Breast cancer risk is influenced by pregnancy, which can stimulate the growth of already initiated cells but conveys long-term protection, perhaps through permanent structural changes to the tissue or other still unknown mechanisms. Circulating high estrogen levels is now well established to increase the risk; the role of other hormones such as prolactin or insulin-like growth factor is less clear but likely to be important. Other established reproductive risk factors include early age at menarche, late age at first birth, and low parity. Established modifiable risk factors include postmenopausal hormone use, moderate alcohol intake, and adult weight gain. Other factors that may decrease the risk of breast cancer include breast-feeding, physical activity, and increased dietary intake of fruits and vegetables, although the evidence here is neither as strong nor as consistent as for other factors. Both high and low penetrance genetic variants alter risk.
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