Young children who retain the ability to digest milk benefit from a big increase in available calories, and this may explain why mutations that allow this spread rapidly in some human populations several thousand years ago.
These extra calories would greatly increase children鈥檚 chances of surviving infections, as undernourished children are more likely to die if they become seriously ill, says Alexandre Fabre, a paediatrician at the Timone Infant Hospital in Marseille, France, who led the study.
Milk is rich in a sugar called lactose. Babies produce an enzyme called lactase in their small intestines that breaks lactose down into glucose and galactose, which can be absorbed by the body.
Advertisement
In around two-thirds of the world鈥檚 population, lactase production starts to decline in the second year of life. By age 5, half of these people have lost the ability to digest lactose and by age 9 most have.
Without lactase, consuming lots of milk can result in flatulence, discomfort and diarrhoea as microbes in the gut feast on the undigested lactose.
After people domesticated animals such as cattle and started using their milk, mutations that result in lactase production persisting throughout life spread rapidly in .
Exactly what drove this has long been debated. Last year, of University College London and his colleagues reported that patterns of ancient milk use alone don鈥檛 explain the evolution of lactase persistence. His team suggested that famine and disease were the key factors, with lactose intolerance becoming life-threatening during hard times.
But not all lactase non-persisters are lactose intolerant, points out Fabre. Many consume moderate amounts of milk products 鈥� especially fermented ones such as yoghurt with lower lactose levels 鈥� without problems.
Fabre thinks the key factor in the evolution of persistence is whether children drank raw milk. His team calculates that a litre of milk can provide 40 per cent of the recommended daily calories for a 3-year-old with lactase persistence, compared with 30 per cent for non-persisters, giving a possible survival advantage. Several studies also show that modern children who are non-persisters drink much less milk than persisters as they get older, the team points out.
When children become ill, lactase production . This is less likely to be a problem in children who have two copies of gene variants conferring lactase persistence rather than just one, because they produce more lactase, potentially giving an additional survival advantage, says Fabre.
Fabre鈥檚 study is the third this year to highlight the extra energy available to lactase persisters. A study in January suggested that an apparent growth spurt in people in Europe between 7000 and 4000 years ago was due to the extra energy available to lactase persisters.
And at University College London and his colleagues also argued that extra energy, rather than intolerance, was the key. 鈥淚t鈥檚 sort of so obvious that people almost feel like we knew that anyway, but actually if you look, we didn鈥檛,鈥� says Curtis.
For decades, it has been taken for granted that the evolution of persistence was driven by lactose intolerance, he says.
However, Curtis isn鈥檛 convinced that childhood was a crucial time for selection. 鈥淟actase persistence gives you an advantage your whole life,鈥� he says. 鈥淚t鈥檚 a massive advantage.鈥�
Extra calories could have been another mechanism acting during famines, says Thomas. 鈥淚 don鈥檛 dismiss the extra calories argument.鈥�
Settling the issue once and for all will not be easy. 鈥淟ooking at healthy people today is no good, and doing experiments on severely malnourished people is unethical,鈥� he says.
BioEssays
