Some adults get a stomachache after drinking milk. This is actually normal. Milk has a sugar called lactose. Babies make a special enzyme to break it down.

But after babyhood, most mammals make less of it. Most adults in the world cannot absorb lactose well. People who can drink milk easily have a special gene change. It spread among people who kept cows and goats.

Many adults feel bloated after drinking milk, and this is actually common among humans. Milk contains a sugar called lactose, which is broken down by an enzyme called lactase. In most mammals, lactase production falls sharply after weaning. Undigested lactose then reaches the large intestine, where bacteria produce gas.

Studies suggest that most adults worldwide do not absorb lactose well, although far fewer adults have this problem in northern Europe. In 2002, Finnish scientists found a small DNA change that keeps lactase working in adults. This change probably spread among groups that kept animals for milk, because milk became a valuable food for them.

Feeling uncomfortable after drinking milk is often seen as a weakness, but from a biological perspective it is the default. Lactose, the sugar in milk, must be split by the enzyme lactase before it can be absorbed, and in most mammals lactase production declines after weaning. Undigested lactose is fermented by bacteria in the large intestine, causing gas, bloating or diarrhea. Estimates suggest that most adults worldwide malabsorb lactose, with much lower rates in northern Europe and higher rates in the Middle East.

However, the 2017 meta-analysis behind many commonly quoted figures was retracted in 2025, so exact numbers are uncertain, and results also vary with the testing method. The ability to digest lactose in adulthood, known as lactase persistence, was linked in 2002 to a single-letter DNA change located some distance from the lactase gene, which appears to keep the gene's switch turned on.

This variant probably spread over several thousand years among herding populations, offering a textbook case of gene-culture coevolution. The symptoms may also be milder than people expect. In a 1995 blinded trial, 30 people who described themselves as severely intolerant had only minimal symptoms after drinking a cup of milk a day.