Many adults in the world are unable to digest lactose, the principle sugar in milk, and for the first time scientists have unravelled a genetic code that has proved that lactose-intolerance is the natural state for adults, rather than a physical disorder. On the contrary, it is a genetic mutation that allows adults to continue to digest lactose after babyhood.

Lactose intolerance can cause nausea, cramps, bloating, gas, diarrhoea and indigestion in those consuming milk, cheese or other dairy products. The condition is found in between 30 million and 50 million North Americans, 75% of African-Americans and 90% of Asian-Americans. Close to 100% of Southeast Asians are unable to digest lactose.

In the majority of people then, the gene for lactase, the enzyme that digests lactose, becomes active at birth but is switched off come the age of weaning.

In Europe, however, only about 5% of the population cannot stomach dairy products. Biologists have explained that with the domestication of cattle and goats about 10,000 years ago, in colder climates where winter crops could not grow, the genetic ability to digest lactose throughout adulthood could be a nutritional advantage. A genetic mutation that prolonged the working of the lactase gene was therefore favoured and gradually spread throughout the population as people who survived harsh winters passed on those genes to their children.

Until now, however, biologists have been confused as to why the gene for lactase and that gene’s promoter are the same both in populations where adults can digest lactose and in those where the adults cannot.

A team of Finnish and American biologists reported in the journal Nature Genetics that they have now identified two single-unit DNA changes that match strongly with the ability or inability of adults to digest lactose.

Geneticist Dr Leena Peltonen, of the University of Helsinki, led the study. She explained that the changes were identified by studying the sequence of DNA units near the lactase gene in nine Finnish families, 196 people, who are lactose intolerant. Around 20% of Scandinavians are lactose intolerant.

The scientists also looked at blood samples from nine Italians, nine Germans and 22 Koreans, all of whom were lactose intolerant, and genetic information from 109 people from France and Utah, USA. They reported finding the gene variation in 41% of the French, 7.6% of white North Americans and 79% of African-Americans.

Peltonen told Reuters: “This is the first time this mutation, the DNA change, is actually identified […] That we found the same DNA variant in all lactose-intolerant people across distant ethnic groups indicates to us that it is very old.”

“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 […] This ‘lactose intolerance’ today is actually the ancient form of the gene.”

“I think it’s fascinating. People think lactose intolerance is a disease, but this is how everyone was initially.”

The finding could allow scientists to develop a more accurate test for identifying lactose intolerance.