Difference Between Lactose Intolerance and Lactase Deficiency
Lactase deficiency refers to the reduced or absent activity of the lactase enzyme in the small intestinal brush border, while lactose intolerance describes the clinical symptoms (abdominal pain, bloating, diarrhea) that occur when someone with lactase deficiency consumes lactose. 1, 2
Key Conceptual Distinction
Lactase deficiency is the underlying biochemical condition; lactose intolerance is the symptomatic manifestation. You can have lactase deficiency without lactose intolerance if you don't consume enough dairy or if your gut microbiome compensates effectively. 2, 3
Lactase Deficiency: The Enzyme Problem
- Lactase is the brush border enzyme that hydrolyzes lactose into glucose and galactose in the small intestine. 1, 4
- Lactase deficiency exists in three forms: 5, 3
- Primary (genetic): Genetically determined decrease in lactase activity after age 2 years, affecting 68% of the world's population (approaching 100% in Han Chinese, but only 2-15% in Northern Europeans). 1, 4
- Secondary: Reversible deficiency due to small bowel mucosal damage from celiac disease, gastroenteritis, NSAIDs, or chemotherapy. 1, 6
- Congenital: Rare primary defect present from birth. 1, 5
- The genetic mechanism involves a C→T polymorphism at the LCT-13'910 locus on chromosome-2, which determines lactase persistence in Northern European populations as an autosomal dominant trait. 4, 2
Lactose Intolerance: The Clinical Syndrome
- Lactose intolerance only occurs when someone with lactase deficiency consumes lactose and develops symptoms. 2, 5
- Symptoms include abdominal pain, bloating, borborygmi, flatulence, and diarrhea due to osmotic effects and bacterial fermentation of undigested lactose in the colon. 7, 5
- Critically, lactose intolerance depends on multiple factors beyond just enzyme levels: 7, 2
- Lactose dose consumed
- Intestinal microbiome composition
- Gastrointestinal motility
- Visceral hypersensitivity (independent of lactose digestion)
- Small intestinal bacterial overgrowth
Clinical Implications
Why This Distinction Matters
- Self-reported lactose intolerance correlates poorly with objective lactase deficiency testing. 1, 6 Many patients who believe they are lactose intolerant actually have IBS with visceral hypersensitivity or broader FODMAP intolerance. 1, 7
- At least 50% of patients with presumed lactose intolerance actually have broader intolerance to FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols). 6, 7
- Only patients consuming more than 280 ml (0.5 pint) of milk or dairy equivalent daily typically need intervention, as those consuming less can generally tolerate moderate quantities even with lactase deficiency. 6
Diagnostic Approach
- The hydrogen breath test diagnoses lactose malabsorption (the consequence of lactase deficiency), not lactose intolerance itself. 1, 6 A positive test (>20 ppm rise in hydrogen) indicates lactose malabsorption but doesn't confirm that symptoms are lactose-related. 1
- False negative rates reach 25% due to hydrogen non-excretion in approximately 18% of subjects. 6 If clinical suspicion remains high despite negative testing, proceed with a 2-week lactose elimination trial. 1, 6
- Never confuse lactose intolerance with cow's milk allergy, which is an immune-mediated response to milk proteins requiring completely different diagnostic approaches (IgE testing, oral food challenge). 8
Management Pitfalls
- Don't assume all gastrointestinal symptoms after dairy consumption are due to lactose. 1, 6 Consider IBS, FODMAP intolerance, or cow's milk protein allergy, especially in children where cow's milk allergy is far more common than primary lactase deficiency. 8
- Yogurt and hard cheeses are generally well-tolerated even in lactose-intolerant individuals due to lower lactose content and bacterial lactase activity. 6
- Secondary lactase deficiency is reversible once the underlying condition (celiac disease, gastroenteritis) is treated. 1, 6