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Rationale: The protective effect of breastfeeding on early respiratory infections is well established, but its relationship to the development of subsequent asthma remains controversial.

Objectives: To clarify these complex issues, we examined the association between lung function and infant-feeding practices.

Methods: In the Tucson Children’s Respiratory Study, feeding practices were assessed prospectively based on questionnaires completed at enrollment and well-child visits. Formula introduction was categorized as having occurred before 2 months (n = 143, "earlyformula introduction"), from 2 and before 4 months (n = 336), or at 4 months and older (n = 200, "longer breastfed"). Lung function was measured at age 11 and 16 years. A random-effects model was used to assess the relationship of infant-feeding practices to measures of lung function.

Measurements and Main Results: FVC by age 16 was increased by 103 40.0 ml (P = 0.01), and the FEV1/FVC ratio was lower (-1.9 +/- 0.6%, P = 0.004) in the longer breastfed children compared with children with early formula introduction. This effect was modified after stratifying by maternal asthma. Compared with children with early formula introduction, longer breastfed children with asthmatic mothers had an FVC that was not increased (P = 0.7) and an FEV1/FVC ratio (-5.7 +/- 2.4%, P = 0.02) that was significantly decreased by age 16. Longer breastfed children with nonatopic, nonasthmatic mothers demonstrated an increased FVC (142 +/- 71.1 ml, P = 0.047) and no decrease in FEV1/FVC (P = 0.7) compared with children with early formula introduction.

Conclusions: Longer duration of breastfeeding favorably influences lung growth in children. However, in the presence of maternal asthma, longer breastfeeding is associated with decreased airflows.