Additionally, a study in 1991 from the European Journal of Cancer found that pre-menopausal women who do not wear bras had half the risk of breast cancer compared with bra users. Additionally, Japanese, Fijians, and women from other cultures were found to have a significantly higher likelihood of developing breast cancer when they began wearing bras. Some, Japanese researchers believe bras to be a problem because they found that bras can lower melatonin production by 60%. Melatonin has anti-cancer properties so it could directly affect chances of developing cancer!
Now this certainly does not mean that bras solely cause cancer, this is simply research implying bras can have a negative effect on breast health, so go braless whenever possible.
Dr. Michael Schachter wrote that bras and tight clothing can impede lymph flow and contribute to the development of breast cancer.
Moreover, Dr. John MacDougall wrote in The Lancet that repeated inflammation from constricting bras are implicated in painful breast cysts and lumps, scar tissue develops, and milk ducts become plugged, all of which is associated with a higher risk of breast cancer.
Again, the reason that bras, especially tight ones, are bad for breast health is because of the restriction of proper lymph flow in the breasts. There are many lymph nodes in the breasts and armpit areas. The role of the lymph nodes is to basically get rid of waste materials and toxins. Bras constrict this regular flow prohibiting the way that we would normally and naturally cleanse carcinogenic chemicals out of the breast area that we are constantly exposed to.