Expectant mums should ensure they get enough vitamin E as low levels during pregnancy increase the risk of asthma in the unborn child, UK experts say.
Vitamin E is found in margarines
Children of mothers who had the lowest intake were over five times more likely to have asthma than peers whose mothers had the highest vitamin E intakes.
Vitamin E has a beneficial effect on the developing lung, the University of Aberdeen researchers believe.
The first 16 weeks of pregnancy could be a critical time, researchers say.
By 16 weeks after conception the airways are fully developed in the embryo.
But lead researcher Dr Graham Devereux said vitamin E most likely had a dual effect - influencing both lung growth and airway inflammation.
He and his team studied 2,000 pregnant women and their children over a five-year period.
The children whose mothers fell into the lowest fifth for vitamin E intake during pregnancy were more likely to develop wheezing and asthma by their fifth birthday.
The children's own nutrient intake at the age of five did not appear to change the researchers' findings, which appear in the American Journal of Respiratory and Critical Care Medicine.
Earlier work by the team showed newborn babies of mothers with low vitamin E intakes had greater responses to dust mites and grass pollen. When these children reached two, they were also more likely to wheeze and have eczema than their peers.
Dr Devereux stressed: "Pregnant women should not take vitamin E just because of this study. They should eat a balanced, healthy diet.
"It is possible that declining intake of vitamin E in the last 50 years may have contributed to the increase in asthma in children," he said.
Currently, 1.1m children have asthma, and it is the most common long-term condition among children in the UK.
Dr Lyn Smurthwaite of Asthma UK said: "Eating a healthy, balanced diet at any time, but especially during pregnancy, makes sense and this study suggests simple modifications in a pregnant mother's diet may help protect her child from developing asthma by the age of five.
"Asthma UK is pleased to have funded this research which has the potential to provide a natural way of reducing childhood asthma."
Dr Keith Prowse, chairman of the British Lung Foundation, said: "This is an important piece of research which indicates that maternal diet may be more important in childhood asthma than previously thought.
"Good nutrition plays an important role in fighting infection, including respiratory infections which can often trigger asthma.
"Eating healthily is especially important for pregnant women, including taking the daily recommended intake of vitamin E.
"We'd like to see more research into the full role of maternal dietary factors in relation to childhood asthma."
Foods rich in vitamin E include vegetable oils, margarine, nuts and sunflower seeds.