More than four in 10 American women are now classified as obese in an alarming new milestone for the nation.

Obesity rates for men and women in the US had been roughly the same for about a decade. But in recent years, women have surged ahead and now just over 40% of women are obese, compared to 35% of men.

The percentages were reported by the Centers for Disease Control and Prevention, in two articles published online on Tuesday by the Journal of the American Medical Association. The numbers are based on a small government survey that is considered the best measure of the nation’s obesity problem.

Though it is not altogether surprising to health researchers because the nation has long been growing more obese, it is “scary” that the statistic has reached 40% for women, said Dana Hunnes, a dietitian who sees obese patients at Ronald Reagan UCLA Medical Center in Los Angeles.

“It’s a really alarming figure, and it’s alarming that it’s continuing to go up despite government calls to action on weight loss and healthy eating,” she said.

Why the problem is getting worse for women faster than for men remains somewhat of a mystery to health researchers. “I don’t know if anyone truly knows for sure,” Hunnes said. Experts say there are a range of possible explanations, including that many women are satisfied with a larger body size.

The rate of obesity in women is also higher than in men across the world, although far lower overall than in the US. According to the World Health Organization, 15% of women worldwide and 11% of men are obese.

Obesity, which means not merely overweight, but seriously overweight, is considered one of the nation’s leading public health problems because it can trigger diabetes and lead to heart disease and other serious health problems. Until the early 1980s, only about one in six adults were obese.

The problem is not increasing as dramatically as it was, but the new numbers show it is clearly not improving, said Dr Felipe Lobelo, an Emory University researcher who focuses on obesity and physical activity.

Researchers looked at obesity rates among different age groups and along racial lines and found wide disparities, mainly ones that have persisted for years.

Obesity continues to be most common among black women. About 57% of black women are obese. In contrast, about 47% of Hispanic women, 38% of white women, and 12% of Asian women are obese.

Among men of different races, obesity rates cluster much closer together — at 35 to 38% for blacks, Hispanics, and whites.