How does breast feeding lower the risk of breast cancer?

Theoretical: No one knows for sure. One theory is that breast feeding makes the cells in the breast do what they are meant to do-- make breast milk. Cancers often arise from cells in our body that haven't matured fully. The hormones your body produces when breast feeding force breast cells to "grow up", making them less likely to become a cancer later on.

Lowers Estrogen: Estrogen fuels majority of breast cancers. Breast feeding lowers estrogen exposure and can reduce risk of breast cancer.

Several theories.: We don't currently know "exactly" how. Breast feeding for six months and longer seems to make a difference. One theory is that milk production keeps the cells busy so that they don't have the energy to mutate into cancer. Another is that toxins are drained with milk production. Another is that hormone levels with milk production are protective - estrogen, progesterone, prolactin, oxytocin etc.