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Could be this next story helps to explain the last few pounds youve gained. When you eat breakfast, the microbes in your stomach are eating too. Now a new study suggests that a high-fat, high-sugar diet can produce big changes in those microbes and the changes might actually contribute to obesity. NPRs Nell Greenfieldboyce has more.

NELL GREENFIELDBOYCE: Our guts are just teeming with microbes. Jeffrey Gordon studies them at Washington University School of Medicine in St. Louis.

Dr. JEFFREY GORDON (Washington University): We have this huge collection of trillions and trillions of microbes that live in our intestines. And one of their functions is to process those components of our diet that we can't digest on our own.

GREENFIELDBOYCE: He says who exactly is living in your gut might affect what you get out of food.

Dr. GORDON: The energetic and nutrient value of food may not be an absolute term but one that is modified in part by the microbes that live in our gut.

GREENFIELDBOYCE: For example, recent studies have suggested certain types of gut bacteria are associated with obesity. Gordon wanted to explore all of this in an easy to use lab animal - the mouse. So his team took completely germ free mice and basically fed them human gut bacteria. The result was mice with microbe communities in their guts that mimic the ones found in people.

Then the researchers started experimenting. The mice first ate a healthy low-fat, plant-rich mouse chow. But then they were switched to a high-fat, high-sugar diet. In less than 24 hours, the researchers saw big changes in the animals gut bacteria.

Dr. GORDON: And we were quite amazed that the community really restructured itself.

GREENFIELDBOYCE: Gordon says the unhealthy diet produced microbe winners and losers.

Dr. GORDON: Certain members of that society of microbes became very dominant, and certain members became more diminutive.

GREENFIELDBOYCE: To see if this change might mean anything in terms of weight gain, the researchers put these altered communities of bacteria into other germ-free mice. Even though these new mice ate the healthy low-fat mouse chow, they accumulated extra body fat over the course of the next two weeks.

The result is reported in the journal Science Translational Medicine. It surprised John Mekalanos. Hes chair of the department of microbiology and molecular genetics at Harvard Medical School.

Dr. JOHN MEKALANOS (Harvard Medical School): This paper addressed squarely the idea that the microbiota affects are ability to metabolize food in a way that might predispose us to obesity.

GREENFIELDBOYCE: He says the ultimate goal would be new therapies for obesity or malnutrition. But to know if thats even possible, scientists have to learn a lot more about what specific bacteria do in our gut. He says this study shows the way ahead, because it used DNA sequencing techniques to identify interesting changes in gut microbes.

Dr. MEKALANOS: The tracking of the biota, if you will, using the state of the art new generation sequencing techniques, is what makes this paper particularly powerful.

GREENFIELDBOYCE: Experiments using these techniques could reveal key microbial players that researchers could then try to manipulate to see what happens. Jeffrey Gordon says theyre now planning similar studies using gut bacteria communities from all kinds of people.

Dr. GORDON: We could sample people living in different cultural contexts, eating different diets, and look at the affects of their communities in mice eating the same diets or even different diets.

GREENFIELDBOYCE: He says this will let them begin to tease out the complex relationship between human gut microbes, diet and weight gain or loss.

Nell Greenfieldboyce, NPR News.

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