It's not just baby fat - overweight kids are far more likely to become overweight adults.

In fact, seven in 10 children who are overweight at age seven will remain that way well into their adult years, the Health Promotion Board (HPB) found in its latest study on obesity.

This flies in the face of the popular belief that chubby kids will eventually outgrow their baby fat, said Dr Annie Ling, who is director of the HPB's policy, research and surveillance division.

"It's not true what grandma says, that (overweight children) are very cute and that they will outgrow it.

"Most of them will not outgrow it - (this is) borne out by our own local data," said Dr Ling.

Her team's findings were part of a large-scale study of the Singapore population over nearly 30 years to find out when people start putting on weight and how obesity trends have changed.

This information can help researchers understand the points in a person's life at which intervention is most needed, so as to prevent overall obesity rates from rising even further.

For example, one in 10 children here are overweight by age five.

And compared with several decades ago, children today find it harder to lose weight.

In 1990, 68.7 per cent of the seven-year-olds who were flagged as overweight during school health screenings remained in the same category when they reached 11 or 12.

But by 2010, the corresponding figures had gone up to 81.3 per cent.

Due in large part to our increasingly sedentary lifestyle, those who put on weight as children no longer lose it as easily as they used to, said Dr Ling.

In fact, gym director Ray Mcgregor said he has seen an increase in the number of parents seeking personal training for their children.

"We hear time and again from parents - and I feel it myself as a parent - that kids are playing very little sport due to their academic curriculum placing such extreme demands," said Mr Mcgregor, who runs a boutique chain called Gym n Tonic. He said poor eating habits and more time spent indoors has also contributed to the problem.

Meanwhile, in order to keep his two young sons healthy, educator Mohamed Zuraidi Mohamed Mukhtar limits their access to screen time.

The 36-year-old said his children - aged two and four - are "naturally active" and he tries to make sure they get time to play outside.

He said: "On weekdays, whenever we get off (work) early, we take them out for activities like cycling or climbing at the playground.

"Weekends... we always take them out for things like walking at parks or swimming - they love the water."