"We have found symptomatic asthma in adults to be associated with a low intake of the dietary antioxidants vitamin C and manganese. The low intake of vitamin C appears to primarily associated with a diet deficient in fruit," wrote corresponding author Dr. Nick Wareham.
This study appears to support a growing body of science that has linked antioxidant intake, particularly vitamins C and E, to the incidence of asthma, a condition on the rise in the Western world and the most common long-term condition in the UK today. And, according to the charity Asthma UK, it affects over four million adults and over a million children.
According to the European Federation of Allergy and Airway Diseases Patients Association (EFA), over 30m Europeans suffer from asthma, costing Europe €17.7bn every year. The cost due to lost productivity is estimated to be around €9.8bn.
The new study, published in the journal Thorax (Vol. 61, pp. 388-393), used a nested case-control design to investigate a potential relationship between fruit and vegetable intake, and corresponding antioxidant intake, and the incidence of both diagnosed and symptomatic asthma for 515 cases and 515 controls with average age 32.
The researchers used registrants in the European Prospective Investigation of Cancer (EPIC)-Norfolk cohort and assigned them as either cases or controls depending on results from a health and lifestyle questionnaire (HLQ). Dietary data was obtained using seven day food diaries.
Dr. Wareham and his colleagues found that dietary intake of vitamin C and manganese were inversely and independently linked to symptomatic asthma, with a 12 per cent reduction in incidence with increasing vitamin C intake, and a 15 per cent reduction in incidence with increasing manganese intake. The researchers divided intakes into five groups (quintiles) ranging from low to high intake, but no quantification of each cut-off point is made.
For diagnosed asthma, only manganese has an effect on the incidence of the condition. Increasing intake of manganese, as a per quintile measure, was associated with a 14 per cent reduction in the risk of diagnosed asthma.
When the researchers looked at the incidence and associated risks of both symptomatic and diagnosed asthma in terms of fruit and vegetable consumption, and particular types of each, it was reported that moderate consumption (between 0.7 and 46.2 grams per day) of citrus fruits decreased the risk of asthma by 12 per cent. High consumption (46.3 grams per day or more) decreased the risk by 41 per cent.
Apples consumption also reported a significant effect on the risk of asthma, both symptomatic and diagnosed, with high consumption (48.1 grams per day or more) associated with a 32 per cent reduction in risk.
"Our observations are consistent with previous reports of an inverse association between dietary fruit and dietary vitamin C and respiratory symptoms," said the researchers.
The mechanism behind the protective effects of vitamin C and manganese appears to be due to their antioxidant nature, with manganese in particular playing a key role in the enzyme superoxide dismutase. Reduced levels of this enzyme have been reported in the lungs and blood of asthma sufferers.
A major strength of this study, argue the researchers, is the comprehensive nature of the dietary data, allowing them to confidently make the link between manganese and vitamin C intake and reduced risk of asthma.
There are several limitations however, as there are with all observational studies. The most notable being that cases may have altered their diets because of their asthma, although possible errors from this are reduced by the fact that only 22 cases reported such a change. There also exists the possibility that other, unaccountable confounders, may have affected the results.
The study does appear to add to a growing body of evidence linking increased antioxidant intake to a reduced risk of asthma. Indeed, a spokesperson for British charity Asthma UK told NutraIngredients.com: "Fresh fruit and vegetables are a good source of anti-oxidants and the results of several studies suggest that a diet high in anti-oxidants may protect against asthma and some other lung diseases.
This research has looked at whether people with a low intake of fruit and antioxidants such as vitamin C have a higher risk of asthma. Further research is necessary before the link between diet and asthma is fully understood.
However, Asthma UK would encourage all people with asthma to strive towards general good health through eating a healthy diet made up of plenty of fruit and vegetables," they said.