May 7, 2012 -- By 2030, 42% of Americans will be obese and 11% of Americans will be severely obese, Duke University and CDC researchers predict.

These shocking numbers actually are conservative, note study researchers Eric A. Finkelstein, PhD, and colleagues.

Finkelstein's team based its calculations on self-reported weight and height from people participating in the CDC's Behavioral Risk Factor Surveillance System. Obesity is defined by body mass index (BMI). People tend to underestimate their weight and overestimate their height. The researchers corrected for this. They also factored in state-by-state trends in factors affecting obesity, such as the number of fast-food restaurants per person and the cost of unhealthy, calorie-dense foods vs. healthy foods.

There's some good news. The study suggests that adult obesity in the U.S. is leveling off, albeit at an unacceptably high level. Previous estimates suggested that 51% of Americans would be obese in 2030. But that figure may be too full, the new study finds.