Is the "breast is best" mantra an oversimplification?

Allergic disease

Asthma. There is evidence of a small protective effect of breastfeeding “ever” on asthma at 5 to 18 years of age in high-income countries (OR=0.90; 95% CI, 0.83-0.97). A family history of asthma or atopy did not affect this finding. The authors note there is some evidence of publication bias in this review, which is the largest and most comprehensive on the topic.⁴⁰

With a lifetime prevalence of asthma in the United States of approximately 13.2%, this association would confer an NNT of roughly 76.⁴¹ Earlier, the literature demonstrated mixed and conflicting evidence, and some experts suggested an effect only when there is a family history of asthma or atopy.³⁶

Eczema. For children younger than 2 years, there is low-grade- and very-low-grade-quality evidence that exclusive breastfeeding longer than 3 to 4 months is associated with a reduced risk of eczema (OR=0.74; 95% CI, 0.57-0.97).⁴⁰

Previously, data suggested that this association existed only in children who had a family history of atopy.³⁵ The protective association, however, exists regardless of family history and does not persist beyond 2 years of age. The authors noted evidence of publication bias, reverse causation, and misdiagnosis of early childhood rashes as eczema as limitations of their findings.⁴⁰

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