The trillions of bacteria that live in the gut — helping digest foods, making some vitamins, making amino acids — may help determine if a person is fat or thin.

The evidence is from a novel experiment involving mice and humans that is part of a growing fascination with gut bacteria and their role in health and diseases like irritable bowel syndrome and Crohn’s disease. In this case, the focus was on obesity. Researchers found pairs of human twins in which one was obese and the other lean. They transferred gut bacteria from these twins into mice and watched what happened. The mice with bacteria from fat twins grew fat; those that got bacteria from lean twins stayed lean.

The study, published online Thursday by the journal Science, is “pretty striking,” said Dr. Jeffrey S. Flier, an obesity researcher and the dean of the Harvard Medical School, who was not involved with the study. “It’s a very powerful set of experiments.”

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