Daycare attendance, breastfeeding, and risk of asthma in the prospective MoBa and DIPP birth cohorts.

Abstract

Background: Daycare attendance and breastfeeding have been associated with asthma risk. However, it remains unclear whether the association between breastfeeding and asthma risk depends on daycare attendance. We investigated whether daycare attendance acts as a modifier in the relationship between breastfeeding and asthma risk.

Methods: We included 72,558 children from the MoBa and 3015 children from the DIPP studies. Current asthma at age 7 years in MoBa and time to asthma by the age 5 years in DIPP were analyzed.

Results: In both cohorts, approximately 4% of participants were diagnosed with asthma. In MoBa, breastfeeding was associated with lower risk of overall asthma (adjusted risk ratio, aRR 0.99 [95% CI, 0.98-1.00]) and nonatopic asthma (aRR 0.97 [95% CI, 0.96-0.99]) in children who had attended daycare by the age of 18 months, and it was associated with higher overall asthma (aRR 1.01 [95% CI, 1.00-1.02]) and atopic asthma risk (aRR 1.03 [95% CI, 1.01-1.06]) in children who had not attended daycare by that age (all interaction p values ≤.05). In DIPP, the duration of breastfeeding was inversely associated with risk of atopic asthma only in an exploratory analysis in those who had started daycare by the age of 12 months (adjusted hazard ratio, aHR 0.75 [95% CI, 0.59-0.95]) but not in children who had started daycare at the age of 13-24 months (adjusted hazard ratio, aHR 1.03 [95% CI, 0.96-1.10]) or had not started daycare by the age of 24 months (adjusted hazard ratio, aHR 1.00 [95% CI, 0.94-1.06]) (interaction p value .04).

Conclusions: While the primary analysis in the MoBa study suggested that age at daycare entry might mildly modify the association of breastfeeding with asthma risk, in the DIPP study an interaction only appeared in an exploratory analysis with limited atopic asthma cases. Consequently, these findings represent inconsistent evidence.