Indeed, some Americans value thinness more than life itself. In a 2007 study, 24 percent of women and 17 percent of men said they would trade three or more years of life to be svelte.

Yet despite the rising personal stakes, a growing body of research shows just how hard it is for the average person to keep the pounds off.

Just before speaking to Reuters, McAfee had exercised for an hour in her Florida pool and had a salad for lunch.

“I work out, I eat a lot of fruits and vegetables, and I’m still not thin,” she said. “So please stop beating the crap out of me: It’s completely counterproductive.”

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By Sharon Begley