Are you really lactose intolerant?

Research is out this week that may suggest that some of us who think we're lactose intolerant- that's about 2/3s of the world's population, have it wrong, and could be missing out on important dietary nutrients for no reason. But what does this mean? Georgia Mills has been investigating, but first wanted to know more about humanity's history with milk...

Georgia - Milk has a very important part to play in  growing up.  All mammals rely on it when they're first born; it provides them with rich nutrients from their mother. Babies can digest this milk easily as they produce an enzyme called lactase which can break down the sugar in milk which is called lactose. Then, as you grow up, most animals stop producing this enzyme meaning if you drink milk it can cause digestive problems.  This is what's known as lactose intolerance. But plenty of people drink milk without any trouble so what makes us so different. To find out I spoke to Dr Mark Thomas, Professor of Evolutionary Genetics at UCL about the story of when we first started drinking milk, which was around 7,000 year ago shortly after we'd started farming cattle and a single mutation happened in our DNA which meant we produced this enzyme into later life...

Mark - And what's more extraordinary is that it spread very quickly. Now a new genetic variant - they arise all the time - and they can spread a little bit but this one spreads so fast that it cannot be explained just by chance. It requires an extra kick and that kick is, of course, very, very strong natural selection.

Georgia - How strong are we talking here?

Mark - Breathtakingly strong! So it's probably the most strongly selected single genetic trait that's evolved, at least in Europeans, in the last 10,000 years and that's also the case in some African and Middle Eastern populations who also continued to produce this enzyme throughout their adult life.

Georgia - What's going on here - why is this such an advantage to be able to digest the sugar and milk?

Mark - I'm glad you asked me that - I don't know. We have lots of ideas but, to my mind, there isn't a single explanation that explains those unbelievably massive selective advantages.

Georgia - What are some of the ideas we've got for starters?

Mark - OK. So the oldest one is related to the whole story of calcium and vitamin D. Most people in the world get most of their vitamin D, not from food, but from the action of sunlight on the skin. Now you need vitamin D to absorb calcium and we all know that calcium is a good idea for bones and various other things. The problem is that if you're at high latitude, then for most of the year, you actually don't get enough sunlight to make decent amounts of vitamin D. They have a problem and that problem gets a lot worse around 10,000 years ago - that's the period when we switched over to farming.

Prior to that we were hunter gatherers. Now hunter gatherers, they do get decent amounts of vitamin D, especially if they're eating a lot of fish but now these early farmers, they've switched over to mostly cereals and cereals are very, very poor in vitamin D. So the argument goes that milk, which has got some vitamin D and lots and lots of calcium, somehow supplemented their diet and it gave them an advantage but that advantage was only there if they could drink it comfortably. And so, evolving the ability to drink milk comfortably by being able to digest the sugar in it, enabled people to drink more milk and, therefore, to supplement their vitamin D requirement.

Now the problem with that theory is we also see strong selection for continuing to produce lactose throughout our adult life. We see strong selection for it in the Middle East, in Africa and in Southern Europe where they definitely have enough sunlight to make enough vitamin D.

Georgia - What other theories are there that might be able to explain that?

Mark - Well, another one is that milk is actually a relatively a good source of uncontaminated fluid. Milk is relatively parasite free and so one idea is that it was a much cleaner fluid and that idea may well be important and, I would suspect, is more important in arid regions, so in desert regions or in regions that don't have much water.

Georgia - So, for whatever reason, it seems that drinking milk gave you the evolutionary edge in many parts of the world but, not everyone can drink milk. In fact, one in five people in the U.K. say they're lactose intolerant but research from China this week suggest it may be something else entirely people are reacting to which could mean they're cutting dairy out of their diets for no reason at all. Dr Anton Emmanual is Senior Lecturer in Neurogastroenterology at UCL and he has been looking into the research...

Anton - The background to this is that a lot of people feel that they can't tolerate milk products and they label themselves as being lactose intolerant. And what we find when we formally test for that is that a lot of people aren't and what this study does is to illustrate something which there is something else in milk other than lactose. There's lots of things in milk but one of those things could be a particular protein which can cause problems.

Georgia - What's this protein?

Anton - So the protein is something called the A2 protein. It's part of something called casein and that protein is something some people can digest perfectly and some people probably don't digest as well.

Georgia - And so some people not digesting this might just identify themselves as lactose intolerant when actually it's something else going on.

Anton - Yes, there's many things in milk as we say and one of those things could be this protein which could be the thing that's causing problems and, when that is not digested properly, that can result in very similar symptoms to lactose intolerance. So things like diarrhoea and rumbly tummies and so it's easy to see why people can readily mistake the two.

Georgia - OK. So how did this study try and separate these two problems?

Anton - There is a variant of cows which can produce this casein in a form which is not a problem to digest and that's the so-called A2 type products. And what this study does then is take both patients to either normal milk, called A1 milk, or this milk which has just got the A2 protein in it. And it did this in a so-called crossover design so patients, unbeknown to themselves, had one or the other in random order and then had the various measurements and symptoms quantified after each exposure.

Georgia - What did they find?

Anton - What they found was that there is a cohort with this patient group who can tolerate the milk fine and if they were exposed to normal milk, in other words when they took A2 milk they were fine, but when they took A1 milk they had the symptoms of diarrhoea, and rumbling tummies, and they had objective measurements of rapidity of movement through their gut.  And their cognitive state, in other words the way their brain processes things quickly, that was impaired when they took the A1 milk but not with the A2 milk.

Kat - Important stuff. That's Anton Emmanual, and before him Dr Mark Thomas. That research was published this week in the Nutrition Journal.