For many years, it was thought that some humans developed the ability to digest lactose permanently after they started drinking milk from animals. A new study points to a different story.
There’s a good chance you have lactose intolerance — and you’re not alone. Only a third of humans can currently digest this sugar present in milk, and 5,000 years ago this percentage was even lower.
Research published Wednesday in the journal Nature, carried out by researchers at the University of Bristol and University College London, found that the ability of some people to digest lactose became more common about 5,000 years after first signs of human milk consumption, dating to approximately 6000 BC
Using new computer modeling methods, they also found that milk consumption was not the reason for the increase in lactose tolerance among humans.
“Milk didn’t help at all,” Mark Thomas, one of the study’s authors and a researcher at University College London, told DW. “I’m excited about the statistical modeling method we’ve developed. As far as I know, no one has done this before.”
What is lactose intolerance?
All babies can digest lactose normally. But for most of them, this ability starts to decrease after they stop consuming breast milk.
About two-thirds of people are non-persistent lactase, which means they lose their ability to digest lactose, the main sugar in milk, throughout their lives.
People who are non-persistent lactase do not produce the enzyme lactase, which breaks down lactose. When this enzyme is absent, lactose makes its way to the colon, in the intestine, where bacteria feed on it.
This can cause unpleasant side effects like cramping, flatulence or diarrhea, some of the symptoms of lactose intolerance.
DNA analysis and medical history
The research results run counter to a widespread belief that the consumption of milk by our prehistoric ancestors led to the evolution of a genetic variation that allowed them to digest lactose in adulthood.
This assumption can be partially attributed to the commercialization of the purported health benefits of lactose tolerance. For years, the dairy industry, physicians and nutritionists have touted milk and dairy products as important vitamin D and calcium supplements.
The researchers dismissed this idea after analyzing a huge pool of DNA and medical information from people in the UK. They found that being lactose tolerant or not had little effect on people’s health, their calcium levels or whether or not they drank milk, Thomas said.
Why did lactase persistence evolve?
Genetic studies show that lactase persistence is “the most strongly selected single genetic trait that has evolved in the last 10,000 years,” said Thomas.
Around 1000 BC, 5,000 years after we started consuming milk from animals, the number of humans able to digest lactose, which is encoded in a gene, began to increase rapidly.
After discovering that milk consumption was not behind this rapid growth, the researchers tested two alternative hypotheses.
One hypothesis was that when humans were exposed to more disease, the symptoms of lactose intolerance combined with the new pathogens could become deadly.
“We know that exposure to pathogens has increased over the last 10,000 years as population densities have increased and people have lived closer to their pets,” Thomas said.
The other hypothesis had to do with hunger. When crops planted by prehistoric lactose-intolerant populations failed to thrive, milk and dairy products became some of their only food options.
“If you’re a healthy person, you get diarrhea. It’s embarrassing. If you’re severely malnourished and you get diarrhea, there’s a good chance you’re going to die,” Thomas said.
The researchers used the same computer modeling methods to examine whether these insights could better explain the evolution of lactase persistence.
“And they explained it much, much better,” Thomas said. “All these theories that ultimately relate to the use of milk don’t seem helpful.”
The study focused primarily on European populations, and further research would be needed for populations on other continents.