The Persistence That Isn't Universal
Lactose intolerance is not a disease — it is the ancestral human norm. All infant mammals produce lactase, the brush border enzyme that cleaves the disaccharide lactose into glucose and galactose for absorption. In most populations, lactase activity drops by 75 to 90 percent after weaning, a genetically programmed decline known as lactase non-persistence. Only populations with a long history of pastoralism — principally Northern Europeans, some East African and Middle Eastern groups — carry mutations near the LCT gene (most commonly the C/T-13910 variant in the MCM6 enhancer region) that keep lactase expression high into adulthood.
Globally, about 68 percent of adults have reduced lactase activity. Prevalence of lactase non-persistence exceeds 90 percent in East Asia and approaches 80 percent in West Africa and South America, while it falls below 10 percent in Scandinavia and the Netherlands. These geographic patterns reflect roughly 7,500 years of convergent evolution driven by dairying cultures — one of the clearest examples of gene-culture co-evolution in humans.
Primary vs Secondary
Primary hypolactasia is the genetically determined decline and accounts for the vast majority of cases. Secondary hypolactasia results from mucosal injury — coeliac disease, Crohn's disease, infectious gastroenteritis, or chemotherapy — that damages the lactase-expressing enterocytes of the jejunal brush border. Secondary forms can be reversible once the underlying cause resolves and the mucosa heals.
What Happens When Lactose Reaches the Colon
Undigested lactose that escapes the small intestine enters the colon, where resident bacteria — particularly Bifidobacterium, Lactobacillus, and certain Bacteroides species — ferment it into short-chain fatty acids, hydrogen, carbon dioxide, and sometimes methane. The osmotic draw of unabsorbed lactose combined with gas production explains the characteristic symptoms: bloating, cramps, flatulence, and watery diarrhoea, typically 30 minutes to 2 hours after ingestion.
The hydrogen breath test exploits this fermentation. After an oral lactose load (typically 25 to 50 grams), a rise in breath hydrogen of 20 ppm or more above baseline indicates malabsorption. Methane-producing archaea can consume hydrogen, so some labs also measure breath methane to reduce false negatives.
Microbiome Adaptation
Intriguingly, regular dairy consumption — even in lactase non-persistent individuals — can attenuate symptoms over time. This is partly a colonic adaptation: the microbiome shifts to favour efficient lactose-fermenting species that produce less gas per gram of substrate. A 2019 study in the American Journal of Clinical Nutrition showed that 3 weeks of daily dairy increased faecal beta-galactosidase activity and altered the Bifidobacterium composition in lactose malabsorbers, with symptom improvement that outlasted the exposure period.
This microbial adaptation means that total dairy avoidance, while effective for symptom control, may not always be necessary. Gradual reintroduction in small doses — particularly fermented dairy such as yoghurt or aged cheese, where bacterial beta-galactosidases have already partially digested the lactose — is often well tolerated and preserves calcium and vitamin D intake.