Background. Studies on the health benefits of breastfeeding in developed countries have shown conflicting results. These studies often fail to account for confounding, reverse causality, and dose-response effects. We addressed these issues in analyzing longitudinal data to determine if breastfeeding protects US infants from developing diarrhea and ear infections.

Methods.  Mothers participating in a mail panel provided information on their infants at ages 2, 3, 4, 5, 6, and 7 months. Infants were classified as exclusively breastfed; high, middle, or low mixed breast- and formula-fed; or exclusively formula-fed. Diarrhea and ear infection diagnoses were based on mothers’ reports. Infant age and gender; other liquid and solid intake; maternal education, occupation, and smoking; household size; family income; and day care use were adjusted for in the full models.

Results. The risk of developing either diarrhea or ear infection increased as the amount of breast milk an infant received decreased. In the full models, the risk for diarrhea remained significant only in infants who received no breast milk compared with those who received only breast milk (odds ratio = 1.8); the risk for ear infection remained significant in the low mixed feeding group (odds ratio = 1.6) and among infants receiving no breast milk compared with those who received only breast milk (odds ratio = 1.7).

Conclusions. Breastfeeding protects US infants against the development of diarrhea and ear infection. Breastfeeding does not have to be exclusive to confer this benefit. In fact, protection is afforded in a dose-response manner. The more breast milk an infant receives in the first 6 months of life, the less likely that he or she will develop diarrhea or ear infection. longitudinal analysis, diarrhea, ear infection, breastfeeding.

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