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The rise of lactose tolerance

Do you like milk? It's likely that your ancestors were cattle herders many thousands of years ago, says Dr Karl.

Last time, I talked about lactose intolerance and how two-thirds of the adults on our planet can't comfortably drink milk.

This is normal for two-thirds of adults. I also discussed the sugar (or carbohydrate) present in milk, which is called lactose. It used to be that only baby mammals had the special enzyme called 'lactase' that could break down lactose in the gut for easy digestion.

It turns out that there are two separate medical conditions related to lactase deficiency.

One is called 'Congenital Lactase Deficiency' (remember that the enzyme 'lactase' breaks down the carbohydrate called 'lactose'). It's a very rare genetic disease in which the lactase enzyme is simply not present — even immediately after birth. The good news is that if the newborn babies can get a milk formula that does not contain the carbohydrate, lactose, they grow up just fine.

A bit more common is a medical condition called 'Secondary, Acquired, or Transient Lactase Deficiency'. It happens occasionally as a result of some kind of injury to the small intestine, such as from intestinal parasites, chemotherapy or acute gastroenteritis. It usually goes away by itself.

Separate again from these abnormal conditions is what most adults on the planet have, lactose intolerance, also called 'Primary Lactase Deficiency'.

So if it's not normal to drink milk as adults, how come about one-third of adults can make lactase and can drink milk? The answer is that they are descended from cattle herders, many thousands of years ago.

Let's talk genetics — specifically, about two genes located on the long arm of Chromosome 2. One gene (LCT) holds the instructions for making lactase — and the other gene (MCM6) can control LCT and turn it on and off. One mutation on the control gene (MCM6) happened (in the geographical region that is now around modern-day Hungary) some 7,000 years ago. It kept the lactase gene turned permanently on, so these cattle herders could now drink the milk from their cows.

And quite separately, on at least three occasions in at least three different locations in Africa, slightly different mutations occurred. These mutations had exactly the same result of keeping the lactase-making gene turned on. And again, these separate and different mutations happened in distinct populations of cattle-raising peoples — some 3,000-7,000 years ago.

The immediate result was an enormous survival advantage. The long-term result was that the cattle-raising people could have almost 20 per cent more fertile descendants than the people who had lost the ability to make lactase after infancy. So, because it was so useful, this ability to digest milk spread very rapidly.

The closer you are to where the mutations arose, the greater the percentage of people who are lactose tolerant. Some 99 per cent of Swedish people and almost all Dutch people are lactose tolerant — but as you move further away, the percentage drops. It's down to 50 per cent in Spanish, French and Arab populations, and to only a few percent in China.

The geneticists see this as an extremely elegant example of 'Convergent Evolution' — when Nature finds several different solutions for the same problem. The cattle-raisers in Europe and Africa each had quite different gene mutations. They each took completely independent pathways that all converged on the same end-point of 'Lactose Tolerance'.

Now you normally think that evolution happens because of changes in the natural environment. For the European and African pastoralists, part of the natural environment was their cattle. These cattle then drove the genetic adaptation in people.

We have found traces of milk proteins in ceramic vessels in present-day Romania and Hungary. These date back 7,500-8,000 years ago. We've also found in Poland some three dozen perforated pottery vessels, also about 8,000 years old, that were used to strain milk to make cheese.

Lactose intolerance is not black and white — in some cases, even though your DNA does not make lactase, sometimes the bacteria in your gut does. Cheese has two advantages over milk for the lactose intolerant — it's lower in lactose, and its higher fat content slows down how rapidly it travels through the gut, and so makes it easier to digest. Fermented cheeses such as cheddar and feta are quite low in lactose, while aged hard cheeses such as Parmesan have hardly any.

So now that we have begun to track the evolution of lactase, maybe we could find the origins of amylase (an enzyme that breaks down starch) or even alcohol dehydrogenase, which lets us break down alcohol.

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