The authors attributed the growing obesity rate to the country's social and economic development, as well as lifestyle changes that had led to excessive energy intake and lack of physical exercise among youngsters.

"Compared with their parents' generation, today's rural children are better fed but spend far less time on physical exercises," said Zhao Jinshan, co-author of the thesis and a nutritionist with the Shandong Provincial Center for Disease Control and Prevention.

The phenomenon is typical among "left-behind children," those who are left in the care of relatives by parents who are busy working in faraway cities.

Cai Shibi, who looks after 20 left-behind children in her village in southwest China's Chongqing Municipality, said at least five of the kids were obese, though their families were not necessarily rich.

One school boy in her care is 110 cm tall but weighs 55 kg. "His parents work far from home and his grandparents spoil him by stuffing him with tasty food," said Cai.

Unlike their parents who ran around having fun and staying fit, many rural children today are couch potatoes, said Zhao.

On the other hand, compared with urbanites, who tend to pay more attention to their dietary balance, most rural residents still prefer meat to vegetables and eat too much salty, oily food, said Zhao. "Dietary habits are a complicated social issue, and rural society needs to enter a certain stage of development for its people to realize the importance of a balanced diet."

Doctors warn that child obesity can lead to hypertension, diabetes and even cardiovascular diseases.