Your Outlook supplement on breast cancer (Nature 485, S49–S66; 2012) does not mention the protective effect of breastfeeding. In most populations, this seems to be even stronger than that conferred by regular exercise (Nature 485, S62–S63; 2012), especially when lactation is extended over several children (S. Ip et al. Evid. Rep. Technol. Assess. 153, 1–186; 2007).
Genetic factors and early puberty both play a part in development of the disease. But these cannot fully account for the increasing incidence of breast cancer in the developed world, given that the genetic make-up of populations is unlikely to have changed much over the past few decades and puberty now starts only slightly earlier.
One change is that Western families have become smaller, so total breastfeeding over a mother's lifetime has declined. Oestrogen exposure, normally suppressed during lactation, is therefore effectively prolonged, increasing the likelihood of developing some types of breast tumour.
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Oudesluys-Murphy, A. Less lactation may explain cancer rise. Nature 486, 473 (2012). https://doi.org/10.1038/486473c