If you can drink milk as an adult without stomach pain, bloating, or diarrhea, your DNA may carry a genetic variant that allows your body to keep producing the enzyme lactase.
Most people do not have this ability. Around two-thirds of humans experience a decline in lactase production after weaning. The enzyme is needed to break down lactose, the main sugar in milk.
When the body no longer produces enough lactase, undigested lactose reaches the large intestine, where bacteria ferment it and can cause digestive problems.
The ability to keep producing lactase into adulthood is known as lactase persistence. According to a genetic review published in Nutrients in 2020, the trait is found in around 91 percent of Swedes outside the Sami population, but only 33 percent of the Sami.
In Asia, the numbers are much lower, at around 15 percent in China and close to zero in South Korea, Viet Nam, and Cambodia.
This means the ability to drink milk as an adult was not always common among humans. It developed later, as some populations began living alongside domesticated animals and consuming their milk.
Humans Drank Milk Before Their DNA Changed
Humans had already been using milk long before lactase persistence became common.
A 2022 study in Nature analyzed around 7,000 traces of fat found on pottery from more than 550 archaeological sites. The evidence shows that people in Europe had been using milk since the Neolithic period, around 9,000 years ago.
The problem was that drinking milk did not automatically make people able to digest it. Ancient DNA studies show that the main genetic variant that later helped Europeans maintain lactase production was still absent from some people who lived during the Neolithic period.
One variant that later became common in Europe is known as -13910 C>T. It is located in a regulatory region near the LCT gene, which contains the instructions for making lactase.
People carrying this variant can continue producing lactase after childhood. If this ability helped them get nutrients from milk, carriers may have had a better chance of surviving and having children.
Over many generations, the variant became increasingly common.
Europe Was Not the Only Place Where This Happened
Europeans were not the only population to develop the ability to digest milk as adults. Several populations developed it through different genetic changes.
A 2007 study in Nature Genetics identified three different variants among populations in Tanzania, Kenya, and Sudan. These variants were also associated with lactase persistence, but they were different from the main variant found in European populations.
A follow-up study in Nature Genetics in 2014 identified five functional alleles, meaning different versions of a particular genetic region, associated with the ability among various African populations. The findings show that the ability to digest milk in adulthood could develop through several genetic routes.
A similar pattern was found in the Middle East. A 2008 study in the American Journal of Human Genetics found that the populations studied did not carry the main European variant. Instead, they had other mutations that could help maintain the regulatory activity of the LCT gene.
Researchers describe patterns like this as convergent evolution. In simple terms, populations facing similar conditions can develop the same ability through different changes in their DNA.
Why Do Many Asians Not Develop This Genetic Trait?
Consuming milk also did not always make lactase persistence variants common.
A 2020 study in PLOS Biology estimated that the -13910 C>T variant first appeared in what is now Ukraine around 5,960 years ago. It then spread across Eurasia and increased sharply in Europe around 3,000 years ago. A similar rise did not occur in Asia.
Yet Central Asian pastoralists had long consumed dairy products. One possibility is that they adapted through culture rather than changes in DNA.
Milk can be fermented into yoghurt and other products, reducing some of its lactose and making it easier to consume for people whose lactase production has declined.
A 2025 study in PNAS also identified an East Asian-specific haplotype, a distinctive stretch of DNA inherited together, associated with changes in LCT expression. In this context, LCT expression simply means how actively cells use the gene's instructions to produce lactase.
However, the researchers concluded that this variation was not directly associated with lactase persistence in the same way as the variants found in European populations.
So, if you can drink milk as an adult without digestive problems, your body likely carries a DNA variant that allows it to keep producing lactase. What feels like an ordinary ability is actually the result of genetic changes that emerged in some human populations over thousands of years, while others adapted to dairy through different means.
