In a 'Pulmonary Perspective' on diet and asthma in the first October issue of the American Journal of Respiratory and Critical Care Medicine (vol 170, pp725-729), UK researchers conclude that in longitudinal and clinical studies only vitamin E has been shown to have a protective effect against asthma; yet, even that vitamin does not generate any real effect on asthma control.
"We have recently reported the largest randomized placebo-controlled clinical trial of vitamin C to date, involving 201 patients randomized to vitamin C or placebo for 16 weeks, and found no effect on clinical asthma control," said Dr John Britton of the University of Nottingham's division of epidemiology and Public Health.
The authors point out that for vitamin E there has been evidence of an inverse association between vitamin E intake and both allergen skin sensitization and total immunoglobulin E levels. In addition, data gathered over a longer period has shown that a high intake of vitamin E is associated with reduced asthma incidence.
However, in a study of vitamin E supplements in 72 patients for six weeks, the authors found no evidence of clinical benefit.
Also, when looking at mineral use research, the authors did not discover any evidence of benefit associated with serum selenium level in connection with airway hyperresponsiveness in adults in the United Kingdom. And in a randomized placebo-controlled four-month study of magnesium supplementation, they demonstrated no evidence of benefit from the metallic element which has been shown to be essential to normal body functioning.
Researchers have been investigating diet in a bid to stem the rise in the numbers of children with asthma. One in eight children in the UK has asthma and this figure has increased six-fold in the last 25 years. In total, 8 million people in the country have the disease - or four times as many people as those with diabetes - and an average 1,400 people die from the disease each year, according to Asthma UK.
Dr Britton noted however that studying diet raises difficulties. "Few if any of the available studies has attempted to control for the full range of nutrients potentially involved, and in our view none has succeeded. Diet is also strongly related to other recognized determinants of asthma risk, such as smoking and socioeconomic status, and depending on the quality of available measures, controlling for these confounders can also be difficult," he said.
If there is benefit associated with diet for asthma sufferers, the authors believe it will come through a 'polypill' approach, delivering a cocktail of potentially beneficial nutrients rather than a single nutrient.
It is also possible, they point out, that if diet has an effect on asthma, it may not be associated with the nutrients widely studied to date. Increased understanding of the role of different nutrients in maintaining antioxidant defences could open up new, more productive areas of research.
They also argue that if diet does play an important role in asthma, the most likely benefit would come from dietary "manipulation to increase intake of natural foods, particularly fresh fruits and vegetables, in a balanced diet throughout life."
"This is not only the most logical and pragmatic interpretation, it is also the strategy most likely to yield benefits in other disease areas," they conclude.