The effect of breast-feeding with and without formula use on the risk of obesity at 4 years of age.

OBJECTIVE: To determine the minimal duration of breast-feeding required to protect against later obesity, whether the concurrent use of formula lessened any protective effect of breast-feeding, and what maternal or child characteristics might modify the association between breast-feeding and child obesity.

RESEARCH METHODS AND PROCEDURES: This was a retrospective cohort study. Participants were 73,458 white and black low-income children followed from birth through 4 years of age. Obesity at age 4 years was defined as measured BMI >or= 95th percentile. Feeding exposure was based on breast-feeding duration and the age of formula initiation. Covariates were obtained from the children's birth certificates.

RESULTS: At age 4 years, the prevalence of obesity was 11.5%. Only 16% of children were breast-fed 8 weeks or longer. Breast-feeding was associated with a reduced risk of obesity only in white children whose mothers had not smoked in pregnancy. In this subgroup, the reduction in obesity risk (adjusted odds ratio, 95% confidence interval), compared with those never breast-fed, occurred only for children who were breast-fed at least 16 weeks without formula (0.71, 0.56 to 0.92) or at least 26 weeks with concurrent formula (0.70, 0.61 to 0.81). Among whites whose mothers smoked in pregnancy and among blacks, breast-feeding was not associated with a reduced risk of obesity at age 4 years.

DISCUSSION: In a population of low-income children, breast-feeding was associated with a reduced risk of obesity at age 4 years only among whites whose mothers did not smoke in pregnancy and only when breast-feeding continued for at least 16 weeks without formula or at least 26 weeks with formula.