Objectives: We reviewed the evidence on the effects of breastfeeding on short- and long-term infant and maternal health outcomes in developed countries.
Data sources: We searched MEDLINE(R), CINAHL, and the Cochrane Library in November of 2005. Supplemental searches on selected outcomes were searched through May of 2006. We also identified additional studies in bibliographies of selected reviews and by suggestions from technical experts.
Review methods: We included systematic reviews/meta-analyses, randomized and non-randomized comparative trials, prospective cohort, and case-control studies on the effects of breastfeeding and relevant outcomes published in the English language. Included studies must have a comparative arm of formula feeding or different durations of breastfeeding. Only studies conducted in developed countries were included in the updates of previous systematic reviews. The studies were graded for methodological quality.
Results: We screened over 9,000 abstracts. Forty-three primary studies on infant health outcomes, 43 primary studies on maternal health outcomes, and 29 systematic reviews or meta-analyses that covered approximately 400 individual studies were included in this review. We found that a history of breastfeeding was associated with a reduction in the risk of acute otitis media, non-specific gastroenteritis, severe lower respiratory tract infections, atopic dermatitis, asthma (young children), obesity, type 1 and 2 diabetes, childhood leukemia, sudden infant death syndrome (SIDS), and necrotizing enterocolitis. There was no relationship between breastfeeding in term infants and cognitive performance. The relationship between breastfeeding and cardiovascular diseases was unclear. Similarly, it was also unclear concerning the relationship between breastfeeding and infant mortality in developed countries. For maternal outcomes, a history of lactation was associated with a reduced risk of type 2 diabetes, breast, and ovarian cancer. Early cessation of breastfeeding or not breastfeeding was associated with an increased risk of maternal postpartum depression. There was no relationship between a history of lactation and the risk of osteoporosis. The effect of breastfeeding in mothers on return-to-pre-pregnancy weight was negligible, and the effect of breastfeeding on postpartum weight loss was unclear.
Conclusions: A history of breastfeeding is associated with a reduced risk of many diseases in infants and mothers from developed countries. Because almost all the data in this review were gathered from observational studies, one should not infer causality based on these findings. Also, there is a wide range of quality of the body of evidence across different health outcomes. For future studies, clear subject selection criteria and definition of "exclusive breastfeeding," reliable collection of feeding data, controlling for important confounders including child-specific factors, and blinded assessment of the outcome measures will help. Sibling analysis provides a method to control for hereditary and household factors that are important in certain outcomes. In addition, cluster randomized controlled studies on the effectiveness of various breastfeeding promotion interventions will provide further opportunity to investigate any disparity in health outcomes as a result of the intervention.