Background: The evidence for a preventive effect of breast-feeding on asthma and other allergic diseases in childhood is inconclusive.

Objective: The aim of this study was to investigate the effect of breast-feeding on asthma and sensitization to airborne allergens among children up to 4 years of age.

Methods: A birth cohort of 4089 children was followed. Exposure data were collected at 2 months and 1 year of age. The total dose of breast milk was estimated by combining periods of exclusive and partial breast-feeding. Outcomes data were collected at 1, 2, and 4 years of age. The response rate at 4 years was 90%, and 73% participated in a clinical investigation, including blood sampling for analysis of specific IgE and lung function testing. Children with onset of wheeze during lactation (n=217) were excluded in some of the analyses to avoid disease-related modification of exposure.

Results: Exclusive breast-feeding for 4 months or more reduced the risk of asthma at the age of 4 years (odds ratio [OR], 0.72; 95% CI, 0.53-0.97), irrespective of sensitization to common airborne allergens ( P=.72). Excluding children with wheeze during lactation tended to strengthen the risk estimate (OR, 0.64; 95% CI, 0.46-0.88). A duration of 3 months or more of partial breast-feeding seemed to offer additional protection; exclusive breast-feeding for 3 to 4 months combined with partial breast-feeding for 3 months or more resulted in an OR of 0.44 (95% CI, 0.21-0.87). The effects tended to be stronger in children without heredity for allergy ( P interaction=.36).

Conclusion: Breast-feeding reduces the risk of asthma during the first 4 years of life.