WASHINGTON — The obesity epidemic may be slowing, but rates are still on the rise.

Today, just over a third of U.S. adults are obese. By 2030, 42 percent will be, according to a forecast released Monday.

That’s not nearly as many as experts had predicted before the once-rapid rises in obesity rates began leveling off. But the new forecast suggests even small continuing increases will add up.

“We still have a very serious problem,” said obesity specialist Dr. William Dietz of the Centers for Disease Control and Prevention.

Worse, the already obese are getting fatter. Severe obesity will double by 2030, when 11 percent of adults will be nearly 100 pounds overweight, or more, concluded the research led by Duke University.

That could be an ominous consequence of childhood obesity. Half of severely obese adults were obese as children, and they put on more pounds as they grew up, Dietz said.

The severely obese are most at risk of developing a host of ailments — and the most expensive to treat. Already, estimates suggest obesity-related problems account for at least 9 percent of the nation’s yearly health spending, or $150 billion a year.

Data presented Monday at a CDC meeting paint something of a mixed picture of the obesity battle. Clearly, the skyrocketing rises in obesity rates of the 1980s and ’90s have ended. But Americans aren’t getting thinner.

Over the past decade, obesity rates stayed about the same in women, while men experienced a small rise, said CDC’s Cynthia Ogden. That increase occurred mostly in higher-income men.

About 17 percent of the nation’s children and teenagers were obese in 2009 and 2010, the latest available data. That’s about the same as at the beginning of the decade, although a closer look by Ogden shows continued small increases in boys, especially African-American boys.

Some larger CDC databases show continued upticks, said Duke University health economist Eric Finkelstein, who led the new CDC-funded forecast. His study used that information along with other factors that influence obesity rates — including food prices, prevalence of fast-food restaurants, unemployment — to come up with what he called “very reasonable estimates” for the next two decades.