In addition, severe obesity - and the serious health problems and extra healthcare costs associated with it - will disproportionately affect women, low-income adults, non-Hispanic black adults and states bordering the lower half of the Mississippi River.
The study projects from past trends, extending them to a state-by-state level and adjusting for the fact that people who respond to surveys on the obesity question often underestimate their weight and overestimate their height, according to chief author Zachary Ward, a PhD candidate at the Harvard T.H. Chan School of Public Health in Boston.
"Obesity is getting worse in every state and especially concerning is severe obesity, which used to be pretty rare and now will be the most common category for a lot of states and subgroups across the country," he told Reuters Health in a telephone interview.
A person with a body-mass index (BMI, a measure of weight relative to height) of 30 or above is considered obese. Having a BMI at or above 35 is regarded as severe obesity.
The average adult male in the U.S. who stands at 5' 9" is obese if he weighs more than 202 pounds and severely obese at more than 236 pounds. The comparable thresholds for obesity and severe obesity for the average U.S. female, at 5' 4," are 174 and 203 pounds. Many online BMI calculators are available, including one from the National Heart, Lung and Blood Institute (here: https://bit.ly/2PySiOB).
The Ward study, published in The New England Journal of Medicine, used more than 20 years of data from 6.3 million adults to project into 2030.
The team forecasts that rates of severe obesity will be particularly pronounced among women (with a rate of 27.6%) and among both non-Hispanic black adults and low-income adults (at rates of 31.7% each).
The state with the highest rate of obesity, the researchers projected, will be Oklahoma, at 58.4%, with Arkansas, Alabama and Mississippi all tied for second place at 58.2%.
The state with the lowest obesity rate will be Colorado, which "has always been one of the best states," Ward said. "People do a lot of outdoors stuff there. Income may be higher. Also a lot of people live at a higher elevation, and there may be something about having to take a little extra energy to do everything at a higher altitude" that keeps weights lower, he noted.
Nonetheless, 38.2% of residents there are expected to be obese by 2030. California and Massachusetts will rank 2nd and 3rd in having the lowest percentage.
The state with the highest rate of severe obesity, the researchers project, will be Arkansas, at 29.6%, with Oklahoma, West Virginia and Louisiana coming in just behind.
Colorado will have the lowest rate in this category at 14.3%. California, Hawaii and New York will rank 2nd, 3rd and 4th in having the lowest proportion of severely obese people by 2030, the researchers calculate.
The margin of error in the estimates is about plus or minus 2 percentage points.
"We hope this will help state and city policymakers see where they're heading because some states are talking about introducing things like a sugar-sweetened beverage tax" to try to address the obesity problem, and the new data may assist in such decisions, Ward said.
The study didn't look at the reasons behind the trends, but "income is a big driver. Age is a big driver. So, where there's more young people in a state, obesity prevalence tends to be lower because people tend to gain weight as they age," Ward said.