The findings prompt the authors to suggest that a fast food diet may be contributing to the rise in these conditions, and if proved causal, could have huge implications for public health, given the popularity of these foodstuffs.

The authors base their findings on data from more than 319,000 13-14 year olds from 107 centres in 51 countries, and more than181,000 six to seven year olds from 64 centres in 31 countries.

All the participants were involved in the International Study of Asthma and Allergies in Childhood (ISAAC), which is a collaborative research project involving more than 100 countries and nearly two million children, making it the largest study of its kind.

The teens and the children's parents were formally quizzed on whether they had symptoms of asthma (wheeze); rhinoconjunctivitis (which produces a runny or blocked nose accompanied by itchy and watery eyes); and eczema; and their weekly diet.

Questions focused particularly on the severity of symptoms over the preceding 12 months -- including frequency and interference with daily life and/or sleep patterns -- and certain types of food already linked to protective or damaging effects on health

These included meat, fish, fruits and vegetables, pulses, cereals, bread and pasta, rice, butter, margarine, nuts, potatoes, milk, eggs, and fast food/burgers. Consumption was categorised as never; occasionally; once or twice a week; and three or more times a week.

After taking account of factors likely to influence the results, the analysis showed that fast food was the only food type to show the same associations across both age groups, prompting the authors to suggest that "such consistency adds some weight to the possible causality of the relationship."

It was associated with current and severe symptoms of all three conditions among the teens -- across all centres in the participating countries, irrespective of gender or levels of affluence.

The pattern among children was less clear-cut, but a fast food diet was still associated with symptoms across all centres -- except for current eczema -- and poorer countries -- except for current and severe asthma.

And this difference might have to do with the fact that children have fewer options about their food choices, suggest the authors.

Three or more weekly servings were linked to a 39% increased risk of severe asthma among teens and a 27% increased risk among children, as well as to the severity of rhinitis and eczema, overall.

On the other hand, fruit seemed to be protective in both age groups across all centres for all three conditions among children -- both current and severe -- and for current and severe wheeze and rhinitis among the teens.

Eating three or more weekly portions was linked to a reduction in symptom severity of between 11% and 14% among teens and children, respectively.

The authors suggest that there are plausible explanations for the findings: fast food contains high levels of saturated and trans fatty acids, which are known to affect immunity, while fruit is rich in antioxidants and other beneficial compounds.

The authors emphasise that their results do not prove cause and effect, but they do warrant further investigation.

"If the associations between fast foods and the symptom prevalence of asthma, rhinoconjunctivitis and eczema is causal, then the findings have major public health significance owing to the rising consumption of fast foods globally," they conclude.