Nursery children need more physical exercise to burn off their energy and stave off the threat of obesity, experts warn.
Experts recommend an hour of activity a day for young children
A Glasgow University team monitored a study group of 545 children who had the recommended adult level of exercise - 30 minutes, three times a week.
But that activity was not enough to affect the children's body mass index, the British Medical Journal reports.
Experts suggest children need an hour of daily exercise and a healthy diet.
One in four children in England is obese, and schools are beginning tests to see if their pupils are overweight.
From next year, pupils will be weighed and measured as they start primary school and again before they leave.
Parents of any obese four- or 10-year-olds can expect a letter telling them their child faces long-term health damage unless they lose weight.
Based on current trends, one million children will be obese by 2010, experts estimate.
The authors of the latest study suggest exercise alone is unlikely to reduce this figure.
Professor John Reilly and his team studied 545 children at 36 nurseries in Glasgow.
The children had their weight and height measured at the beginning of the study to work out their body mass index (BMI) - a measure of what is a healthy and unhealthy weight for a person.
Half of the children participated in the exercise sessions for the next 24 weeks, and their parents were also given guidance on increasing physical play at home. The other children had no intervention.
The researchers then measured the children's BMI again at six months and then a year later.
Although the increased level of exercise did appear to help the children improve their movement skills, it had very little effect on BMI.
An hour a day of activity
Professor Reilly and his team say children probably need to be even more active than this.
Dr Mike Knapton, of the British Heart Foundation, said it was "crucial" to encourage good exercise habits in young children, even if the benefits were not immediately obvious.
He said: "Children get less active as they get older so it's vital that youngsters get regular physical activity to lay the foundations for good health as they grow up.
"What this study does reinforce is that we need to try and get the whole package right from the earliest years, not just one lifestyle aspect.
"We recommend that parents encourage their toddlers and young children to eat a healthy diet and have lots of active play time - at least an hour a day for kids over five - as a vital investment for long-term heart health."
Mr Tam Fry, honorary chairman of the Child Growth Foundation, said: "Half an hour of physical activity should be a daily event for pre-school children. To measure its true impact on BMI the study should have required seven sessions a week- not three."