I have a question about intolerance to dairy and alcohol. I've read that the gene for digesting dairy products is on chromosome 2 whilst the gene for metabolising alcohol is on chromosome 12. If these aren't linked, why do both of these intolerances often tend to occur simultaneously and what is the most reliable way of determining whether a person has a true allergy to alcohol or just an intolerance to perhaps the ingredients of or preservatives in alcohol?
We put this to Giles Yeo, from MRC Metabolic Diseases Unit...
Giles - Ah. They are not linked genetically and their apparent linkage in the world in various ethnicities, is entirely coincidental and it actually has to with the history of agriculture within that particular region. So milk: most of the world cannot handle milk - people donít know this. So most people are, actually, lactose intolerant or are unable to handle lactose once they become adults.
The reason why the ability to lactose has evolved is because three different populations, in particular, Northern Europeans and two different African tribes figured out that if you donít kill the cow, the camel or the goat immediately and eat the meat but drink the milk, you actually get a lot more calories out of them. Thatís the first thing.
The second thing is that milk provides a very safe drinking source and so, for people who are able to take advantage of this, they thrive.
You might ask why a lot of the world are not able to handle milk. Iím Chinese, so I will use the Chinese example. And the reason why is they had their own other sources of calories and the Chinese, for example, were the first people to domesticate chickens, and so then, chickens came with eggs and they were able to do that. And the Chinese were the first to domesticate pigs and we donít drink pig milk, eek!. So, therefore they do that.
Chris - I thought you were going to say they were the first to come up with crispy duck as well because that is also fantastic!
Giles - Poultry or in general. In terms of alcohol. Alcohol, once again post agriculture, and people were able to drink and people have always drunk it for cultural reasons. The reason why some cultures are able to handle it more than others however, has to do with safe drinking water. So the Northern Europeans and the people in the fertile crescent, so the Middle East, chose to purify their water by adding alcohol and so that it was then became safer to drink itÖ
Chris - Choice!
Giles - Mmm. So they had to be able to metabolise it well enough to drink it otherwise they would die. The Chinese, amongst other groups, choice to boil the water and end up sticking some twigs in it and call it tea. Although they did drink alcohol, their survival did not rely on alcohol.
Kat - So those genetic variations for lactose tolerance, and alcohol tolerance, have just become selected in the populations that have loads of dairy and loads of booze.
Giles - Correct.