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Human germs from obese people make mice fat

Vanessa Ridaura handles bacteria cultures from twins that will be transplanted into mice as she discusses the research with Jeffrey I. Gordon, MD, director of the Center for Genome Sciences and Systems Biology at Washington University.
Vanessa Ridaura handles bacteria cultures from twins that will be transplanted into mice as she discusses the research with Jeffrey I. Gordon, MD, director of the Center for Genome Sciences and Systems Biology at Washington University.E. Holland Durando / Washington University/ St. Louis

Gut bacteria from obese humans made mice pack on fat, researchers reported Thursday in the latest finding showing the bugs in your tummy can affect weight.

Bacteria taken from thin people did not make mice fat, the researchers report in the journal Science. 

In fact, bacteria from thin people appeared to protect mice from getting fat, perhaps by outcompeting "fat" germs.

They hope their findings will help eventually narrow down just which bacteria may affect how much energy people absorb from their food.

The hope is to eventually manufacture probiotics – pills, perhaps, or food -- that can help people maintain a healthy weight.

Researchers have found that bacteria and fungi that live in and on our bodies affect health in many unexpected ways. They can prevent disease, cause dandruff, make feet stinky or sweet-smelling. And there’s a big growing body of evidence that the balance of bacteria in the intestines can affect how the body absorbs and uses energy from food.

Fecal transplants can help beef up a patient’s gut microbiome and cure a deadly diarrheal infection called Clostridium difficile.

This population of microbes may also affect an individual’s metabolism.

For the latest study, Vanessa Ridaura, Jeffrey Gordon and colleagues at Washington University in St. Louis found sets of human twins with a special characteristic – one twin was fat and one was lean. They took poop samples from each.

There’s some evidence that people inherit much of their gut microbiome when they are born and then keep a pretty similar population for most of their lives. But there’s also evidence that diet can change it a little over time – people who eat a fatty, Western diet have a different balance of bacterial species living inside them than people eating a plant-based, lean, diet in the developing world.

Twins have the same genetics and would have had the same environment at birth and in childhood. But as they grow, that might change and the team wanted to see the differences in their gut bacteria.

They fed the poop samples to mice bred to have no gut bacteria at all. Mice that got feces from the fat twins grew fatter, while mice given waste from the thin twins stayed lean. They also saw differences in the way the mice metabolized food.

But it wasn’t a simple matter of bacteria squeezing more calories out of every morsel of food. Instead, they found a very complicated change in how energy from food becomes fat in a fat cell.

Then they set up a natural experiment to see which microbes would come out on top. They put the fat and lean mice together. "Mice—delicately put—exchange their microbes readily," said Gordon. They eat one another’s poop, in effect performing their own fecal transplants.

The skinny poop came out on top. The mice that had been colonized with microbes from fat humans, and which had themselves become tubby, adopted more lean microbes. Their metabolisms also changed. But the lean mice did not seem to absorb the “fat” germs.

"So, why isn't there an epidemic of leanness in America?" Gordon asks. It might be down to diet.

When they fed all the mice healthy, high-fiber diets, the lean bacteria seemed to win out. But when they gave the mice junk food, the fat mice stayed fat.

“Eating a healthy diet encourages microbes associated with leanness to quickly become incorporated into the gut,” Gordon said. “But a diet high in saturated fat and low in fruits and vegetables thwarts the invasion of microbes associated with leanness."

“It may be that future microbiota-based therapies for an obese individual will require an alteration in diet to aid colonization by beneficial microbes. Alan Walker and Julian Parkhill of Britain’s Wellcome Trust Sanger Institute wrote in a commentary in Science.

There are already some spas offering fecal transplants with the promise of losing weight. But experts say it’s important to narrow down just which bacteria play a role in weight loss or weight gain. And it’s always possible that someone might get sick from unregulated poop transplants – so the Food and Drug Administration has stepped in to regulate the practice.