A regular sized wheelchair (L) is pictured alongside an oversized one in the children's and women's maternity ward at the University of Alabama Hospital in Birmingham, Alabama, April 26, 2012. (Reuters) (MARVIN GENTRY/REUTERS)

In 2030, 42 percent of American adults will be obese, and about one-quarter of that group will be severely obese, a condition that shortens life and incurs large medical expenses, a new study predicts.

This view into the future is less ominous than one published four years ago that predicted that 51 percent of the population would be obese in 2030. Nevertheless, the trend fortells a huge drag on the health and economic welfare of the United States.

“If we don’t do anything, this is going to really hinder any efforts to contain future health-care costs,” Justin G. Trogdon, an economist and one of the authors of the projection, told experts Monday at the start of the two-day “Weight of the Nation” conference in Washington.

However, if obesity stays at its current prevalence — 34 percent of adults — and does not increase, the savings in projected health-care costs will be considerable, about $550 billion, the authors said. The most recent evidence, in fact, suggests that obesity rates are plateauing.

“Regardless which is correct, we still have a very serious problem,” William H. Dietz, head of the Centers for Disease Control and Prevention’s obesity program, said of the scenarios.

Obesity related ailments — diabetes, heart disease, kidney failure — consume at least 9 percent of health-care spending the United States. Some researchers believe the cost may be twice that estimate. Total health spending is about $2.6 trillion a year.

The new study, published in the American Journal of Preventive Medicine, used obesity prevalence data from 1990 through 2008 to extrapolate future trends. The information came from the Behavioral Risk Factor Surveillance System, a federally funded telephone survey. People underestimate their weight when asked on the phone; that fact was compensated for in the mathematical model.

The researchers also incorporated variables, measured in each state, that affect obesity rates. These included the price of gasoline, which discourages walking when it is low; access to the Internet (and other technologies), which encourages sedentary behavior as it increases; and restaurants per 10,000 people, which increases eating out and weight gain when the number goes up.

In 2030, 42 percent of people are projected to be obese, and 11 percent severely obese. Obesity is a body mass index (BMI) of 30 or more, which is 186 pounds for someone 5 feet, 6 inches tall. Severe obesity is a BMI of 40 or more — 248 pounds for someone that height.

Cynthia L. Ogden, an epidemiologist at the CDC, told the conference that, in general, obesity rates changed little in the 1960s and 1970s, rose steeply in the 1980s and 1990s, and have been leveling off in the past decade.

For men, obesity prevalence doubled but has changed little in the past eight years, with no difference between blacks, whites and Mexican Americans (which are the three groups for which there are good data). For white women, the obesity prevalence has not changed in 12 years. It has risen slightly in black women and Mexican American women, although that increase mostly occurred early in that 12-year period.

There are some exceptions to this general picture of stability.

Obesity is rising in higher-income men. Severe obesity is increasing in both sexes. It was 6.2 percent in women in 1999 and 8.1 percent in 2010. For men, it was 3.1 percent in 1999 and 4.4 percent in 2010.

Eric A. Finkelstein, a researcher at Duke University who led the new study, said that just in the past 50 years has it been possible for millions of people to be both sufficiently inactive and to have access to enough food to become severely obese.

“The world has changed in ways that allow people to be that overweight,” he said.

The reason for the plateauing of the obesity prevalence is uncertain. It almost certainly reflects many factors, including an approach to a natural limit of the epidemic and the success of efforts to fight it by encouraging exercise and educating people about better eating habits.