Researchers asked nearly half a million teenagers aged 13-14 and children aged six and seven about their eating habits and whether in the previous year they had experienced wheezing, eczema or an itchy, blocked nose when they did not have cold or flu.

The questionnaires -- completed by a parent or guardian for the younger children -- were distributed in scores of countries.

It marks the latest phase in a long-running collaborative programme, the International Study of Asthma and Allergies in Childhood (ISAAC), which was launched in 1991.

The investigators filtered out factors that could skew results, such as maternal smoking during pregnancy, sedentary lifestyle and body-mass index, in order to focus purely on diet.

They found that fast food was the only food type that could be clearly linked to asthma severity.

Three or more weekly servings of fast food were associated with a 39% increase in the risk of severe asthma among teens and a 27% increase among younger children.

It also added to the risk of eczema and severe rhinitis.

In contrast, eating three or more weekly portions of fruit led to a reduction in symptom severity of between 11% and 14%, respectively.

The study, which appears in the British Medical Association journal Thorax, noted that to prove an association is not to prove a cause -- but argued that a further inquiry was clearly needed.

"If the associations (are) causal, then the findings have major public health significance, owing to the rising consumption of fast foods globally," the authors said.

Previous research has found that the saturated and "trans" fatty acids trigger an inflammatory response from the immune system, the paper noted.