Milk is easily digested by only about one-third of the global adult population. For the majority of people, consuming milk causes digestive distress because their bodies cannot process the sugar it contains. The ability to continue digesting milk past infancy is not the biological default. Rather, this trait, known as lactase persistence, is a relatively recent genetic change that spread rapidly in specific populations, demonstrating how human culture and genetics can co-evolve.
Lactose Digestion: The Standard Human Condition
For all mammals, milk nourishes the young. The milk sugar, lactose, is a disaccharide that must be broken down in the small intestine into simpler sugars, glucose and galactose, before absorption. This breakdown is performed by the enzyme lactase, which is produced by cells lining the small intestine.
In most humans, lactase production begins to decline shortly after weaning, typically between the ages of two and twelve years. This decline is called lactase non-persistence, and it represents the ancestral human condition. When a person with this condition drinks milk, undigested lactose travels to the large intestine. There, gut bacteria ferment the sugar, producing gases and drawing water into the colon. This process results in the common symptoms of lactose intolerance, such as bloating, gas, cramps, and diarrhea.
The Genetic Change That Causes Persistence
The ability to digest milk throughout adulthood is an autosomal dominant trait, meaning only one copy of the associated gene variant is needed for expression. This continued production of the lactase enzyme results from a specific genetic variation. The most well-known variation in European populations is a change from a Cytosine (C) to a Thymine (T) base, designated C/T-13910.
This genetic change does not occur within the LCT gene itself, which instructs the making of the lactase enzyme. Instead, the change is located in a regulatory region, or enhancer, found within the nearby MCM6 gene on chromosome 2. This enhancer acts like a genetic switch for the LCT gene, controlling its activity. The ancestral C version allows the gene to turn off after childhood, while the T variant keeps the gene perpetually active, ensuring lactase production continues into adulthood.
The Evolutionary Advantage of Dairying
The genetic change for lactase persistence first arose independently in multiple human populations within the last 10,000 years, coinciding with the domestication of livestock. This timeline suggests gene-culture co-evolution, where dairying created selective pressure for the genetic trait. Individuals who possessed lactase persistence gained a survival advantage over those who did not.
Milk provided a calorie-dense, protein-rich, and relatively pathogen-free food source. In early dairying communities, such as in Northern Europe and parts of Africa, milk consumption offered survival during periods of famine or crop failure. Furthermore, digesting fresh milk enhanced calcium absorption, which was beneficial in low-sunlight environments where Vitamin D deficiency was common. This selective pressure meant individuals with the persistent trait were more likely to survive and reproduce, causing the gene variant frequency to increase rapidly within a few thousand years.
Global Distribution and Modern Health Implications
The frequency of lactase persistence varies across the globe, reflecting the history of dairying and population movements. In Northern European countries, where the trait is most common, up to 90–95% of adults can digest lactose without issue. Conversely, in populations with no historical tradition of consuming unfermented milk, such as in East Asia, the frequency of lactase persistence can be as low as 5–10%.
This genetic difference has practical health implications. For those with lactase non-persistence, consuming fresh milk can lead to gastrointestinal symptoms. However, modern food science offers solutions to manage this reality. Many non-persistent individuals can still consume fermented dairy products, like yogurt and hard cheeses, because bacterial action significantly reduces the lactose content. Additionally, over-the-counter lactase enzyme supplements and lactose-free milk products allow people to enjoy the nutritional benefits of milk without digestive distress.