(CNN)

America is getting fatter, according to a new report, and bulging waistlines will rack up big health care expenditures within the next two decades.

The report, from the Trust for America's Health and the Robert Wood Johnson Foundation, draws on previously published government data to make predictions about what consequences an upward obesity trend would have for individual states.

It also projects that the health of the country -- and the dollars spent on the health care system -- would benefit from even a 5% reduction in the average body mass index. The report is called "F as in Fat: How Obesity Threatens America's Future 2012."

The U.S. Centers for Disease Control and Prevention found, in data published in August, that Mississippi is the country's leader in adult obesity, at 34.9%. That number could rise to 66.7% by 2030, the new report found.

The new analysis also projected that obesity rates in 13 states could rise above 60% among adults by 2030. By that year, every state in the nation may have adult obesity rates above 44%, including 39 states with rates higher than 50%, the report said.

This is consistent with a 2012 study in the American Journal of Preventive Medicine, which concluded that by 2030, 42% of adults will be obese. That study forecast $550 billion in health care spending from now to 2030 as a result of rising obesity rates.

Just how fat?

But some experts are skeptical about how accurately obesity trends can be predicted. Methods of calculating how fat Americans will be in the future vary greatly, and there's no accepted standard of determining it, said David B. Allison, director of the Nutrition Obesity Research Center at the University of Alabama at Birmingham, who was not involved in the new study.

"I don't mean for a moment that we should not be taking steps to reduce obesity," Allison said. "If it increases in prevalence, it would be a more serious problem. And even if it decreases in prevalence, without us intentionally doing anything in the immediate term, I'd be shocked if it's going to vanish."

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Health care costs to bulge along with obesity