At least 900 children in Wales attended a slimming club this year alone prompting a call for nursery-aged youngsters to be taught healthy eating.
During one week in July, nearly 170 children aged between 11 and 15 attended sessions in Wales run by a weight-loss club.
Public Health Wales figures showed over 28% of five-year-olds are overweight, with 12.5% of children obese.
The National Obesity Forum said healthy eating needs to be taught earlier.
Tam Fry, who is also chair of the Child Growth Foundation, told BBC Radio Wales: "What we haven't done in this country is start young enough.
"There's a lot of emphasis on teaching children how to eat healthily in mainstream school, in primary and secondary. But actually it should begin much earlier."
Mr Fry explained that in Scotland they are starting to teach nursery age children about healthy living and what is good food and what is bad.
"That's the time to get the messages across because the younger these children are the more able they are to take on these messages and you hope that they'll keep those messages for the rest of their life."
He also said he was not surprised at the numbers of children attending Slimming World classes in Wales.
"If you look at the problem in the country as a whole, it's a very, very small number," said Mr Fry.
"I think it's a great idea if there's a facility, but it's only a good idea if the end result is that children don't put weight back on once they come off the courses."
"It's the long term effect which will prove the value of the exercise but I think the exercise is worth it."
A report on childhood obesity published by the National Assembly for Wales in July said the rates of childhood obesity in Wales are the highest in the UK, with about 35% of children aged under 16 being overweight or obese in 2011.
At diet classes adults learn about how to eat healthily and they are weighed to check their progress.
Slimming World said its Free2Go sessions attended by 11 to 15-year-olds encourage them to eat fruit, vegetables and lean meats with the aim of making healthy lifestyle choices.
A Slimming World spokesperson said: "The emphasis is on praising young people for healthy eating and increased physical activity and improving their self-esteem and confidence rather than weight loss."
The company says it supports around 21,000 11-15 year olds a year across the UK with 6,000 being members of a group at any one time.