A single genetic mutation allows people to tolerate milk after they leave babyhood, and is virtually the same in people of Asian, European and African descent, researchers reported Sunday.
Finding the tiny change in the genetic code should allow scientists an easy test for lactose intolerance, a painful digestive condition, and also offers insights into how some groups of people evolved a milk-drinking culture, the team of U.S. and Finnish researchers said.
People who have lactose intolerance - most of the people in the world - cannot digest large amounts of lactose, the main sugar found in dairy products.
If they eat milk, cheese or other dairy products they develop nausea, cramps, bloating, gas and diarrhea. Between 30 million and 50 million North Americans are lactose intolerant - 75 per cent of African-Americans and 90 per cent of Asian-Americans.
It affects about 5 per cent of Northern Europeans and close to 100 per cent of Southeast Asians, said the researchers, who reported their findings in the journal Nature Genetics.
Lactose intolerance was known to be genetic, caused by a recessive gene, meaning that a person has to inherit a "faulty" copy from each parent to be lactose intolerant.
"This is the first time this mutation, the DNA change, is actually identified," said Dr. Leena Peltonen, a geneticist at the University of California Los Angeles, who led the study. "This paves the way to DNA testing."
Peltonen and colleagues first looked at nine Finnish families, 196 people, who had lactose intolerance. They narrowed it down to a gene that regulates the gene responsible for making the enzyme that breaks down lactose.
They found two changes, one in every person with lactose intolerance and another in all the Finns.
Then they looked at blood samples from nine Italians, nine Germans and 22 Koreans, all of whom had been diagnosed with lactose intolerance, as well as genetic information from 109 people from Utah in the United States and France.
They found the gene variation in 41 per cent of the French, 7.6 per cent of white North Americans and 79 per cent of African-Americans.
Peltonen said babies are born with the ability to digest lactose - it is found in breast milk - but they lose this ability after weaning.
"That we found the same DNA variant in all lactose-intolerant people across distant ethnic groups indicates to us that it is very old," she said.
"We believe that the variant we identified in patients is the original form of the gene, which mutated to tolerate milk products when early humans adopted dairy farming," she added.
"This 'lactose intolerance' today is actually the ancient form of the gene."
In cold climates where winter crops cannot be reaped, a gene mutation allowing adults to digest milk would help people survive better. People who survived would pass on those genes to their offspring.
"Ten to twelve thousand years ago, when human populations started to use dairy culture - cattle, goats - around that time the mutation happened and made some individuals lactose tolerant," Peltonen said.
"I think it's fascinating. People think lactose intolerance is a disease, but this is how everyone was initially."