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If Americans keep getting heavier at the current rate, 42% of the population will be obese by 2030, a new study says. The increase accounts for an additional 32 million obese Americans and a whopping $549.5 billion in medical expenditures over the same time frame.

The rise in obesity rates has slowed over the past decade or so, settling at about 36% — or 78 million U.S. adults — in 2010. The new public health report presented on Monday at the Centers for Disease Control and Prevention’s (CDC) Weight of the Nation conference in Washington, D.C., predicts that unchecked, that rate could increase by 33% by 2030.

“Should these forecasts prove accurate, the adverse health and cost consequences of obesity are likely to continue to escalate without a significant intervention,” said study author Dr. Justin Trogdon of RTI International, a nonprofit research organization, in a statement.

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The number of people with severe obesity — those with a body mass index (BMI) greater than 40, or who are about 100 lbs. over their healthy weight — is predicted to rise to 11% by 2030, from 5% in 2010. (Obesity is defined as having a BMI of 30 or higher.) Severely obese people are at greater risk for weight-related diseases like Type 2 diabetes, heart disease, stroke, sleep apnea and cancer and, therefore, account for a greater proportion of medical costs to the country.

“They also have a much shorter life expectancy and generate greater lifetime medical costs, suggesting that future health care costs may continue to increase even if obesity prevalence levels off,” the authors say in the study.

Previous projections had estimated that more than 50% of Americans would be obese by 2030, but the authors of the current study thought that figure was high. To gain a better perspective on obesity in the U.S. and to make a more accurate prediction, lead researcher Dr. Eric Finkelstein, an associate research professor at the Duke Global Health Institute, and his team used data on more than 100,000 people involved in the 2008 Behavioral Risk Factor Surveillance System (BRFSS) — a state-based, cross-sectional telephone survey conducted by the CDC and state health departments.

The survey captured people’s self-reported height and weight. The researchers then also factored in variables that influence obesity rates like the prices of food, alcohol and fuel, annual unemployment rates, household income, the number of fast-food restaurants, and the rate of Internet use.

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“We found that obesity is still increasing, but increasing at a decreasing rate,” said Dr. Finkelstein in a teleconference on Monday.

The data did not include children, so the estimates could be conservative. Obese children typically become obese adults.

The researchers hope their findings will spur nationwide initiatives to curb weight gain. “We know more than ever about the most successful strategies that will help Americans live healthier, more active lives and reduce obesity rates and medical costs,” said Dr. William Dietz, director of the CDC’s Division of Nutrition, Physical Activity and Obesity, in a statement. “People need to make healthy choices, but the healthy choices must first be available and accessible in order to make them.”

Strategies that are known to help people stay fit include building recreational facilities, improving urban design, increasing anti-obesity social marketing programs, adding workplace health promotion programs and developing new drugs and technologies.

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The authors note that their estimate is just that — an estimate. “We’re almost surely wrong in our forecast,” Finkelstein told CNN. “That’s the thing about forecasts — they are guesses. This is our best guess. The world changes, if new drugs or technology comes out, if the government comes out and makes major changes, lots could happen. If in fact we’re wrong and obesity rates are less, I’d be happy.”

The study is published online by the American Journal of Preventive Medicine.