There's a big divide between the world's overweight and underweight children

The weight problems that preoccupy Americans typically are about how to lose weight, not gain it. But a study published in the Lancet on Tuesday night provides a sobering look at how much the relationship children globally have with food and weight depends on where they are growing up.

The study reports that the number of obese children has increased more than tenfold in the past four decades – from 5 million girls in 1975 to 50 million in 2016, and from 6 million boys in 1975 to 74 million in 2016.

Overall, one in every five children on the planet is either obese, meaning more than two standard deviations from the median on growth charts, or overweight, meaning more than one standard deviation.

The analysis, led by Imperial College London in collaboration with the World Health Organization, involves data on nearly 129 million children ages 5 to 19 in 200 countries. Study author Majid Ezzati, a researcher at the college's School of Public Health, and his collaborators say it is the most comprehensive database ever assembled on this topic.

But there's a flip side to this story.

Despite the big increases in obesity, there are still more children who remain moderately or severely underweight, especially in the poorest corners of the world. An estimated 75 million girls and 117 million boys are moderately or severely underweight, meaning greater than one standard deviation from median on the WHO charts. Almost two-thirds of these children live in South Asia, where some governments' ability to feed their citizens has been unable to keep up with countries' booming populations.

"We have wide and widening inequalities. Even though we may see some signs of improvement, we cannot be complacent, and we need to ramp up our actions much more significantly to act across the life-course and across all of society," said Harry Rutter, a researcher at the London School of Hygiene and Tropical Medicine.

Rutter and the WHO's Fiona Bull, another co-author, said on a call with reporters that the solution lies in not only targeting individual behavior – including the quantity and quality of meals and physical exercise – but in looking more broadly at agricultural policies as well as the marketing, packaging, pricing and availability of food.

While being overweight is associated with earlier onset of cardiac and metabolic issues and some cancers, being underweight also can carry serious consequences. It is associated with a higher risk of infectious disease and linked to cognitive issues. In girls, it can create problems with pregnancy, including preterm birth and maternal death.

Most highly developed countries have a significant percentage of overweight children, but the same trend is accelerating in middle-income countries, especially in Southeast Asia. Large increases also are being seen in North Africa, Latin America and the Caribbean.

"While it's happening also in the high-income world, especially in Europe and certainly in Japan and some of its neighbors like South Korea, it has been a slower process," Ezzati said. Overall, obesity and overweight rates are plateauing in those places and the United States – where 20 percent of children are obese.

The percentage of boys and girls who are underweight correlates with poverty. Starting in the late 1970s and continuing through the 1980s, much of sub-Saharan Africa, South Asia and parts of Latin America were affected. "As the clock goes forward, it becomes more and more concentrated in South Asia and in the poorest pockets of Africa," Ezzati said.

These days, the issue is very regionally concentrated – part of what one researcher called "this polarization of the world." In 2016, the average body mass index for both boys and girls was the lowest in Ethiopia but also low in Niger, Senegal, Myanmar, Cambodia, Bangladesh and India. In many parts of Africa and India, 30 percent to 50 percent of boys were underweight.