Telling children to eat up their greens – and giving them a helping of liver – may be the best way to prevent them from developing asthma.
Researchers who examined the influence of diet on asthma found that people who did not get enough vitamin A or C from their food had a higher risk of becoming asthmatic.
Vitamin A is found in dark green and yellow vegetables such as broccoli, spinach and turnip greens, carrots, squash and sweet potatoes. It is also found in liver, milk, butter, cheese and eggs. Vitamin C is found in many of the same vegetables and in fruits including oranges, lemons, pineapple and strawberries.
Low levels of vitamin C in the blood were associated with a 12 per cent rise in incidence of the disease. Analysis also showed that those with asthma had an average daily intake of vitamin A which was between a quarter and a third of the recommended level.
Those with severe asthma had the lowest levels of vitamin A. No association was found with vitamin E.
The findings are from one of the largest reviews of research into the link. Scientists from the University of Nottingham found 40 relevant studies conducted around the world between 1980 and 2007.
Jo Leonardi-Bee, from the university's Department of Public Health, who led the study published in the journal Thorax, said: "It does appear that there is a link between diet and respiratory disease. It is unclear what the link is but it is probably to do with the anti-inflammatory properties of vitamins."
Asthma, which affects an estimated five million people in Britain, is characterised by oversensitive airways in the lungs which react to irritants in the air such as pollution and tobacco smoke.
In an asthma attack, the bronchioles (tiny airways) constrict and release histamine, which causes inflammation and the release of mucus, making it harder to breathe. Vitamins are thought to help neutralise this process by countering the inflammatory response.
The researchers say that their findings do not prove cause and effect and experiments that involve giving asthma sufferers vitamin supplements have proved disappointing.
"It is hard to know what dose to give, which vitamin and whether you are giving it for long enough," Dr Leonardi-Bee said.
Jeremy Laurance: But how do you get your kids to clean their plates?
Some years ago, a cancer charity dreamed up an idea to encourage children to develop a love of vegetables: they would dip them in chocolate. Accordingly, the charity joined forces with a frozen food retailer to produce "Chocobroccoli" and similar delights. I tried the icy nuggets of soggy veg smeared with chocolate sauce on my own children, who reacted with utter disgust. They thought that it was cheating, as I recall.
Although that attempt failed, it illustrated an essential truth. In the struggle that every parent faces to persuade their children of the virtues of veg, presentation is all. A child who rejects a dull cooked carrot may devour it when it is presented raw and sliced into sticks. A plate of fruit may be left untouched – until it is sliced, peeled and re-arranged to make a face which can be eaten, ears first.
Adults need help, too. As a nation, we eat about half the recommended "five a day" – essential for respiratory health as well as general health. The most common complaint is that fruit and veg are boring. So innovate.
The other night a visitor to our house, in her 20s, who had hated tomatoes all her life, ate a plateful enlivened with a few basil leaves and a dash of balsamic vinegar. Delicious, she declared. The only regret was that it had taken her two decades to discover their delights.