(CNN) The Mediterranean diet is well-known for its positive effects on cardiovascular and metabolic health. But according to new data from the World Health Organization, childhood obesity rates in the Mediterranean region are among the highest in the world.

The new WHO report , presented Wednesday at the European Congress on Obesity in Vienna, indicated that of 34 countries in the European region, the countries of Cyprus, Greece, Italy, Malta, San Marino and Spain had the highest rates of childhood obesity. In these countries, approximately one in five boys was obese (18% to 21% of boys). Rates of obesity among girls were only slightly lower.

Childhood obesity is more prevalent in this region than in the United States, where approximately 17% of children are obese, according to the US Centers for Disease Control and Prevention

"Obesity in the majority of the member states affects 1-2 in every 10 children. The problem is more serious among southern European countries," Dr. João Breda , head of the WHO European Office for Prevention and Control of Noncommunicable Diseases, which authored the report, wrote in an email.

"We believe it is due to the loss of the traditional Mediterranean diet patterns in the south [and] to the increased intake of sugars and energy dense foods combined with particularly low levels of physical activity," Breda added.

The Mediterranean diet is typically characterized by a high intake of plant-based foods and olive oil; a moderate intake of fish and poultry; a low intake of dairy products, red meat and sweets; and a moderate intake of wine, according to a 2018 WHO report

Exhaustive evidence suggests that adherence to a Mediterranean diet can have prolonged benefits on cardiovascular and metabolic health. A 2011 study , for example, showed that adherence to the Mediterranean diet for four years reduced the incidence of Type 2 diabetes by 52% in older adults.

Childhood obesity was over twice as prevalent in southern European countries than in northern European countries such as Denmark, Ireland and Norway, where rates of obesity in boys and girls ranged between 5% and 9%, according to the report.

The latest data come from the WHO Childhood Obesity Surveillance Initiative, which has tracked obesity and overweight prevalence among European children ages 6 to 9 for the past 10 years. For the new report, height and weight measurements of approximately 250,000 children from 34 countries were collected between 2015 and 2017.

Some larger nations -- such as the United Kingdom and Germany -- did not partake in the study but have their own surveillance systems to track overweight and obesity prevalence in children.

"This is a unique initiative in global terms and the biggest worldwide with such huge datasets," Breda said. "Given its dimension and critical mass, we believe this study brings significant insights into the global picture."

The increased prevalence of childhood obesity in southern Europe compared with the rest of the continent is probably due to a combination of dietary and lifestyle changes, according to Dr. Bruce Y. Lee , executive director of the Johns Hopkins Global Obesity Prevention Center, who was not involved in the WHO report.

"It's a combination of factors," Lee said. "There are changes in the food system -- more artificial foods, more processed foods, foods with additives, salt, sugar, et cetera -- and that's an issue worldwide."

"There's also decreasing physical activity that's occurring. We're currently in the middle of a physical inactivity epidemic, or pandemic, because people have more sedentary lives," Lee added.

San Marino, which had the fifth highest rate of childhood obesity in the new report, paradoxically also has the fifth highest average life expectancy of any country in the world, according to the CIA World Factbook . The average life expectancy there in 2017 was 83.3 years.

"One of the issues is of course that the Mediterranean diet was part of a lifestyle with lots of spontaneous physical activity -- [that is] why it always had a generous amount of calories," Breda said.

"In recovering the Mediterranean diet, we must ensure the provision of calories is adjusted for modern life and physical activity levels are increased -- basically adjusting and recovering the Mediterranean diet to the 21st century," he added.

Changes to diet and lifestyle in the Mediterranean region may also be the result of increased urbanization, according to Aviva Must , professor and chair of public health and community medicine at Tufts University School of Medicine, who was not involved in the new report.

"The Mediterranean diet is under threat by urbanization and globalization," Must wrote in an email. "Traditional southern European food habits, particularly for children, are changing, with more processed foods, more animal sources, less fiber and more calories overall."

Breda says the new report does not mean the Mediterranean diet is less healthy than previously believed. However, the results suggest that, in many parts of the Mediterranean region, many people's diets and lifestyles are no longer what they once were, he said.

"The Mediterranean diet in children of the Mediterranean countries basin is a myth -- it is gone unfortunately," Breda said.

"But the good news is that there are strong movements and initiatives in these countries to recover it. We hope these are successful and the principles of this traditional diet will be available and become the default for all in years to come in this part of Europe," he added.

Changes in diet may also be linked to increased rates of poverty in some southern European countries. Many of the countries with the highest rates of childhood obesity have among the highest rates of poverty in Europe, according to the CIA Factbook 2011-2015

In Greece, for example, nearly 36% of the population lives below the poverty line -- the highest rate on the continent.

"In times when childhood obesity has risen, there has been a disproportionate rise among lower income neighborhoods and lower income populations," Lee said. "Many low income populations have poor access to healthy foods, because if you think about it, living a healthy lifestyle can cost money."

Childhood obesity increases the risk of a number of adverse health outcomes in adulthood. Studies suggest that a child with obesity is five to seven times more likely to be obese as an adult than children who are not obese, according to Breda.

"Childhood obesity increases the risk of adult obesity, diabetes and cardiovascular disease later in life," Breda said. "In many countries, obesity affects more children in underprivileged groups and adds to the striking health inequalities reinforcing a vicious cycle of poverty and noncommunicable diseases."

Obesity also has negative effects on children's physical and mental health before adulthood, according to Lee.

"We're now seeing chronic diseases that were once thought of as adult diseases affecting children. So now we're seeing Type 2 diabetes in children, where typically when you talk about diabetes in children, you used to think about Type 1 diabetes," Lee said.

Despite the high rates of childhood obesity in much of southern Europe, the prevalence has leveled off or slightly decreased in many of these countries since the previous WHO report, according to Breda.

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"The decline has been estimated from 2-7% in countries like Spain, Italy, Portugal and Greece," Breda said. "We need more studies and more rounds to be sure about the size of the decrease and we are already preparing for the 5th round next year."

The new data could be good news, then, for those working to tackle the increasing rates of childhood obesity in the region.

"The study clearly highlights the importance of addressing the problem of childhood obesity in a comprehensive and systematic fashion using educational and regulatory tools," Breda said. "But it also conveys a strong sign of hope that if we do the right things and implement powerful solutions, the problem can be solved."