JACKSON (AP) — Mississippi is America’s fattest state for the third year in a row, and adult obesity rates across the nation are climbing steadily despite initiatives to combat the epidemic, a new report says.

Nationally, adult obesity rates rose in 37 states in the past year, according to the report released Tuesday by Trust for America’s Health and the Robert Wood Johnson Foundation.

In 18 of those states, including Mississippi, it’s the third consecutive increase, which researchers say shows that some wellness policies being implemented on the state level aren’t working. They say a national strategy is needed.

The report recommends the federal government, industries and local organizations invest in community-based disease prevention programs and improve access to affordable nutritious foods by providing incentives for grocery stores and farmers’ markets to operate in poor communities.

‘‘There’s a responsibility in all parts of society to really take on this issue,’’ said Jeff Levi, executive director of the nonprofit Trust for America’s Health. ‘‘Some states and communities have taken positive steps but overall we are not treating the obesity epidemic with the seriousness it deserves.’’

The report was based on data gathered by the U.S. Centers for Disease Control. The CDC sampled people in all states and calculated their body mass index from their height and weight. A 5-foot, 9-inch adult who weighs 203 pounds would have a BMI of 30, which is considered the threshold for obesity.

The report said Mississippi’s adult obesity rate is 31.7 percent — the nation’s highest. It was followed by West Virginia with 30.6 percent; Alabama with 30.1 percent; Louisiana with 29.5 percent and South Carolina with 29.2 percent.

Colorado was the leanest state with a rate of 18.4 percent, followed by Hawaii with 20.7 percent and Connecticut with 20.8 percent.

There was no decrease in obesity in any state. Of the 10 states with the highest rates, nine were in the South. Seven of the top 10 have the nation’s highest unemployment rates. That’s not unusual as obesity is more prevalent among the poor who often buy cheaper but calorie-dense processed foods.

Pete Smith, spokesman for Mississippi Gov. Haley Barbour, said the state has taken steps to encourage residents to live a healthier lifestyle, including requiring students to exercise 30 minutes each day and promoting a statewide walking campaign.

Smith said the governor knows the strain obesity places on the economy ‘‘in terms of rising healthcare costs and loss of productivity in the work place.’’

The report said Arkansas, Kentucky and Tennessee are among the states that tax junk food. And California recently became the first state in the nation to bar artery-clogging trans fats from restaurant foods.

But Levi said there’s a lag time between the implementation of policies and a change in the statistics. Another problem is enforcement. For instance, 50 states have legislation related to physical education in schools, but only 13 have enforcement language.

‘‘There needs to be a sense of urgency and outright alarm,’’ said James Marks, senior vice president of the Robert Wood Johnson Foundation. ‘‘This is the fifth ’F As In Fat’ report and each year we see more evidence that our nation’s obesity epidemic continues to gain speed and destructive force.’’