For the first time in 30 years of national obesity reporting, levels of adult obesity have held steady in all but one U.S. state, according to a new study.

A positive takeaway from the news is that by large, Americans are not getting any fatter. Arkansas was the only state in the study to report a gain in obesity levels.

But the figures are still sobering. In 1980 no state had more than 15 percent obesity.

Today, 13 states have obesity rates higher than 30 percent, 41 states have rates of at least 25 percent and every state's population is more than 20 percent obese.

America's most obese states are Louisiana (34.7 percent), Mississippi (34.6 percent) and Arkansas (34.5 percent). For the first time in eight years Mississippi was not at the top of the list.

Colorado was the trimmest, with obesity at 20.5 percent.

The data is published in the report "F as in Fat: How Obesity Threatens America's Future," which was produced by Trust for America's Health and the Robert Wood Johnson Foundation.

Jeffrey Levi, executive director of Trust for America's Health said that stable rates of adult obesity may be a sign that obesity prevention efforts are starting to work, but that the nation's level of obesity remains "extremely high."

"Even if the nation holds steady at the current rates, Baby Boomers-who are aging into obesity-related illnesses-and the rapidly rising numbers of extremely obese Americans are already translating into a cost crisis for the healthcare system and Medicare."

Of the 20 states with the highest obesity rates, only Pennsylvania was not in the South or Midwest. The report also notes that instances of extreme obesity, where a person's body mass index is 40 or higher, have grown exponentially in the last 30 years, from 1.4 percent to 6.3 percent -- a 350 percent increase.

However, there is also some evidence that overall obesity has been slowing in America, the report contends. In 2005, for instance, all but one of the US states saw an increase in obesity rates. In 2008, 38 states reported an increase in obesity rates, lowering to 28 states in 2010. By 2011 obesity rates increased in only 16 states.

"After decades of unrelenting bad news, we're finally seeing signs of progress. In addition to today's news about the steady rates for adults, we've seen childhood obesity rates declining in cities and states that were among the first to adopt a comprehensive approach to obesity prevention," said Risa Lavizzo-Mourey president and CEO of the Robert Wood Johnson Foundation. "But no one should believe the nation's work is done. We've learned a lot in the last decade about how to prevent obesity. Now it's time to take that knowledge to scale."