How some adults keep making lactase
For most people, the body gradually turns down lactase production after weaning. Lactase is made by cells lining the small intestine, where it splits lactose—the sugar in milk—into simpler sugars that can pass through the gut wall and enter the bloodstream. When too little lactase is available, some lactose continues into the large intestine, where gut microbes ferment it. That can cause gas, bloating, or diarrhea.
In some populations, inherited DNA changes alter this developmental switch. They help keep the nearby lactase gene active beyond childhood, a trait called lactase persistence. The changes do not create a different enzyme; they affect how strongly the body continues to produce the same one.
A small regulatory change, a different adult experience
Genes are not simply on or off for life. Cells use regulatory DNA—stretches of genetic material that help control when and how much a gene is used—to adjust production as the body develops. Variants associated with lactase persistence affect regulation of the lactase gene, so intestinal cells continue making enough enzyme for many adults to digest milk without symptoms.
The variants arose in different populations, rather than from one change shared by everyone who can digest milk as an adult. Adult lactose digestion is therefore common in some ancestries and less common in others, but ancestry alone cannot predict an individual’s tolerance. Symptoms also depend on how much lactose someone consumes and other aspects of digestion.
The practical consequence is that the same glass of milk can be easy for one adult to digest and uncomfortable for another. The difference often begins with whether the intestine keeps producing a childhood enzyme.
