People exercise as part of treatment at the Aimin Fat Reduction Hospital in Tianjin, China.

The world is on an eating binge.

According to a new study, body mass index levels are spiking worldwide — so much so that obese people now outnumber the underweight population for perhaps the first time in global history.

And there’s no silver lining: The underweight population may have dropped, but it’s a slow decline compared to the rapid rise in obesity and the related health problems that come with excessive body fat.

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“Both sides of this are worrisome,” said Majid Ezzati, an environmental health researcher at Imperial College London who led the study published Thursday in the Lancet.

Ezzati and his colleagues analyzed data from 1975 to 2014 across 19.2 million adults from 186 countries. Over the 40-year span, they found, the proportion of obese men worldwide more than tripled, to roughly 11 percent, and the proportion of obese women more than doubled, to about 15 percent.

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At the current pace, the researchers estimate, 18 percent of men and 21 percent of women worldwide will be obese by 2025.

The study also showed that over the same four decades, the proportion of underweight men dropped from around 14 percent to 9 percent, and the proportion of underweight women fell by a similar amount, from 15 percent to 10 percent.

Ezzati said that the numbers of underweight and overweight women reached a rough equilibrium around 2004, and for men the crossover happened in 2011.

As a group, high-income, English-speaking countries had the highest average BMI — and the United States led that group. More than 25 percent of the world’s severely obese men and almost 20 percent of the world’s severely obese women are American.

But it’s the impact of the obesity epidemic on developing nations that’s especially troubling to Eric Feigl-Ding, an epidemiologist and health economist at the Harvard T.H. Chan School of Public Health.

“People always think obesity is a first-world problem, but it’s becoming just as bad, if not worse, in developing areas,” Feigl-Ding said. “In America, you can ameliorate your diet or blood sugar, or take cholesterol medicine, but in these developing countries, once things get bad, the mortality rates can’t be checked.”

The world’s highest BMI populations are found in the island nations of Polynesia and Micronesia, where, for the region as a whole, more than 38 percent of men and more than half of women are obese.