From Scientific American:
Two years ago Katherine M. Flegal, a researcher at the Centers for Disease Control and Prevention, did a new statistical analysis of national survey data on obesity and came to a startling conclusion: mildly overweight adults had a lower risk of dying than those at so-called healthy weights.
Decades of research and thousands of studies have suggested precisely the opposite: that being even a little overweight is bad and that being obese is worse. The distinction between overweight and obese—which are sometimes both classified under the rubric of obesity—can be confusing. It relates to the measure called body mass index (BMI), derived by dividing one’s weight in kilograms by the square of one’s height in meters. A myriad of Internet-based calculators will handle the math for you. The only thing to remember is that a BMI of at least 25 but less than 30 is considered overweight, and one of 30 or more is characterized as obese.