Lactose Intolerance

How it works

Lactase breaks down lactose, the sugar in milk. When lactose isn't digested, gut bacteria ferment it, which can cause bloating, gas, cramps and diarrhea.

The ability to keep producing lactase in adulthood is called lactase persistence. It's controlled by variants in a regulatory region of the MCM6 gene, next to the lactase gene. People without these variants gradually make less lactase, a pattern called lactase non-persistence, which is the most common pattern globally.

Genetics is only part of the story

• Not everyone with lactase non-persistence has symptoms, and many tolerate small amounts of dairy.
• Lactose intolerance can also be "secondary", caused by damage to the gut lining from conditions such as celiac disease, infections or inflammatory bowel disease. A genetic test won't detect this.
• Different MCM6 variants are associated with lactase persistence in different ancestral populations.