Across much of the world, most adults are lactose-intolerant, meaning that after being weaned from breast milk, they gradually lose the ability to digest animal milk and many dairy products without experiencing digestive issues. In contrast, most people of European ancestry—particularly those from northern and central Europe—retain the ability to digest milk into adulthood. This trait, known as lactose tolerance, is linked to a genetic mutation that allows for the continued production of lactase, the enzyme in the small intestine that breaks down lactose, the sugar in milk.
Several explanations have been suggested for why this mutation became widespread in certain populations. One theory involves famine and the availability of liquid milk. Thousands of years ago, long before refrigeration, milk in warmer regions would spoil quickly and turn into yogurt. Since bacteria in yogurt break down lactose, even lactose-intolerant individuals could consume it without difficulty. In cooler climates such as northern Europe, however, milk stayed fresh longer instead of fermenting. During periods of famine, people without the lactase mutation who drank fresh milk may have suffered from severe diarrhea, which combined with malnutrition could have been fatal. Those with the lactase-persistence gene, however, could safely consume milk, gaining vital nourishment and increasing their chances of survival.
