Water deliveryman Zhang Feng is worried about the 8-year-old son he left in the care of his parents in his hometown in Shandong province when he moved to Beijing.

"They love him, but they give him whatever he likes to eat or drink, including unhealthy stuff such as soft drinks," he said.

"It is very normal for couples in my age group, those born after 1980, to ask their parents to look after their child, since they are too busy with their jobs."

Zhang's fears are backed up by a new study. Researchers at the UK's University of Birmingham found that leaving children in the care of their grandparents can put the youngsters at greater risk of becoming obese.

The study, conducted at four communities in two Chinese cities - Guangzhou in Guandong province and Hechi in the Guangxi Zhuang autonomous region - concluded that grandparents are a significant factor in the growing level of childhood obesity in the country.

The study, published in the International Journal of Behavioral Nutrition and Physical Activity, found that grandparents contribute to the problem because of a lack of knowledge.

Many think fat children are more healthy than thin ones, and they tend to overfeed youngsters in their care and excuse them from taking part in physical exercise.

Chinese people have become taller and fatter over the past 10 years, according to a report on nutrition and chronic diseases released last month by the National Health and Family Planning Commission.

Excessive consumption of fat has caused increasing obesity problems, and more than 30 percent of people age 18 and older in China were overweight in 2012, causing a rising incidence of chronic diseases such as hypertension, according to the report.

Nearly 10 percent of children and teenagers between 6 and 17 years old were overweight, compared with 5.1 percent in 2002.

Wu Guangchi, a nutrition researcher at the Capital Institute of Pediatrics in Beijing, said obesity among adults and children is a growing problem in many parts of China and has spread from the cities to rural areas.

"In general, grandparents suffered food shortages when they were young, so they give plenty of food to their grandchildren," he said.

Lin Hang, a drama teacher at a primary school in Beijing, said many of her students are from other parts of China and their parents lack the time to take care of them.

"Grandparents tend to satisfy them through material means such as providing more than enough food," she said.

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