Dr. Sue Decotiis, a medical weight-loss specialist and internist at NYU Medical Center, says now there's proof as to why her patients are successfully shedding pounds.
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's involved in obesity," said Decotiis.
Researchers say 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," said Decotiis.
Decotiis says 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 really is the purpose of giving it," said Decotiis.
Toni Castellucci says she's following Decotiis' program.
"I really was concerned about achieving optimal health and physical wellbeing," said Castellucci.
Castellucci says she went from a size 12 to a 4 in three months. She 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," said Castellucci.
She says the recent study is convincing, but the bottom line isn't just about weight. It's about feeling healthier.
Decotiis says many things can contribute to gut bacteria, like long-term use of the birth control pill, hormones in food, junk food and chemicals in the environment.