Myth: High fructose corn syrup causes weight gain.

Reality: "There's probably nothing particularly evil about high fructose corn syrup, compared to regular old sugar," Rosenbloom says.

She explains that this diet myth arose in 2003, when researchers noticed that obesity was rising along with the use of high fructose corn syrup. "They speculated that ... maybe we handle [high fructose corn syrup] differently than we do sugar," but "there really isn't any evidence to support that," she says.

The American Medical Association recently concluded that high fructose corn syrup doesn't contribute to obesity beyond its calories.