Is asthma prevention possible with dietary manipulation?

To the Editor: In the abstract of his article on asthma prevention with dietary manipulation,1 Mellis states that "we know" that the major modifiable dietary environmental risk factors for childhood asthma are not having been breastfed and low intake of omega-3 fatty acids.

In his discussion of the evidence, Mellis suggests that breastfeeding may be protective and, importantly, acknowledges the controversy. He further states (in the abstract) that observational studies have shown a reduction in childhood asthma in children who eat fish regularly (that is, have a high intake of omega-3 fatty acids), similar to those who were exclusively breastfed for three months. However, he provides no references for these observational studies, and nor does he discuss any specific evidence in support of including omega-3 fatty acids for reducing childhood asthma.

While there are some suggestions of such an association, the evidence is extremely limited compared with the extensive literature on the potential for the protective effect of breastfeeding. Further, there are substantial methodological issues associated with the few studies that do exist, not the least of which is the measurement of the relevant dietary parameters.

Australian studies have suggested a protective influence of at least two fish meals per week on bronchial hyperresponsiveness in 7–11-year olds2 and of eating oily fish3 on the prevalence of childhood asthma. However, neither of these studies had the capacity to measure omega-3 fatty acid nor fish intake in a valid way. These limitations were acknowledged by the authors of the studies, and have been noted by others;4 they need to be included in any discussion of a putative protective effect. It should also be noted that the biological plausibility of such an association has been challenged.4 There are many valid reasons for promoting the consumption of omega-3 fatty acids, but shouldn't we wait for the outcome of the randomised clinical trial currently under way before accepting the statement that "we know" that a low intake of these fatty acids increases the risk of childhood asthma?