Sufferers of lactose intolerance – something which affects around 5 per cent of the UK, Irish, North European and American population – lack lactase (the enzyme which breaks down the milk sugar lactose), and can experience bloating, abdominal pains and diarrhoea, as a result of excessive lactose consumption.
Research has established that lactose intolerance sufferers often confuse their condition with a food allergy (an altogether different disorder, which activates the immune system) and furthermore, most sufferers self-diagnose – using information based on unfounded preconceptions.
‘Around 45 per cent (27 million) of the UK population claim to be lactose intolerant, when in fact only 2 per cent (1.2 million) have actually been clinically diagnosed,’ commented Michele Stephens, communications manager for the UK Dairy Council.
Boosting their case, researchers at Cornell Universitycompiled data on lactose intolerance from 270 indigenous African and Eurasian populations in 39 countries, from southern Africa to northern Greenland.
Publishing their findings in a forthcoming issue of Evolution and Human Behavior, the scientists conclude that it is primarily people whose ancestors came from places where dairy herds could be raised safely and economically, such as in Europe, who have developed the ability to digest milk.
On the other hand, most adults whose ancestors lived in very hot or very cold climates that could not support dairy herding, or in places where deadly diseases of cattle were present before 1900, such as in Africa and many parts of Asia, do not have the ability to digest milk after infancy.
“This is a spectacular case of how cultural evolution – in this case, the domestication of cattle – has guided our biological evolution.”
On average, researchers Paul Sherman, a professor of neurobiology and behaviour at Cornell and Gabrielle Bloom found that a massive 61 per cent of people studied were lactose intolerant, ranging from 2 per cent in Denmark and 100 per cent in Zambia.
They also found that lactose intolerance decreases with increasing latitude and increases with rising temperature, particularly with the difficulty in maintaining dairy herds safely and economically.
Although all mammalian infants drink their mothers’ milk, humans are the only mammals that drink milk as adults.
But most people, about 60 per cent and primarily those of Asian and African descent, stop producing lactase, the enzyme required to digest milk, as they mature.
People of northern European descent, however, tend to retain the ability to produce the enzyme and drink milk throughout life.
A major challenge in interpreting the data, Sherman noted, was to resolve the puzzle that about 13 lactose-tolerant populations live side-by-side with lactose-intolerant populations in some parts of Africa and the Middle East.
“The most likely explanation is nomadism,” Sherman concluded. All thirteen of the populations that can digest dairy yet live in areas that are primarily lactose intolerant were historically migratory groups that moved seasonally, he added.
“Their nomadism enabled them to find suitable forage for their cattle and to avoid extreme temperatures. The fact that these groups maintained small herds and kept them moving probably reduced the pathogen transmission rate,” he concludes.