Researchers raised the alarm Wednesday about an obesity explosion among children in rural China as a Western-style diet high in sugar and carbohydrates starts taking its toll.

A 29-year survey of kids in China's eastern Shandong province revealed that 17 percent of boys younger than 19 were obese in 2014, and nine percent of girls -- up from under one percent for both genders in 1985.

"This is extremely worrying," the European Society of Cardiology's Joep Perk said of the study published in the European Journal of Preventive Cardiology.

"It is the worst explosion of childhood and adolescent obesity that I have ever seen."

The data comes from six government surveys of some 28,000 rural school children (aged 7-18) in Shandong.

The percentage of overweight boys had grown from 0.7 percent to 16.4 percent, and girls from 1.5 percent to nearly 14 percent, it found.

The study used different measures of Body Mass Index (BMI) for overweight and obesity than the World Health Organisation (WHO) standard.

BMI is a ratio of weight-to-height squared.

For the UN's health body, a BMI of 25-29.9 is classified as overweight, and from 30 upwards obese. The study authors used a stricter cut-off of 24-27.9 for overweight, and 28 and above for obese.

This means it would be difficult to compare the numbers to other countries, but does not invalidate the fattening trend observed within China itself, said Perk.

"China has experienced rapid socioeconomic and nutritional changes in the past 30 years," study co-author Ying-Xiu Zhang of the Shandong Centre for Disease Control and Prevention said in a statement.

"In China today, people eat more and are less physically active than they were in the past. The traditional Chinese diet has shifted towards one that is high in fat and calories and low in fibre."

- Fat will 'cost lives' -

The WHO says being overweight is a major risk factor for heart disease and stroke, diabetes, and some cancers.

"China is set for an escalation of cardiovascular disease and diabetes, and the popularity of the Western lifestyle will cost lives," said Perk.

The study found that the trend was growing faster in children aged seven-12 than in adolescents.

And they speculated that the higher prevalence among boys could be the result of a "societal preference" for males which "could result in boys enjoying more of the family's resources".

A 2005 National Youth Risk Behaviour Surveillance had found that 4.3 percent of boys and 2.7 percent of girls frequently enjoyed soft drinks. Nearly 13 percent of boys and 4.3 percent of girls spent more than two hours per day playing computer games.

"The adoption of Western foods, notably American junk food high in calories and sugary drinks, is the cause of this phenomenon," observed French obesity expert David Nocca.

The authors warned the findings had implications for the entire nation, with almost half of its 1.36 billion population living in rural areas in 2014.

"The rises in overweight and obesity coincide with increasing incomes in rural households and we expect this trend to continue in the coming decades in Shandong province and other regions of China," said Zhang.

"This is a wake-up call for policymakers that rural China should not be neglected in obesity interventions. We need to educate children on healthy eating and physical activity, and monitor their weight to check if these efforts are making a difference."

Last year, a Chinese national report said adult obesity rates had reached 9.6 percent in 2012, more than doubling in a decade.

A study in the Journal of the American College of Cardiology in March 2015 said three out of four people in China were in poor cardiovascular shape.

Cardiovascular disease has become the leading cause of death in China, and the prevalence of diabetes has more than doubled in 10 years.

China's expansion is part of a global one: a major survey in April, published in The Lancet medical journal, said one in five adults in the world could be obese by 2025.