More than one third of U.S. adults are obese, according to the Centers for Disease Control and Prevention. Now, we're learning what could be a major factor when it comes to how much you weigh.

Dr. Sue Decotiis said there's now proof as to why her patients are successfully shedding pounds.

"Now, we know that this really works," said Decotiis, medical weight loss specialist and internist, NYU Medical Center.

A new study reveals that gut bacteria may help determine whether a person is fat or thin.

"It's one of the many factors that are involved in obesity," Decotiis said.

Researchers said they took bacteria from human twins — one overweight and the other thin — and transferred it to mice. Mice with bacteria from a thin human stayed thin and mice with bacteria from an obese human gained weight

"The whole study is fascinating because we’re learning that we can really prove that there is a difference," Decotiis explained.

Decotiis said she's already been altering the gut bacteria of her patients to help them lose weight by prescribing medicinal grade probiotics.

"It contains a lot of species that perhaps you don’t have if you’re obese, so that is the purpose of giving it," Decotiis said.

Toni Castellucci said she's following Decotiis' program.

"I really was concerned about achieving optimal health and physical well-being," Castellucci said.

Castellucci said she went from a size 12 to a four in three months.

"It works for me," said Castellucci, who expects to go back in the field for the second time with the Peace Corps at age 72. "It has reset my, not so much my goals, but my expectation for myself.  I can do it."

Castellucci said that recent study is convincing, but the bottom line isn't just about weight. It's about feeling healthier.

Decotiis said many things in our environment contribute to gut bacteria, like long-term use of the birth control pill, hormones in food, junk food, and chemicals in the environment.

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