The secret to thinness might not be what people eat, but how their bodies manage to digest food differently from those who are overweight, a new medical study shows.

While conducting research on irritable bowel syndrome and Crohn’s disease, medical experts decided to test the effects of gut bacteria. They transferred the bacteria from human twins—one lean, the other obese—into mice.

The result: mice with bacteria from fat twins grew fat, and those that received bacteria from lean twins stayed lean.

Michael Fischbach of the University of California, San Francisco, who was not involved with the study, called the findings “the clearest evidence to date that gut bacteria can help cause obesity.”

Fischbach said he was “very excited” by the work, and insisted the next step should be to use gut bacteria to treat obesity by transplanting feces from thin people.

But other experts, including those involved in the study, said hold on.

Dr. Jeffrey I. Gordon of Washington University in St. Louis, the senior investigator for the study, told The New York Times he wants to determine which bacteria are responsible for the effect in order to give people pure mixtures of bacteria instead of feces.

Another option, Gordon said, would be to figure out what the bacteria produce that induces thinness and provide that as a treatment.

-Noel Brinkerhoff

To Learn More:

In Gut Research’s Latest Advance, Bacteria From Thin Humans Can Slim Mice Down (by Gina Kolata, New York Times)

Gut Microbiota from Twins Discordant for Obesity Modulate Metabolism in Mice (by Vanessa K. Ridaura, Jeremiah J. Faith, Federico E. Rey, Jiye Cheng, Alexis E. Duncan, Andrew L. Kau, Nicholas W. Griffin, Vincent Lombard, Bernard Henrissat, James R. Bain, Michael J. Muehlbauer, Olga Ilkayeva, Clay F. Semenkovich, Katsuhiko Funai, David K. Hayashi, Barbara J. Lyle, Margaret C. Martini, Luke K. Ursell, Jose C. Clemente, William Van Treuren, William A. Walters, Rob Knight, Christopher B. Newgard, Andrew C. Heath, Jeffrey I. Gordon; Science) (abstract)

FDA Committee Endorses Controversial Weight-Loss Drug to Combat Alleged Obesity Epidemic (by Noel Brinkerhoff, AllGov)

Obesity May Cost U.S. $215 Billion a Year (by Noel Brinkerhoff, AllGov)