Why doesn't lactase (enzyme that breaks down lactose) work for some individuals with lactose intolerance, causing stomach pain?

Why Lactase Supplements May Not Work for Lactose Intolerance

Lactase enzyme supplements often fail to relieve symptoms because at least 50% of patients with presumed lactose intolerance actually have broader intolerance to FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols), not just lactose—meaning the lactase enzyme only addresses one component of their carbohydrate malabsorption problem. 1, 2, 3

Primary Reasons for Lactase Supplement Failure

Misdiagnosis of the Underlying Problem

• Self-reported lactose intolerance correlates poorly with objective evidence of lactase deficiency, meaning many people who think they're lactose intolerant are actually reacting to something else entirely 1, 2
• Symptoms of bloating, abdominal pain, and diarrhea are not specific to lactose intolerance and overlap significantly with irritable bowel syndrome (IBS) and other functional gastrointestinal disorders 1, 3
• At least 50% of IBS patients have broader FODMAP intolerance, which includes not just lactose but also fructose, fructans, galactans, and polyols—lactase only breaks down lactose 1, 2, 3

Insufficient Enzyme Dosing or Timing

• The degree of lactose malabsorption varies greatly among patients, and standard lactase supplement doses may be inadequate for the amount of lactose consumed 4, 5
• Most lactose-intolerant individuals can tolerate 12-15g of lactose (approximately 1 cup of milk) without symptoms, but consuming larger amounts may overwhelm even supplemented lactase activity 5

Secondary Causes Not Addressed

• Reversible lactose intolerance can occur from underlying conditions including celiac disease, gastroenteritis, NSAID use, small bowel disease, or chemotherapy—lactase supplements won't help if the primary problem isn't addressed 1, 2
• Small intestinal bacterial overgrowth (SIBO) can cause similar symptoms and won't respond to lactase supplementation 3

The Role of Colonic Microbiome

Why Stopping Lactose Makes Things Worse

• In lactase-deficient individuals, regular lactose feeding supports the growth of lactose-digesting bacteria in the colon, which enhances colonic lactose processing through a process called "colonic adaptation" 6
• Withdrawing lactose from the diet results in loss of colonic adaptation, which lowers the threshold for intolerance symptoms when lactose is reintroduced—this explains why people who avoid lactose completely become MORE sensitive over time 6
• Intestinal lactase expression itself does not depend on dietary lactose (it's genetically determined and cannot be induced), but the colonic bacteria ARE adaptable 6

Practical Clinical Algorithm

Step 1: Confirm the Diagnosis

• Only test patients consuming more than 280 ml (0.5 pint) of milk or dairy equivalent daily, as those below this threshold rarely experience symptoms even with lactose malabsorption 7, 1, 2
• Use hydrogen breath test for objective confirmation (25-50g lactose load, breath samples every 15-30 minutes for 3 hours, positive if hydrogen rises >20 ppm from baseline) 1, 2
• Consider a 2-week dietary restriction trial as a simple, economical alternative diagnostic approach 1, 2

Step 2: Rule Out Secondary Causes

• Screen for celiac disease (antiendomysial antibodies), gastroenteritis, NSAID use, and small bowel disease before accepting primary lactase deficiency 7, 1, 2
• Consider thyroid function testing and stool microscopy if diarrhea is prominent 7

Step 3: Implement Appropriate Dietary Management

• If lactose intolerance is confirmed, start with temporary reduction (not complete elimination) of lactose to maintain some colonic adaptation 1, 6
• Yogurt and hard cheeses should NOT be eliminated as they contain negligible lactose and are generally well-tolerated even in lactose-intolerant individuals 2, 8
• If symptoms persist despite lactose restriction, implement a low FODMAP diet trial to address the broader spectrum of fermentable carbohydrate intolerance 1, 2, 3

Common Pitfalls to Avoid

• Don't rely on patient self-reports alone—objective testing or systematic dietary elimination is essential 1, 2
• Don't ignore the possibility of IBS—visceral hypersensitivity can cause symptoms independent of lactose malabsorption 1
• Don't recommend complete lactose avoidance—this eliminates colonic adaptation and makes patients MORE sensitive when they inevitably consume lactose again 6
• Don't assume lactase supplements will work for everyone—they only address lactose, not the other FODMAPs that affect at least half of these patients 1, 2, 3