< Fat Bacteria in Human Guts Tied to Obesity

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Heard on All Things Considered

December 20, 2006 - MICHELE NORRIS, host:

You might be worried about putting on a few pounds during the holiday season. If you are, you're not alone. But when you reach for a Christmas cookie, keep in mind that you're not the only one who's going to enjoy that treat. It will also get eaten by the bacteria living in your gut. And it turns out that the kind of bacteria living there may affect how much weight you gain.

Here's NPR's Nell Boyce.

NELL BOYCE: No matter what you tell the maitre d', there is no such thing as a table for one.

Dr. JEFFREY GORDON (Washington University): You never dine alone. You dine with a few trillion friends.

BOYCE: Jeffrey Gordon studies bacteria at Washington University at St. Louis. He says these trillions of bacteria help us break down certain foods that otherwise we couldn't digest.

Dr. GORDON: They're able to persist in the gut despite this continuous flow of Diet Coke, burgers, etc. So there's able to maintain a foothold in this very dynamic ecosystem, called our gut.

BOYCE: Now, until a couple of years ago, scientists didn't have the tools to figure out exactly who is living in a person's digestive tract. But with new genetic probes, they can do a kind of census.

So Gordon thought, let's compare the gut bacteria of fat and skinny mice. He asked Ruth Ley, a member of his lab, to compare the guts of mice that had a gene for obesity with their siblings that didn't have the gene.

Ms. RUTH LEY (Washington University): I didn't think it was a worthwhile experiment.

BOYCE: You didn't want to?

Ms. LEY: Well, no. I shouldn't say that. I was perfectly happy to, but I was extremely skeptical.

BOYCE: And then surprised, because there was a difference. The obese mice had a smaller proportion of a kind of bacteria known as Bacteroidetes. Next, Ley looked at people, twelve obese people. When she ran her tests, she thought -

Ms. LEY: Oh my goodness, it's just like the mice. And what's more as the 12 people lost weight over a year, their gut populations changed. Their profile became more and more like the one in the skinny mice. Ley says she looked at the this data and thought -

Ms. LEY: There's some link between the microbial community composition and obesity, and we're starting to see it. And we need to understand it.

BOYCE: The researchers faced a chicken and egg problem. Was the change in bacteria causing the weight loss or the other way around? So the lab took mice that had never been exposed to any bacteria, their guts were germ-free. Half of them got bacteria taken from skinny mice. The other half got bacteria from fat mice. Both groups put on body fat. But guess which one gained more?

PETER TURNBALL(ph) (Washington University): The mice that were actually given the community from an obese individual gained more fat over the course of the experiment.

BOYCE: That's Peter Turnball, another researcher in Gordon's lab. He says there's a reason why the obese bacteria might lead to more weight gain. It turns out this bacteria have genes that make them better at harvesting calories from food.

Randy Seeley reviewed these studies, which appear in the journal, “Nature.” He's an obesity researcher at the University of Cincinnati. Initially, he was very skeptical.

Dr. RANDY SEELEY (University of Cincinnati College of Medicine): I had a hard time believing that this could really make a difference.

BOYCE: But he's starting to think that Jeff Gordon's lab could really be on to something.

Dr. SEELEY: This is not something that's been on the radar screen. It's exceptionally novel. And if it's correct, over the long road, it would have large-scale implications.

BOYCE: Still, Seeley says we don't know what makes the bacteria change when people lose weight.

Dr. SEELEY: How did the gut microbes know whether they're in a obese person versus a lean person?

BOYCE: And we don't know much of a difference microbes might make compared, to say, exercise. Jeff Gordon says it's too early to try to use these ideas to treat obesity.

Dr. GORDON: I'm worried that people will start trying to treat themselves with antibiotics to shift their microbial ecology. And I think that that would be very dangerous and very premature.

BOYCE: He says it will take much more work to learn whether changing a person's existing gut bacteria can make them lose or gain weight.

Nell Boyce, NPR News.

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