The high saturated fat levels in food such as burgers lower children's immune systems, it is believed.
A research project involving more than 50 countries found that teenagers who ate junk food three times a week or more were 39 per cent more likely to get severe asthma. Younger children were 27 per cent more at risk.
Both were also more prone to the eye condition rhinoconjunctivitis, according to The Sun newspaper
But just three weekly portions of fruit and vegetables could cut that risk by 14 per cent in the younger group and 11 per cent among the teens, it is believed.
Researchers from New Zealand's Auckland University looked at the diets of 181,000 youngsters aged six to seven and 319,000 aged 13-14.
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