Does fat run in your family?

Blame your gut bacteria.

Researchers at Cornell University have identified a family of microbes called Christensenellaceae that appear to help people stay lean—and having an abundance of them, or not, is strongly genetic.

Someday, it may be possible to have the Christensenellaceae clan adopt you, however. Mice that received transplants of the bacteria gained less weight than untreated mice eating the same diet. The study was published in the journal Cell this month.

There has been an explosion of research into how bacteria affect human health, and body weight is one of the most intriguing areas. There is growing speculation that rising rates of obesity may be due in part to increased use of antibiotics, which may be wiping out bacteria that help humans convert food into energy efficiently.

Babies are born without any bacteria and eventually play host to approximately 100 trillion of the tiny micro-organisms, which outnumber human cells by about tenfold. Bacteria coat every inch of skin, the mouth, the nose, the ears, the genitals and particularly the gastrointestinal track. They not only digest food and help fight off invaders, but also produce vitamins and chemicals that help regulate the immune system, metabolism—even mood.

“In the past, the main bacteria we saw were the nasty guys, the ones that kill you. We haven’t been looking at the thousands of nice guys that help us and keep us thin,” says Tim Spector, a genetic epidemiologist at King’s College London, who also contributed to the study.

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