A diet low in vitamins A and C can increase the risk of developing asthma, researchers claim.
A review of 40 studies found that people with low levels of the key vitamins were more likely to suffer from asthma than those without the condition.
Each vitamin was independently linked with an increase in the risk of asthma, with people with the lowest levels of vitamin A appearing to have more severe asthma than other sufferers.
The findings, published in the journal Thorax, contradict a large-scale study published last year which found no link between levels of antioxidants in the diet and asthma.
But the latest researchers claim theirs is a more robust study. The review of studies that measured dietary intake of the vitamins, or levels of vitamins in the blood.
A total of 13 studies related to children while the rest were in adults or were unspecified. The authors found that, on average, men with asthma were lacking 26 per cent of their daily requirements of vitamin A while women with asthma were lacking 30 per cent.
Leanne Male, assistant director of research at Asthma UK, welcomed the review, but added: “However the jury is still out as to how exactly vitamin intake and asthma are related, and how vitamins might be able to help the 5.4 million people in the UK with asthma.
“Until further clinical trials have been carried out to establish the exact role of vitamins in both the development of asthma and treatment of asthma symptoms, we would advise that everybody eats a healthy balanced diet, especially if they have a history of asthma and allergies in their family, or are pregnant.”
Glenys Jones, Nutritionist, MRC Human Nutrition Research, said: “The data provided is interesting, but inconclusive of low vitamin A & C intake increasing asthma risk, as there are many factors such as smoking, physical activity and socio-economic status that have not been taken into account. Therefore more research is required to investigate a causal relationship. This research does however support the consistent advice of following a good, balanced diet, as poor quality diets of limited variability are associated with lower nutrient intakes.”
Experts say people should be able to get their daily supply of vitamins A and C from a healthy diet, without necessarily taking vitamin pills or other supplements.
Good sources of vitamin A include cheese, eggs, oily fish (such as mackerel), milk, fortified margarine and yoghurt. Recommended intakes are 0.7mg a day for an average man and 0.6mg a day for a woman, according to the Food Standards Agency.
Vitamin C is found in a wide variety of fruit and vegetables. Good sources include peppers, broccoli, Brussels sprouts, sweet potatoes, oranges and kiwis.
It is recommended that adults have 40mg a day.