An Australian study followed 2,187 children from birth to age 6 years and found that the introduction of milk other than breastmilk before 4 months of age was a significant risk factor for asthma (odds ratio 1.25; 95% CI 1.02-1.52) after adjustment for confounders. It was also a risk factor for wheeze three or more times since 1 year of age (1.41; 1.14-1.76), wheeze in the past year (1.31; 1.05 to 1.64), sleep disturbance due to wheeze within the past year (1.42; 1.07-1.89) and positive skin prick test reaction to at least one common aeroallergen (1.30; 1.04-1.61).

Oddy WH et al. (1999) Association between breastfeeding and asthma in 6 year old children: findings of a prospective birth cohort study. BMJ 319: 815-819.