Thomas Broomhall, who is eight-years-old, has lost two stone
Childhood obesity is set before the age of five, ministers will hear from researchers later.
Compared to children in the 1980s, today's youngsters are fatter and most of their excess weight gain happens before school age, they will say.
This suggests initiatives to prevent childhood obesity should be started before school, suggest the authors.
The EarlyBird Diabetes study of 233 children from birth to puberty is being published in the journal Pediatrics.
One in four children aged four to five in England are overweight, latest figures show.
Disease 'of our time'
At birth, the children in the study were of similar weight to babies 25 years ago, but had gained more fat by puberty compared with children of the same age in the 1980s.
When they reach the age of five the die seems to be cast
Professor Terry Wilkin Peninsula Medical School
The bulk of this excess weight was gained before the children were five.
Weight at five years bore little relation to birth weight, but closely predicted weight at nine years old.
Before an obese girl reaches school age she will have already gained 90% of her excess weight, and boys will have gained 70% of their excess weight.
Lead researcher Professor Terry Wilkin, of the Peninsula Medical School, Plymouth, said: "When they reach the age of five the die seems to be cast, at least until the age of puberty.
"What is causing it is very difficult to know."
He said there must be a factor now that was not there 25 years ago which is making today's children obese.
And, given the young age, this is likely to be in a child's home rather than school environment and linked to upbringing rather than schooling.
Rather than lack of physical exercise, he believes diet could be to blame.
"It is entirely possible that the calorie density of food and portion sizes could be higher."
Obesity is one of the few medical problems that can be reversed very, very quickly
Sir Liam Donaldson Chief Medical Officer for England
He said strategies to prevent childhood obesity and its associated health problems, such as type 2 diabetes, might do better to focus on pre-school children.
Professor Wilkin said there had been a lot of focus on school meals, PE time, school runs, television viewing and computer games in the development of childhood obesity, but these are all issues for school age children.
But he said the mandatory measurement of the height and weight of all children in England on school entry at the age of four or five could be helpful, not only as a record of national obesity trends, but also as a pointer to future risk for the individual child.
Sir Liam Donaldson, the chief medical officer for England, said it was no exaggeration to describe soaring rates of obesity as an "impending crisis".
He said: "We need to get in early and build the foundations to healthy living from a very early stage."
However, he added: "It is never too late. Obesity is one of the few serious medical problems that can be reversed very, very quickly."
Sir Liam said eating five portions of fruit and vegetables a day was one of the most important elements of a healthy diet.
David Haslam, of the National Obesity Forum, said: "It is never too late or too early to intervene. The earlier the better in terms of long-term outlook."
He said early childhood obesity was likely to be down to environment and learned behaviours.
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