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Why You're Fat

Your Fat Bacteria Is Why You're Fat (And May Be The Key To An Obesity Cure)

Guess what happens if you take the bacteria from a skinny person's intestines and put them into a mouse. A few scientists decided to find out.

Your Fat Bacteria Is Why You're Fat (And May Be The Key To An Obesity Cure)

[Image: Intestines via Shutterstock]

Weight gain can be infectious—just ask anyone tempted by their coworkers’ constant snacking. But our tendency to be either lean or heavy might be far more infectious than we've realized, new evidence shows.

In recent years, scientists have learned that the unique microbial communities in our guts have a role in determining our body composition, just like diet and exercise. Actually harnessing the power of bacteria to fight obesity, however, has always been a long ways off.

In a series of experiments reported in the journal Science today, a team of researchers from Washington University in St. Louis showed how traits like "obese" or "lean" can be transmitted from humans to mice via gut bacteria and took "a step toward the ultimate goal" of developing anti-obesity therapies from simple bacteria, as a commentary published alongside the study put it.

The scientists started by finding four pairs of human twins, where one twin was obese and one was lean, and then implanted a sample of each of the twins’ gut "microbiota" into mice that were raised in sterile conditions. Even though all mice were fed the same exact diet, the mice populated by microbes from the obese twin grew fatter than mice who got the microbes from the lean twin and their metabolism changed, too.

Another stage of the experiment led to an even more interesting result: Combined with a healthy diet, "lean" microbial communities can be catching.

Remarkably, when the obese and lean mice were later put in the same cage for 10 days, the obese mice began to show the same "leaner" metabolic profiles of their cagemates (through feces, mice exchange bacteria when they share a cage, but the researchers wanted to find out which bacteria would dominate). But while this transformation happened when mice were fed their normal diet and a "healthy" human diet, it actually did not occur when the mice were fed a "typical" American diet that is low in fiber and high in saturated fat.

The upshot is that we may be able to use bacteria to treat obesity, but it probably would need to happen in concert with a healthy diet-a lab finding that pretty accurately reflects the intertwined factors that experts have observed in public health studies.

"Together, these results emphasize the strong microbiota-by-diet interactions that underlie invasion and illustrate how a diet high in saturated fats and low in fruits and vegetables can select against human gut bacterial taxa associated with leanness," the study says.

Today, probiotics are all the rage in food marketing, with more than 500 products hitting the shelves within the last decade and many labeled with a wide range of healthy claims. Whether "fights obesity" could ever be added to the list remains to be seen—especially because the study was done with mice only and the same interactions may not happen in humans—but the results offer hope.

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