Abstract
Systematic reviews suggest that breastfeeding is associated with a lower risk of asthma, although marked heterogeneity exists. Using UK Millennium Cohort Study data (n = 10,126 children, born 2000-2002), we examined the association between breastfeeding duration and wheezing in the previous year, first for each age group separately (ages 9 months, 3 years, 5 years, 7 years, and 11 years) and then in terms of a longitudinal wheezing phenotype: "early transient" (wheezing any time up to age 5 years but not thereafter), "late onset" (any time from age 7 years but not beforehand), and "persistent" (any time up to age 5 years and any time from age 7 years). The association between breastfeeding and wheeze varied by age (2-sided P for interaction = 0.0003). For example, breastfeeding for 6-9 months was associated with lower odds of wheezing at ages 9 months, 3 years, and 5 years but less so at ages 7 years and 11 years (adjusted odds ratios = 0.73, 0.78, 0.79, 0.84, 1.06, respectively). There was a strong dose-response relationship for breastfeeding per month and early transient wheeze (adjusted odds ratio for linear trend = 0.961, 95% confidence interval: 0.942, 0.980) but no clear trend for late-onset or persistent wheeze. Our results identified heterogeneity in the association between breastfeeding and wheezing according to age at wheezing and wheezing phenotype.