With the U.S. confronting an obesity epidemic that shows no signs of abating, some doctors and scientists are divided on just how dangerous extra weight is.

The debate is a critical one when you consider that about two-thirds of U.S. adults are classified as either overweight or obese, meaning they have a body-mass index (calculated as weight in kilograms divided by height in meters squared) of 25 or higher.

While most everyone agrees weight loss is a good goal for those who are heavy, some researchers now believe that fitness is more important than fatness when it comes to long-term health and longevity.

They cite studies showing that people in the overweight category—or those with a BMI from 25 to 29.9—actually have better survival rates in some situations than their normal-weight counterparts, a finding known as the “obesity paradox.”

Others say the risks related to obesity are being underestimated. They say most of the studies supporting the obesity paradox are flawed in that they don’t take into account people’s weight history.