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.

Severe Obesity a Severe Problem

Whatever comfort that good news may bring is more than made up for by the finding that severe obesity -- BMI of 40 or more -- is skyrocketing. Severely obese people are at the highest risk of diabetes, heart disease, and other obesity-related conditions.

Once rare, severe obesity now afflicts 1 in 20 Americans. By 2030, the study suggests, more than 1 in 10 Americans will be severely obese.

"This is a group at risk of really great health implications, yet they are increasing at even a greater rate than overall obesity," study co-author Justin Trogdon, PhD, of the Research Triangle Institute, said at a news teleconference held to announce the findings.

The cost of obesity is ballooning along with our waistlines. It's no secret that health care costs are getting higher and higher. Obesity costs account for about 9% of annual medical costs. That's a $147 billion per year price tag, even if predictions are wrong and obesity remains at 2010 levels.

This means efforts to reign in obesity can pay big dividends. Just a 1% decrease in the predicted trend would mean 2.9 million fewer obese adults in 2030, resulting in $4.7 billion annual cost savings.

If Americans could become no more obese than we were in 2010, the U.S. would save $549.5 billion over the next two decades.

Might this happen? Only if there are big changes.

"Things may change. Time will tell," Trogdon said. "But if our predictions are not accurate, it will be because things have changed in the environment."

Finkelstein and colleagues reported their findings today at the CDC's Weight of the Nation conference in Washington, D.C. The study also appears in the current issue of the American Journal of Preventive Medicine.

SOURCES:Finkelstein, E.A. American Journal of Preventive Medicine, manuscript received ahead of print.News teleconference from the CDC's Weight of the Nation conference, held May 7-9 in Washington, D.C.News release, Duke University.

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