Lactose intolerance is the biological baseline for adult humans
Drinking milk into adulthood is often viewed as normal, but globally it is an evolutionary anomaly. Around 65 to 70 percent of adults worldwide cannot digest lactose. All mammal infants produce lactase to digest mother's milk, but the gene normally switches off after weaning. Lactase persistence—the ability to digest dairy as an adult—is a genetic mutation that emerged only in the last 10,000 years among pastoralist cultures in parts of Europe, Africa, and the Middle East.
The Mammalian Default
In the broader context of mammalian biology, weaning marks a permanent transition in diet. For virtually every mammal on Earth, maternal milk serves as an exclusive and essential source of nutrition during early life. To process this milk, infant digestive systems manufacture an enzyme called lactase. Lactase is produced by the cells that line the small intestine, where it performs a singular, critical job: breaking down lactose, the primary sugar found in milk, into two simpler sugars called glucose and galactose. These simple sugars pass through the intestinal wall directly into the bloodstream to be used as fuel.
Once an infant mammal weans and begins eating solid food, the biological need for lactase vanishes. In nature, adult mammals never consume milk again. As a consequence of natural regulation, the body gradually shuts down lactase synthesis after infancy. This scheduled reduction in enzyme activity is known as lactase non-persistence. Far from being a pathology or digestive defect, the cessation of lactase production is the ancestral, baseline state for humans and all other mammalian species.
What Happens When Lactase Declines
When lactase levels drop, the body can no longer efficiently break down dietary lactose in the small intestine. If an adult with lactase non-persistence consumes fresh milk, the complex sugar travels intact through the digestive tract until it reaches the large intestine, or colon. In the colon, undigested lactose encounters a dense population of resident bacteria that readily ferment the sugar as an energy source.