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Childhood obesity continues to rise around the world, and the World Health Organization has called it “one of the most serious public health challenges of the 21st century.”

Yet the prevalence of childhood obesity appears to vary across countries.

Island nations in the Pacific, such as Nauru and the Cook Islands, appear to have the highest obesity rates among children 5 to 19, but the countries Ethiopia and Burkina Faso appear to have the lowest rates.

“There are still more children that are underweight in the world than there are obese, but that’s likely to change pretty soon,” said Tiago Barreira, an assistant professor in the Department of Exercise Science at Syracuse University in New York who has studied childhood obesity on a global scale.

The prevalence of child and adolescent obesity is expected to surpass the prevalence of moderate and severe underweight by 2022, according to a study published in the journal The Lancet in 2017.

The study estimated that in 1975, there were 11 million children 5 to 19 with obesity, and that number increased to 124 million in 2016.

The number of obese or overweight children 5 and younger climbed from 32 million globally in 1990 to 41 million in 2016, according to WHO data. If current trends continue, the number of overweight or obese children in that age group could increase to 70 million by 2025.

Where childhood obesity is most and least prevalent

The highest prevalence of obesity in children 5 to 9 is in the Pacific Islands, at around 30% for both boys and girls, said Juana Willumsen, an expert in WHO’s Department of the Prevention of Noncommunicable Diseases.

Among children 5 to 9, Nauru appeared to have the highest obesity rate at 36.3% in 2016, according to WHO data.

Based on that 2016 data, Nauru is followed by the Cook Islands at 36.1%, Palau at 35.5%, Niue at 33.3%, the Marshall Islands at 31.2%, Tuvalu at 31.1%, Tonga at 30.2%, Kiribati at 27.5%, Micronesia at 25.2% and Samoa at 24.9%.

Among children 10 to 19, Nauru still appears to have the highest obesity rate at 31.7%, followed by the Cook Islands at 30.3%, Palau at 29.4%, Niue at 27.6%, Tuvalu at 25.3%, Tonga at 24.9% and the Marshall Islands at 24.4%, according to WHO data from 2016.

“However, these are small countries,” Willumsen wrote in an email. “The next highest is Kuwait.” That Middle East country appears to have an obesity rate of 23.1% among children 5 to 9 and 22.8% among children 10 to 19, based on that WHO data from 2016.

On the other hand, “a number of countries have childhood obesity prevalence below 1% for boys,” Willumsen said, including Uganda and Rwanda among ages 5 to 9 and Niger, Burkina Faso and Ethiopia among ages 10 to 19, based on WHO data from 2016.

Several countries also have childhood obesity rates below 2% for girls, Willumsen said, including Cambodia and Burkina Faso among ages 5 to 19, according to 2016 data.

In the United States, the prevalence of childhood obesity was 18.5%, and it affected about 13.7 million children and adolescents in 2015 and 2016, according to the US Centers for Disease Control and Prevention.

A report from the Robert Wood Johnson Foundation, released last year, showed significant state-by-state differences in obesity rates among children 10 to 17. The report, based on combined data from 2016 and 2017, revealed that Mississippi had the highest childhood obesity rate at 26.1% for that time, and Utah had the lowest at 8.7%.

Across Europe, there seems to be similar rates of and differences in childhood obesity prevalence. A WHO report last year showed that of 34 countries in the European region, Cyprus, Greece, Italy, Malta, San Marino and Spain had the highest rates of childhood obesity. In these countries, about 1 in 5 boys was obese, and rates of obesity among girls were only slightly lower.

Denmark, France, Ireland, Latvia and Norway were among the countries with the lowest rates, ranging from 5% to 9% in either boys or girls, according to WHO. Those findings were based on 2015-17 data among children ages 6 to 9 from the WHO Childhood Obesity Surveillance Initiative.

‘The definition of obesity for children is not universal’

Obesity is defined by body mass index or BMI, which is a measure of the relationship between weight and height. Most health groups – including WHO – measure the condition in adults as when their body mass index is 30 or higher.

Yet in children, identifying and measuring obesity can be difficult, as their BMI drastically changes as they grow and standards around those measurements change with growth.

“The definition of obesity for children is not universal like the adults’,” Barreira said.

Also, “global data is a little problematic because the methods to collect data – self-reported or measured – and to analyze data differ. So some of the information that we have is not identical depending on who is reporting,” he said. “For children – depending on who is reporting – the World Health Organization has a standard, the CDC has another standard, so obesity rates vary.”

The CDC, for instance, defines childhood obesity as having a body mass index at or above the 95th percentile of a child’s age group in the CDC’s sex-specific growth chart.

Meanwhile, the WHO defines childhood obesity according to the WHO growth reference for school-age children and teens, so a body mass index that is two standard deviations above the average for a child’s age group and sex would be considered obese.

Overall, most health groups agree that there are several risk factors for childhood obesity, including eating high-calorie and low-nutrient foods and beverages; not getting enough exercise; sitting too much, such as watching television or other screen devices; medication use; and getting inadequate sleep.

WHO has noted that “replacing traditional foods with imported, processed food has contributed to the high prevalence of obesity and related health problems in the Pacific islands.”

Where a child lives can influence some of those risk factors, but overall, the link between risk factors and obesity has been well-established, Barreira said.

When it comes to exercise, a study published in the journal Obesity in 2017 found that across 12 countries – Australia, Brazil, Canada, China, Colombia, Finland, India, Kenya, Portugal, South Africa, the United Kingdom and the United States – physical activity was a stronger predictor for childhood obesity than how much a child weighed when born.

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“The main thing that we found was that there was a relationship between physical activity level – especially moderate to vigorous physical activity – and obesity in all those different places,” said Barreira, who was involved in the study. “So that relationship exists everywhere.”