The world’s highest levels of childhood obesity are on the Pacific islands of Polynesia and Micronesia © AFP

The number of obese children and teenagers across the world has increased 10-fold over the past four decades and is about to overtake the number who are underweight, according to the most extensive analysis of body weight ever undertaken.

The study, led by Imperial College London and the World Health Organization, used data on 31.5m children and adolescents worldwide to estimate trends in body mass index (BMI) from 1975 to 2016. The results are published in the Lancet

Over this period the number of obese girls, aged 5 to 19, rose from 5m to 50m, while the total for boys increased from 6m to 74m.

The definition of child obesity is relative and depends on age, with obese children exceeding the healthy standard by twice the amount that weight normally varies around the average. An average 7-year-old girl who is 125cm tall would have to weigh more than 31kg to be obese, for example.

The world’s highest childhood obesity levels are in the Pacific islands of Polynesia and Micronesia. Nauru has the highest prevalence for girls and the Cook Islands for boys: both above 33 per cent.

Among wealthy countries, the US has the highest obesity rates for girls and boys of about 20 per cent. Levels in most of western Europe are in the 7 per cent to 10 per cent range. 

A further 213m children are overweight but not sufficiently so to meet the WHO’s obesity criteria, which vary by age. Forty years ago, 0.8 per cent of the world’s children were obese; now the prevalence is close to 7 per cent. 

“Over the past four decades, obesity rates in children and adolescents have soared globally, and they continue to do so in low- and middle-income countries,” said Majid Ezzati of Imperial, the project leader. “More recently, they have plateaued in higher-income countries, although obesity levels remain unacceptably high here.” 

The study also looked at adult obesity, which increased from 100m people in 1975 to 671m in 2016. A further 1.3bn adults were overweight (with a BMI above 25) but below the threshold for obesity (BMI above 30). 

But the authors are most concerned about the findings about childhood obesity, because of their implications for public health many decades into the future.

Excessive weight “is setting up children for a lifetime of ill health that is very hard to reverse”, said Harry Rutter of the London School of Hygiene and Tropical Medicine. It is associated with a higher risk and earlier onset of many chronic diseases such as type-2 diabetes and heart disease. 

Michael Bloomberg, who tried to ban the sale of large sugary drinks when he was mayor of New York and is now an ambassador for the WHO, said the report was a reminder “both of the scope of the crisis and the need for a more robust and urgent response”. He added: “Anti-obesity policies like sugary drinks taxes are working and the faster we spread them, the more lives we can save.” 

More than 20 countries around the world have introduced sugary drink taxes, said Fiona Bull of the WHO, “and the evidence shows that they are beginning to work”. 

The authors express particular alarm at accelerating childhood obesity levels in rapidly urbanising middle-income countries. This begins among wealthier members of society and then spreads to poorer families.

Eventually, in high-income countries, a strong inverse correlation emerged between wealth and obesity, said Prof Rutter. In the UK, obesity levels are beginning to decline in children from wealthy backgrounds — and are now well below those in deprived families. “In the high-income world it is impossible to solve the obesity problem without solving it in the poor,” he said. 

Although the study focuses on obesity, the researchers point out that 75m girls and 117m boys are still moderately or severely underweight. Two-thirds of these children who do not receive enough food are in south Asia and especially in India. In south Asia, 20.3 per cent of girls and 28.6 per cent of boys are moderately or severely underweight — an improvement on 1975 when the comparable figures were 23.0 per cent and 37.8 per cent, respectively. 

The nutritional world was increasingly polarised between overweight and underweight, said Prof Bull, as the proportion of the world’s children who were a normal healthy weight fell. 

“There is a continued need for policies that enhance food security in low-income countries and households, especially in south Asia,” said Prof Ezzati. “But our data also show that the [change] from underweight to overweight and obesity can happen quickly in an unhealthy nutritional transition, with an increase in nutrient-poor, energy-dense foods.”

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