SYDNEY, Oct 11 - Global analysis of 200 countries published in The Lancet shows the number of children and adolescents with obesity has ballooned in the past four decades, increasing from five million girls in 1975 to 50 million in 2016, and from six million to 74 million boys.

Rates of child and adolescent obesity were highest - above 30 per cent - in some islands in Polynesia and were around 20 per cent higher in the USA.

"Rates of child and adolescent obesity have increased significantly over the past four decades in most countries in the world," says study author Dr James Bentham, University of Kent.

In some places, including Australia, New Zealand and North America the average body mass index had recently plateaued, however the authors warned against complacency.

The authors call for governments to make healthy food more affordable.

"While there have been some initiatives led by governments, communities or or schools to increase awareness about childhood and adolescent obesity, most high income countries have been reluctant to use taxes and industry regulations to change eating and drinking behaviours to tackle child obesity," said Professor Majid Ezzati, study author from Imperial College London.

"Most importantly, very few policies and programs attempt to make healthy foods such as whole grains and fresh fruits and vegetables affordable to poor families. 

"Unaffordability of healthy food options to the poor can lead to social inequalities in obesity, and limit how much we can reduce its burden."

Despite the rise in childhood obesity rates, there are still more children and adolescents who remain underweight. 

Almost two thirds of the world's children either moderately or severely underweight live in south Asia.