The scientists, from New Zealand, Spain, Australia, Germany and Britain, found young teenagers in particular were nearly 40 per cent more likely to have severe asthma if they ate burgers and other types of fast food more than three times a week.

They said their study could have ''major public health significance owing to the rising consumption of fast foods'' if the link turns out to be causal.

The study also has the potential to overturn the so-called "hygiene hypothesis" - that our obsession with cleanliness is behind the mysterious rise in allergic conditions around the world.

But the good news is eating fruit helped reduce risk of the conditions, with the severity of symptoms cut by about 11 per cent among those who ate three or more portions weekly.

The head of respiratory trials at The George Institute for Global Health, Christine Jenkins, said evidence was mounting that fast food put children at risk of asthma and allergic disease.

She said it was particularly convincing because the study covered so many children from so many countries: 319,000 13- to 14-year-olds from 51 countries and 181,000 six- to seven-year-olds from 31 countries.

"Often a mother with a family history of asthma will ask 'what can I do to reduce the chance of my child having asthma', and I think you could say … that children who eat more fruit and vegetables are more likely to have good lung function," said Professor Jenkins, who is also a thoracic physician at Concord Hospital.

The study found the risk of severe asthma increased by 27 per cent in the younger children who regularly ate fast food, and they were also more likely to get severe eczema and rhinitis.

Charles Mackay, a Monash University expert in the role of diet in human disease, said the data was a "tour de force", and supported other emerging research linking allergies to diet rather than hygiene.

Laurence Macia, also from Monash, said people who ate a lot of fast food could be missing out on the protective effects from healthy foods.

"We do believe that fibre plays a very good anti-inflammatory role in the body," she said.

The research, published in the journal Thorax, came from the biggest study of children, called the International Study of Asthma and Allergies in Childhood.

The paper said the link between fast food and asthma and allergies could be related to higher saturated fatty acids, trans fatty acids, sodium, carbohydrates, sugar or preservatives. Fast foods have high levels of industrially hydrogenated vegetable fats such as margarine, which can be a source of trans fatty acids - ''and there is some evidence that dietary intake of trans fatty acids is associated with asthma and allergy'', the authors said.

In the teenagers, eating butter, margarine and pasta was also associated with asthma symptoms.

with Guardian News & Media