Reduced progesterone levels explain the reduced risk of breast cancer in obese premenopausal women: a new hypothesis.

Understanding the complex relationship between obesity and breast cancer is fundamental to our knowledge of the etiology of this malignancy; changes in the composition of the hormonal milieu are implicit in this process. Estrogens are synthesized from androgens by aromatase in the gonads and in peripheral tissues, principally, adipose tissue. Obesity in women, regardless of their age, leads to more aromatase and more extra-glandular estrogen production. In postmenopausal women, in whom ovarian estrogen production is absent, the increased incidence of breast cancer in women with high body mass index has been attributed to the relatively high plasma levels of estradiol from subcutaneous fat. In contrast, obesity in premenopausal women is associated with a previously unexplained reduced incidence of breast cancer. In obese premenopausal women, the cumulative effect of higher levels of estrogens synthesized in the peripheral tissues, together with ovarian estrogen production, results in a negative feedback on the hypothalamic pituitary controlled release of gonadotrophins and a resultant diminution in ovarian steroid production. As a consequence, the normal balance of estrogen and progesterone levels is disrupted: while estrogen levels are normalized, progesterone production is markedly decreased. Progesterone is a promoter of proliferation in the breast. The low levels of progesterone in obese premenopausal women are consistent with, and we propose, are responsible for, the reduction in breast cancer incidence in these women.