It has been widely lauded as one of the healthiest ways to eat.
But the Mediterranean diet is increasingly being shunned by southern Europe, even as other countries try to follow the region's lead.
It means children in Italy, Greece, Cyprus and Spain are now the fattest on the continent.
He blamed inactivity and junk food for fuelling obesity in southern Europe
Researchers found in these countries more than four in ten primary school kids were overweight or obese.
This compares with around one in three for primary school children in the UK.
Experts from the World Health Organisation (WHO) found that today youngsters in Sweden were much more likely to be following the Mediterranean diet than their counterparts in Italy or Greece.
Dr Joao Breda, who is the head of the European office for prevention and control of non-communicable diseases at WHO, said: "The Mediterranean diet for children in these countries is gone.
"In southern Europe, kids are really far from that – and those closer to the Med diet were Swedish kids."
He blamed inactivity and junk food for fuelling obesity in southern Europe, saying children were snacking more but doing little exercise.
There is growing evidence that a Mediterranean diet – one that is high in fresh vegetables, fruit, whole grains, olive oil and fish – can help to ward off heart problems and also lower the risk of cancer.
More recent studies have also found that it could also cut the chance of dementia and may also protect against age-related macular degeneration, which causes blurred vision.
The latest data from 2015 to 2017 looked at the height and weight of children, aged from seven to nine years old, in 38 countries.
The research found that in Italy, Cyprus, Spain, Malta, Greece and San Marino, one in five boys were obese.
France, Norway, Ireland, Denmark and Latvia were among the countries with the lowest rates, at between 5 and 9 percent for both sexes.
Children in these countries ate more fruit and vegetables, and less high-fat foods such as pizza, chips, sausages and burgers, consuming them either never or just one to three times a week.
Worryingly, most parents of obese children thought that their youngsters were either slightly overweight or normal, when asked.
Dr Breda added: "It is crucial to increase the consumption of fruit and vegetables in children, while reducing their intake of sweets and particularly sugary soft drinks.
"It is also very important to increase the awareness of parents and families to the problem of child obesity, given that our data shows that many mothers do not recognise their children as overweight or obese." (Daily Mail)