 8
• Don't overlook medication excipients - lactose is a common excipient in many pharmaceutical compounds, though amounts are typically <12g and rarely cause symptoms 4, 1

Nutritional Considerations

• Lactose-intolerant patients must ensure adequate calcium intake through lactose-free dairy products, calcium-fortified foods, or supplements 7, 2
• If dairy products are eliminated, other dietary sources of calcium or calcium supplements need to be provided 7
• The long-term effects of a dairy-free, low FODMAP diet on nutritional health and the fecal microbiome are not well defined 5