Peeking at poo: how some stay slim

A study comparing the bugs living in the guts of identical and non-identical twins has helped scientists flush out a strain of bowel bacteria that are controlled by the genes that you carry and which appear to help some people stay slim.

Naked Scientist Chris Smith spoke with Ruth Ley who made the discovery at Cornell University in the States with help from a British bunch of twins...

Ruth -   We're very interested in the kinds of microbes that live in people's guts and we know that there's hundreds of thousands of different kinds living there.  And we know that everybody has got their own special mix.  And up until now, the thought has been, it's what you eat or your lifestyle or who you hang around with, that really is the key to your personal mix.  But what we wanted to know was, what if it's your own genetic makeup?  Does that have any effect?

Chris -   How can you disentangle though the fact that all of us are different?  And we all have different lifestyles, we all have different parents, we all have different preferences for food, drink, and so on.  How did you get that away, that lifestyle effect away from the genetic effect then?

Ruth -   You can use twins.  There's two kinds of twins.  There's identical twins and the identical twins have exactly the same genetic makeup.  They're essentially clones of each other.  And then the other kind of twin, fraternal twins.  And so, fraternal twins are basically siblings that are born at the same time.  because the twins are all born together, they'll be exposed with the same things, they'll grow up in the same house.  And so, those environmental influences are going to be the same.  And so, if you find that something is more similar for the identical twins, you know that it's not just a shared environment that's done that.

Chris -   Indeed.  So, what did you find then when you did this?

Ruth -   So, what we had to do is ask these twins.  I know hundreds of them, to send us some of their poo.

Chris -   Was that in the form of soiled nappies or just samples in parts?  How did this come to you?

Ruth -   Well, these are all English people actually.  These English twins, I used to working with the UK Twin Registry in London.  So, they're coming once a year for an annual visit.  This time, they brought with them some poo.  The poo is about half bacterial cells.  Each one of those cells has got DNA in it.  We can tell what kinds of bacteria there by sequencing a particular gene that tells us what kind of bacteria that came from.  And then what we can do is tally them all up.  We do that for all the twins and then we see if the identical twins have more similar profiles than the fraternal twins.

Chris -   Well, do they?

Ruth -   Well, they do overall and then we found that there's particular kinds of bacteria for which that's particularly true.  So, there's particular kinds of bacteria that are very much influenced by the host genetic material whereas others are not at all.

Chris -   What fraction of the bugs as a whole were determined by the genes of the person carrying them?

Ruth -   It's a small fraction but we think it's a very important fraction because what we noticed is that they tend to be more abundant in thin people compared to obese people.  We did some experiments with very special mice that are born and bred in bubbles.  They don't have any bacteria or any microbes in or on them. 

So, what we did was give them poo from an obese person that didn't have these special bacteria that we're interested in and then we took another set of mice and gave them the same poo, but this time, we did add our special bacteria that we're interested in.  What we noticed after 3 weeks is that the mice that got the poo plus this special bacterium was thinner than the mice that got poo from the obese people without the special bacteria.  And so, what that told us was that these special bacteria are able to make the mice thinner than if they're not there.

Chris -   Did you compare how much the fat and thin mice ate?  In other words, could you actually say, "Well, these mice are slimmer despite eating the same amount" or were they just eating less because they didn't feel so good?

Ruth -   No, they did eat the same.  So, when we see these bacteria more abundant in thin people, we think maybe people are thinner in part because they've got these special bacteria helping them maintain that healthy weight.

Chris -   Would your interpretation be then that you have a certain genetic makeup, your genetic makeup determines or influences what sorts of microorganisms you carry in your intestines and that in turn influences how you tend to handle energy - food that comes into your body - whether you turned it into fat in other words?

Ruth -   We do think that something exactly like that is happening.  That the genes are influencing the types of microbes that we have and the microbes are then influencing our body type.  Now, whether that is actually through how we handle food or some other way, that is something that we're still trying to understand now.

Chris -   There's a lot of emphasis being placed on the phenomenon of 'transpoosion' now, isn't there where people actually give poo transplants from one person to another to help them to recover from various intestinal diseases.  Do you think then that one approach might be to get together with a few thin people if you're trying to shed a few pounds and ask them for a 'transpoosion'?

Ruth -   I don't think that's necessarily a very good idea, but I think that eventually, we might want to have this particular bug that I'm talking about, these special bacteria and just take those directly in a capsule.