I know, I know. You're all tired of my finding silly things that people say on the Net and correcting them. (Like the doctor - a doctor! - who wrote "Only IndoEuropeans (and one small tribe in Uganda) are able to digest milk without problems." Aarrgghhh! Every culture has at least some people who are lactose tolerant. And there are many tribes in Africa who have historic milk cultures and so can drink milk.)
So it's really nice when I can find somebody who's knowledgeable and can explain things and who even says something new and interesting that I didn't know before.
Like Dr. Barry Starr of Stanford University whose Ask a Geneticist column answers a question I get frequently.
I read that lactose intolerance is caused by a genetic mutation. I previously consumed dairy, but have been a vegan for over a year now. I have heard some vegans say that they couldn't return to drinking dairy after being vegan for a long time because their body built up an immunity to it.
But if lactose intolerance is genetic, how is this possible? Are there multiple ways to build or weaken your tolerance for dairy?
Starr gives a good basic introduction to LI and then gets to the guts of the question.
So, what might be going on with your vegan friends?
I can think of a few possibilities. First off, lactase is only made in certain cells in the lining of your intestine. These cells are the only ones that can read the lactase recipe.
So if anything happens to them, then you can get temporary lactose intolerance until the cells return to normal. Things like gastro-enteritis or being allergic to wheat can cause this form of lactose intolerance. I haven't heard of diet affecting these cells though.
Another possibility is maybe something happened to the bacteria in their gut. Even people who make no lactase can tolerate a little milk in their diet. Most likely this is because of some beneficial intestinal bacteria.
These bacteria can digest milk for us in a way that doesn't cause discomfort. Certainly things like antibiotics can give someone temporary lactose intolerance. How might a diet change affect these bacteria?
Our gut is a battleground—a place of ferocious competition between lots of different kinds of bacteria. They are all trying to crowd each other out and they'll use any advantage they have.
Let's say some of the bacteria in your gut can easily digest milk. When you drink milk, they have an advantage over the other bacteria and so happily divide, making more of themselves.
Once you stop drinking milk, their ability to easily digest milk may become a liability. To digest milk, bacteria actually have extra DNA that lets them do it. The extra DNA means it takes a little longer for that bacterium to divide because it needs to copy all of its DNA first.
Now this is no big deal when they are awash in milk. The ability to digest milk more than compensates for the extra DNA. But when there is no milk, they slowly lose out to bacteria that can't digest milk. Or they get rid of that extra DNA and lose the ability altogether.
Either way, when you stop drinking milk, you may lose your ability to digest it because your bacteria lose their ability. I am not sure how big an impact something like this might have on someone's ability to drink milk. But it is a possibility...
Finally, it may be that it is a coincidence, the timing of the vegan diet and lactose intolerance. As I said before, people lose their ability to digest milk at different times of their lives. It may just be that some of your vegan friends happened to become lactose intolerant over the course of the time they were vegan.
That is the same answer that I always give people (phew!) and like Starr I think the real answer is either the second or third possibilities. That bit about the extra DNA on the bacteria was new even to me, though. It helps to be a expert expert.