Children of migrants to Chinese cities have lower rates of obesity than youngsters in more affluent established urban families -- probably because their grandparents are not around to over-feed them, a new study has found.

Fewer opportunities for unhealthy snacking and less pressure for academic achievement, leading to more active play, also contribute to migrant children's lower obesity rates.

Large-scale migration sees millions of Chinese families leave the countryside and settle in the country's biggest cities in search of economic prosperity.

However, migrant children are still at risk of increasing obesity because, unlike youngsters from affluent families, lack of parental supervision after school and unsafe neighbourhoods cause them to eat unhealthily and limit opportunities for active play.

Researchers at the University of Birmingham interviewed parents, grandparents and teachers at schools in the city of Guangzhou, in southern China. Their study -- published today in the journal PLOS ONE -- explored the differences in perceived causes of childhood obesity between local and migrant communities.

They worked in partnership with the Guangzhou Centre for Disease Control and the Guangzhou Health Care Promotion Centre for Primary and Middle Schools to carry out the first qualitative study to explore and identify these differences in urban China.

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Dr Bai Li, from the University of Birmingham, said: "Childhood obesity is a global public health crisis -- particularly in China, yet the health of children who migrate with their parents to major Chinese cities has rarely been explored.

"It is clear that an important step towards preventing the rise of obesity in migrant children is understanding the perceptions of parents, grandparents and teachers on the causes of childhood obesity."

She added that 15 per cent of Chinese children and adolescents, aged 7-18, are overweight or obese -- accounting for 30.43 million individuals. Live-in grandparents in local families often took responsibility for looking after their grandchild, but many viewed a fat child as a symbol of health and success and, as a result, overfed them.

However, grandparents of migrant children remained living in their hometown and had no influence on childcare.

Migrant children had fewer opportunities for unhealthy snacking after school, because they had to catch buses home -- getting on and off the vehicles within their school. Yet, local children often bought food from unlicenced street traders or were met at the school gates by their grandmothers bearing snacks.

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"Although childhood obesity in major Chinese cities is currently more prevalent among local children than migrant children, recent trends suggest a steeper increase among youngsters coming to the cities from rural areas," commented Dr Li.

"The results of our study highlight the need for tailored interventions to prevent a significant rise in the number of migrant children who are overweight or obese and the associated short and longer term health consequences.

"Future interventions for local communities should include education for grandparents, enforcement of regulations limiting illegal food traders outside schools and education policies that re-balance academic focus with increased physical activity. Within migrant communities on the other hand, interventions should focus on supporting parents and providing more physical activity opportunities outside of school."

Earlier research by Birmingham and Guangzhou researchers showed that the prevalence of obesity in resident children in Guangzhou was 20% compared with 14.3% in migrant children. Migrants now comprise up to 50% of the population in major cities across China. In general, they tend to have lower levels of income and education than city residents.

The earlier study found that the obesity among resident urban children was higher in boys than compared with girls. It also increased as children grew older and per-capita household income and maternal education increased.

Dr Weijia Liu said: "Childhood obesity is an important public health problem in China. With the cooperation of the University of Birmingham, the Guangzhou Center for Disease Control is able to deepen the research of factors contributing to childhood obesity in China.

"Our research does not only provide a scientific basis for formulating effective intervention for childhood obesity in the city, but also strengthens the friendship and cooperation between the two cities."