5 Sep 2013

Are bugs to blame for obesity?

You’re not “big boned”, you’ve got the wrong bacteria – or so suggests a new study showing that the microbiology of our guts has a major influence on obesity.

Bacteria in a petrie dish (Getty)

Writing in the journal Science American researchers describe how transplanting bacteria from the gut of a fat person into the guts of mice made them fat. Bugs from the gut of a thin person made mice thin.

“This wasn’t attributable to differences in the amount of food they consumed, so there was something in the microbiota that was able to transmit this trait,” said Professor Jeffrey Gordon at the Washington University school of medicine in St Louis, Missouri who co-authored the study.

Small but vital

The finding is part of a new era of biological research highlighting the importance of the bacteria that live in us and on us.

While our the human body is made up of around 10 trillion cells, our guts and skin are colonised by around 100 trillion microbes.

The make-up of this community of bacteria that outnumber our cells ten-to one has been been implicated in our susceptibility to diseases like irritable bowl syndrome, disorders like diabetes, and even complex illnesses like autism.

To demonstrate gut bacteria’s potential role on obesity, the researchers recruited four pairs of twins – one of each pair obese, the other thin. Their gut microbiota was then transplanted into the guts of mice reared in sterile conditions since birth meaning they had no bacteria in them at all.

As well as showing that bugs from the fat twin’s gut made mice fat and vice-versa, they found something more suprising.

Mice that had received gut bugs from the thin donor could pass on their beneficial effects just by being housed with a mouse which had recevied the “fat” gut bugs. How can that be? It it’s a dirty business – mice aren’t too fussy about what they eat.

“Mice – delicately put – exchange their microbes readily,” said Professor Gordon. But this “battle of the microbiota” as Gordon calls it is not a fair fight.

No easy answers

Bacteria transplanted from the obese twin didn’t move the other way. Fat mice did not make thin mice fat – suggesting the interactions between bacteria and the gut is a complicated topic.

So could a simple dose of slimming bacteria from another human mean you never have to diet again?

The effect was only observed when the mice were being fed a “healthy” low-fat, high-fibre diet. When mice were fed a “western” high-fat, low-fibre diet they gained weight regardless of which human gut bacteria they were given.

So it’s not simple to explain why bacteria might influence obesity.

In fact the species of bacteria more involved in causing the mice to slim down have been previously implicated in causing obesity in humans. But the findings could point to important relationships between our guts, their bacteria and the food they help us digest.

“In the future, the nutritional value and the effects of food will involve significant consideration of our microbiota – and developing healthy, nutritious foods will be done from the inside-out, not just the outside-in,” said Gordon.