RE: ASTHMA PREVENTION: BREAST IS BEST?

28 July 2004 Dear Editor, Kemp and Kakakios recently published an annotation in this journal examining the evidence for and against breast-feeding as a means of preventing asthma and allergic disease in children.1 They begin by stating that ‘It has been an article of faith amongst pediatricians and allergists that breast-feeding is beneficial for the prevention of allergic disease’. After reviewing some of the recent literature, including our own study from New Zealand,2 they end by stating ‘we believe that pediatricians can advocate breast-feeding for its preventative effects on asthma and atopy in childhood in addition to other demonstrated benefits on childhood growth and development’. Between these opening and concluding expressions of belief, the authors have made a number of statements that deserve clarification and comment. As in many of the discussions regarding breast-feeding and the likelihood of development of atopy and asthma in childhood, the authors have failed to adequately consider the issue of duration of follow up when examining outcomes. There are numerous studies of the impact of breast-feeding on wheezing syndromes in early childhood, fewer studies reporting objective allergic outcomes and fewer still providing well-documented outcomes of subsequent childhood or adult asthma with objective measurement of lung function, airway responsiveness and atopy to validate the diagnosis. Studies with outcomes examined over the first few years of life, including that from Tucson,3 generally indicate that breast-feeding protects against early childhood wheezing and to some degree against atopic dermatitis. Studies examining outcomes in later childhood and adult life, including again the Tucson4 and the New Zealand2 studies, conclude that breast-feeding does not protect against long-term development of allergy and asthma. These studies showed that infant breast-feeding may, in the long term, be associated with increased risk in specific subpopulations of children as in the Tucson cohort2 or in the entire population of exposed children as in the New Zealand study.2 It is essential that those reviewing the literature examine critically the time point at which evaluations of outcome are made. The authors state that it is ‘important to be sure that any decision not to breast-feed made in the light of possible adverse effects of the development on allergic disease is well founded’. We reiterate the statement made at the end of our initial report in the Lancet,2 and in repeated subsequent rebuttals of criticisms,5-7 that breast-feeding should be encouraged for many reasons. Our study suggested that long-term prevention of allergic disease and asthma can no longer be cited as a benefit of breast-feeding. However, there are many other well-substantiated benefits of breast-feeding, far overshadowing the lack of protection against asthma and allergy, that provide ample justification for advocating breast-feeding. We have never suggested that children should not be breast-fed on the basis of our findings. There are some minor factual errors regarding the New Zealand study in the review. The authors state that children were atopic on skin testing at all ages from 13 to 26 years, but in fact skin tests were performed at 13 and 21 years only. The authors question the accuracy of recall of breast-feeding as these data were collected at age 3 years, but as previously published we did verify the accuracy of reported breast-feeding from prospective records where available, with 98% accuracy being shown.6 We have addressed the issues of duration and exclusivity of breast-feeding in other correspondence,5-7 and noted that any impact of non-exclusivity should have decreased the effect we found, not increased it. With respect to socioeconomic status (SES), the authors queried whether parents of children with lower socioeconomic status, who were slightly less likely to breast-feed, may have subsequently underreported asthma on questionnaire. Even if that were true, this would not explain the allergy skin test data which was measured objectively at ages 13 and 21 years. The reading of skin tests is not impacted by parental socioeconomic status. Furthermore, in our Lancet paper,2 we examined the relation between breast-feeding and asthma or wheeze accompanied by objectively documented airway hyperresponsiveness, which again would not be impacted by SES. We have recently shown that SES is not related to asthma in this cohort,8 and consider that many previous studies of the relation of SES to asthma are likely to be confounded by breast-feeding. We agree that we lack adequate information on allergy and asthma in the early years in this cohort, and have no direct information on development of atopic eczema. The authors hypothesize that mothers of children who developed atopic eczema in early life may have been more likely to continue breast-feeding because of the possible protective effects. We have no data for or against this hypothesis, but as atopic eczema is very rare before age 4 weeks, this should not have impacted the decision to breast-feed beyond 4 weeks, which we took as the cut point for established breast-feeding.2 The authors question why the effects of breast-feeding reported in the Lancet paper2 were not featured in the recent paper from the same cohort published in the New England Journal of Medicine (NEJM).9 The Lancet paper used all 1037 study members to examine breast-feeding as a risk factor for atopy as an outcome. The emphasis of the NEJM paper was on long-term outcomes of wheeze and asthma, and analysed data from 613 study members who had been seen and assessed at every survey from age 9 to age 26. In this paper, atopy was analysed as a risk factor for the outcome of persistent and relapsing wheezing. When both breast-feeding and atopy were included in the multivariate analysis, the effect of breast-feeding was excluded by that of atopy. However, analysis of the NEJM paper subset confirms that the prevalence of breast-feeding was significantly higher among those with persistent, relapsing or remitting asthma than in other study members included in the NEJM analysis (56.8% vs 48.3%, P = 0.038). Ideally, studies of the benefits of breast-feeding would ensure prolonged exclusive breast-feeding in large groups of otherwise comparable subjects. Given that a randomized controlled study cannot be undertaken, and that there are numerous reasons for advocating breast-feeding which ethically would preclude such a study, we are limited to analysing data from large longitudinal studies. Similar data from another US longitudinal study were recently reported, indicating that atopy was increased by 50% in children who were exclusively breast-fed.10 We would suggest that the last statement of Kemp and Kakakios should be modified to read ‘we believe that paediatricians can advocate breast-feeding for its many demonstrated benefits on childhood growth and development, despite the fact that it does not have long-term preventative effects on asthma and atopy’.

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