Hawaii’s 2011 adult obesity rate is the second lowest in the nation, but that could change dramatically for the worse in 20 years, according to a report released Tuesday by two public health groups.

Hawaii ranked 50th with a 2011 obesity rate of 21.8 percent, according to the report titled "F as in Fat: How Obesity Threatens America’s Future 2012," by the Trust for America’s Health and the Robert Wood Johnson Foundation. The state with the lowest rate was Colo­rado at 20.7 percent. The District of Columbia is also ranked.

But the number of obese adults has grown dramatically in Hawaii over the past 15 years and is expected to grow even more in the next 20 years, the report said. The groups warned Hawaii’s obesity rate is projected to climb to 51.8 percent in 2030 if the Aloha State continues on its current trajectory. In 1995, Hawaii’s obesity rate was 10.6 percent.

"By 2030, obesity-related health care costs in Hawaii could climb by more than 12.3 percent, which could be the 38th highest increase in the country," the report said. "In the United States, medical costs associated with treating preventable obesity-related diseases are estimated to increase by $48 billion to $66 billion per year by 2030."

This is the first time the annual report forecasts 2030 adult obesity rates.

If body mass index would be reduced by 5 percent, Hawaii’s rate in 2030 would instead be 45.5 percent, the report said.

Hawaii has one of the lowest obesity rates in the nation for 10- to 17-year-olds. Hawaii tied for 46th with Iowa at 11.2 percent.

Mississippi is expected to retain its crown as the fattest state in the nation for at least two more decades. The report predicts 67 percent of that state’s adults will be obese by 2030, compared with Mississippi’s current 35 percent obesity rate.

Trust for America’s Health’s dismal forecast goes beyond the 42 percent national obesity level that federal health officials project by 2030. The group predicts every state would have rates above 44 percent by that time, although it didn’t calculate an overall national average.

About two-thirds of Americans are overweight now. That includes those who are obese, a group that accounts for about 36 percent. Obesity rates have been holding steady in recent years. Obesity is defined as having a body-mass index of 30 or more, a measure of weight for height.

Trust for America’s Health officials said their projections are based in part on state-by-state surveys by the Centers for Disease Control and Prevention from 1999 through 2010. The phone surveys ask residents to self-report their height and weight; people aren’t always accurate about that.

The researchers then looked at other national data tracking residents’ weight and measurements and made adjustments for how much people in each state might fudge the truth about their weight. They also tried to apply recent trends in obesity rates, along with other factors, to make the predictions.

Trust for America projects that by 2030, 13 states would have adult obesity rates above 60 percent, 39 states might have rates above 50 percent and every state would have rates above 44 percent.

Even in the thinnest state — Colo­rado, where about one-fifth of residents are obese — 45 percent would be obese by 2030.

The report didn’t detail why some states’ rates were expected to jump more than others’. It also didn’t calculate an average adult obesity rate for the entire nation in 2030, as the CDC did a few months ago. But a researcher who worked on the Trust for America’s Health study acknowledged that report’s numbers point toward a figure close to 50 percent.