Student research

by
Linda Hodge
Master of Science in Medicine, The University of Sydney
Supervisors: Robert Loblay, Kwok Yan
March 1993

Although the existence of food chemical sensitivity in a sub-group of asthmatics has been reported for at least half a century, the long term effect of removal of those food chemicals from the diet of susceptible individuals has not been well documented. The two studies described herein both address this aspect of diet and asthma.

The aim of the first study (pilot study) was to document food chemical sensitivity in selected asthmatics and to assess them for objective improvement in asthma over a period of four months. Subjects were required to adhere strictly to an elimination diet, which excluded food chemicals known to cause food intolerance, for one month before, and also during, challenge with those food chemicals most likely to exacerbate symptoms of asthma. A modified diet was individually prescribed for each subject following the challenges which eliminated only those chemicals to which there was a positive reaction as determined by clinical observations and spirometric measurements. Subjects were then requested to adhere to this modified diet for a minimum period of three months. Bronchial responsiveness to histamine was used as a measure of asthma severity before entry to the study, after one month of the elimination diet and after three months of the modified diets. There was a statistically significant mean improvement in bronchial responsiveness at both one month on the elimination diet and three months on the modified diets compared with initial readings.

The results of this pilot study suggested that elimination of food chemicals could reduce bronchial responsiveness to histamine, and thus the severity of asthma, which could be sustained. However, the method of assessment was long and tedious and clinical experience has shown that many patients are unwilling to undertake extended periods of severe dietary restriction in spite of the potential benefits. Recent research in England showed that, in some asthmatics with a history of asthma following ingestion of specific foods, bronchial responsiveness to histamine was significantly increased immediately following ingestion of the relevant food chemical. They suggested that this increased bronchial responsiveness was indicative of sensitivity and could be used in the diagnosis of food chemical sensitivity in asthmatics. An important advantage of this alternative method was that the subject was not required to adhere to an elimination diet prior to challenge. Thus, the aim of the second study was to assess the reliability of this alternative method by comparing it with the results of food chemical challenges using the tried and proven method as outlined in the pilot study. In order to compare the two methods, each subject was required to complete both.

There was no correlation between the results of the two methods with respect to metabisulphite-sensitive asthmatics. However, it was not possible to extend this observation to other food chemicals because there were insufficient responders to support a statistical analysis. Another finding of this study was that, unlike the pilot study, there was no statistically significant mean improvement in bronchial responsiveness after one month on the elimination diet or after three months on the modified diets. A comparison of subjects in the two groups revealed that the only significant difference was more frequent use of bronchodilators in the subjects of the pilot study. This could indicate greater instability and thus more scope for improvement.