Professor Li, who runs a clinic for obese children, says parents and schools play very important roles in children's attitudes towards life.

'Parents are role models. If they don't exercise or they eat a lot of junk food, their children will copy their behaviour,' he says.

Regina Lee, assistant professor of nursing at Hong Kong Polytechnic University, agrees.

'Parents are busy working nowadays so both adults and children have become less active. Children do little exercise and have bad eating habits because they learn from their parents.'

Dr Lee says parents who have obese children mistakenly believe the children have inherited bad genes.

In the past, fast-food chains were relatively expensive and seen as occasional treats. Now, prices have dropped and the number of outlets has increased. This means children have easy access to unhealthy snacks, Dr Lee says.

With both parents working, fast food is seen as an easy alternative to home cooked meals.

Children are also spending more time studying, watching TV and using the computer than going out to play, says Daphne Wu, a dietician at Matilda International Hospital.

'Some parents prefer their children to study or take extra classes rather than exercise in their spare time. After studying children watch TV or go online. There's not much physical activity,' says Ms Wu.

Although some schools have improved their canteen menus and physical education programmes in recent years, experts think such measures are inadequate.

Dr Lee says most lunchboxes still have fattening sauces and lack fruit and vegetables.

Teachers could also do more to promote healthier lifestyles.

'The teachers' schedules are very tight and they don't have spare time to promote health at school,' she says.