For the first time, children with obesity outnumber those who are underweight across the globe, according to a new report on child nutrition from UNICEF, a United Nations agency.

It’s a dramatic but unsurprising milestone, said Johanna Ralston, the chief executive of the World Obesity Federation. Since 2000, the share of underweight children has dropped to 9.2 percent from 13 percent, while global childhood obesity rates have climbed. One in 10 children now has obesity, and one in five is overweight.

While Ms. Ralston hopes the new report will galvanize action at the upcoming U.N. General Assembly, multinational organizations and governments around the world are “phenomenally underprepared” to tackle the childhood obesity crisis, she said.

Part of the problem is a longstanding and false assumption that poor countries struggle only with hunger, while rich countries face obesity alone. But since 2000, the number of overweight children has more than doubled in low- and middle-income countries, compared with a 20 percent increase in high-income countries. In 2022, low- and middle-income countries accounted for 81 percent of overweight children, according to the report.

The risks of obesity tend to build over time, since the disease is tied to more than 200 other conditions such as Type 2 diabetes, heart disease and various forms of cancer. Childhood obesity can be a gateway to decades of poor health and even early death, said Dr. Sriram Machineni, an obesity medicine specialist at Montefiore Einstein Medical Center in the Bronx.

Sedentary lifestyles, chronic stress and genetic risk contribute to rising childhood obesity rates. But the report specifically highlights the global shift in “food environments,” as cheap, ultraprocessed foods, pushed by relentless advertising, have flooded children’s lives. These foods tend to be calorie-dense, with combinations of sugar, salt and fat rarely found in nature, making them almost addictive and easy to overconsume, Dr. Machineni said.