Image caption Fast food often contains high levels of "unhealthy" fat

Eating fast food three times a week may lead to asthma and eczema in children, say researchers who have looked at global disease and dietary patterns.

Data from more than 500,000 children in more than 50 countries suggests poor diet may be to blame for rising levels of these allergy-related conditions.

Those who ate fast food, such as take-away burgers, risked severe asthma, eczema and itchy, watery eyes.

Eating plenty of fruit appears to be protective, Thorax journal reports.

Fast food often 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, say the researchers.

In the study, children in their early teens who ate three or more weekly servings of fast food had a 39% increased risk of severe asthma.

Six- and seven-year-olds had a 27% increased risk.

Eating three or more portions of fruit a week cut the risk of severe asthma, eczema and rhinoconjunctivitis by between 11% and 14%.

The study authors, Prof Innes Asher, from the University of Auckland in New Zealand, and Prof Hywel Williams, from the University of Nottingham in the UK, said: "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."

Generally, people with asthma do not have to follow a special diet.

In some cases, certain foods, such as cow's milk, eggs, fish, shellfish, yeast products, nuts, and some food colourings and preservatives, can make symptoms worse.

Malayka Rahman of Asthma UK, said research suggests that a person's diet may contribute to their risk of developing asthma and that eating healthily may have a beneficial effect.

"Evidence suggests that the vitamins and antioxidants found in fresh fruit and vegetables have a beneficial effect on asthma therefore Asthma UK advises people with asthma to eat a healthy, balanced diet including five portions of fruit or vegetables every day, fish more than twice a week, and pulses more than once a week."