Breastfeeding and the prevalence of allergic diseases in schoolchildren: Does reverse causation matter?

Infants at higher risk of allergic diseases might be breastfed for longer periods compared with infants at lower risk in the hope that breastfeeding might reduce the risk of atopic disorders. Therefore, this intention could manifest as an apparent allergy-promoting effect of breastfeeding or reverse causation. To analyze the effect of breast feeding on the prevalence of allergic diseases at school age, a large questionnaire survey was administered to the parents of schoolchildren aged 7-15 yrs. 13,215 parents responded (response rate, 90.1%). Prevalence rates of allergic diseases were compared according to the type of feeding in infancy (either complete breastfeeding, mixed feeding or complete artificial feeding). In both univariate and multivariate analysis, compared with those with complete artificial feeding, those with mixed and complete breastfeeding showed a significantly lower prevalence of bronchial asthma (BA) (p = 0.01 and 0.003, respectively). On the other hand, in univariate analysis, the prevalence of atopic dermatitis (AD) and food allergy (FA) were significantly higher in those with complete breastfeeding (p = 0.04 and 0.01, respectively). There was a significantly higher proportion of complete breastfeeding among those with greater risk of allergic diseases (presence of family history, either eczema or wheeze within 6 months after birth, or FA in infancy). Therefore, our multivariate analysis included these risks as confounding factors, and we found that the promoting effects of breastfeeding on AD and FA disappeared. In conclusion, our data clearly showed the inhibitory effect of breastfeeding on the prevalence of BA at school age. The apparent promoting effect of breastfeeding on the prevalence of AD and FA is most likely because of reverse causation.