© 2008 American Thoracic Society
Is Reverse Causation Responsible for the Link between Duration of Breastfeeding and Childhood Asthma?To the Editor:In the article by Dr. Guilbert and colleagues (1), the authors conclude that "longer breastfed children of mothers with asthma demonstrate no improved lung growth and significant decrease in airflows later in life." The potential for reverse causation to have influenced this association has not been considered by the authors. Reverse causation would occur if early signs of allergic disease in children influence the duration of breastfeeding rather than the reverse. Although the authors sought to control for the presence of allergic disease in study children at 6, 11, and 16 years of age, they were unable to address the issue of reverse causation. To address this possibility, an accurate measure of infant allergic disease at the time of breastfeeding is needed. In a study of 620 atopy-prone children, our group (2) found evidence of prolonged exclusive breastfeeding in infants with eczema (a 28% reduction in the risk of ceasing breastfeeding compared with infants without eczema). In a separate study (3), we found that infants manifesting early allergic disease have more significant eczema and were at increased risk of progressing to asthma later in life. In the study by Guilbert and colleagues, early signs of allergic disease in the infant are likely to have increased the duration of breastfeeding and to have increased the risk of asthma and subsequent poor lung function. This effect would probably have been greater among mothers with asthma who are aware of the presumed beneficial effects of breastfeeding on allergic disease outcomes in the infant. Thus, the longer-breastfeeding group may comprise more allergic and more severely allergic children than the other breastfeeding groups, independent of any "effects" of breastfeeding. The onset of eczema commonly occurs in very early life when decisions relating to breastfeeding are being made, introducing the possibility of reverse causation. The potential role of reverse causation must be considered when exploring the apparent link between duration of breastfeeding and subsequent allergic disease in children.
The University of Melbourne Carlton FOOTNOTES Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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