More than three hundred years ago, Italian physician Bernardino Ramazzini noticed that nuns developed breast cancer more often than other women. He suspected that their lack of childbearing accounted for the difference, and he theorized that reproductive hormones affect a woman’s risk of the disease.
Evidence now supports his theory, as natural estrogen from the menstrual cycle has been associated with a higher breast cancer risk. Risk is higher for women who were younger at menarche, older at menopause, and older at the birth of their first child. Risk is also higher for those who are childless or who have never breastfed.
Alcohol use, a lack of physical exercise, a higher body mass after menopause, and weight gain after menopause—all factors that affect hormone levels—are associated with a higher risk as well; conversely, the removal of the ovaries, the body’s source of estrogen, before menopause substantially reduces risk. Estrogen- and progestin-based pharmaceuticals, including birth control pills and hormone replacement therapy, further increase breast cancer risk.
Given the weight of the evidence that endogenous and pharmaceutical estrogens affect breast cancer risk, other hormonally active compounds—including synthetic estrogens in consumer products and pesticides, natural phytoestrogens in food, and other compounds that affect hormone signaling—deserve careful study. Silent Spring Institute has placed priority in its research on several classes of chemicals: mammary carcinogens, endocrine disrupting compounds, and developmental toxicants.
Since 1994, Institute scientists have undertaken a number of initiatives aimed at piecing together how environmental exposures may be linked to breast cancer. The scientists now collaborate with researchers from a range of universities, as well as the U.S. Centers for Disease Control and Prevention. The collaborative spirit extends beyond the scientific community to include activists and others concerned about women’s health. Together these collaborators are working to identify and break the links between the environment and women’s health, especially breast cancer.
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