Just what is estrogen?
Estrogens, as a group, promote cell proliferation. i.e. they make things grow.  They stimulate the uterine lining in preparation for pregnancy, prompt breast tissue growth, stimulate the menstrual cycle, and initiate the bodily changes that occur at puberty. However, if estrogen is left unchecked, cells can multiply in an unpredictable way, in some cases leading to cancer. The most commonly discussed estrogens are estrone, estradiol, and estriol.

 

Estrogen dominance
When an excess of any hormone exists, an imbalance develops, and health problems can arise. When there is too much estrogen and not enough progesterone to counteract its effects, the situation is called estrogen dominance. Estrogen dominance is caused by exposure to excess environmental xenoestrogens, use of synthetic estrogens like birth control pills and hormone replacement therapy, anovulation (i.e., lack of ovulation during menstrual cycle, which is not uncommon among women older than 35), digestive issues, unrelenting stress (which strains the adrenals and the thyroid), poor diet, and negative lifestyle factors (including smoking).

How does estrogen dominance specifically alter women's health? It has been linked to a wide range of conditions, including PMS, uterine fibroids, and endometriosis. In addition, numerous studies demonstrate that excess estrogen can cause breast, uterine, and ovarian cancers (which are all considered estrogen-dependent cancers), and cervical dysplasia, a precancerous condition characterized by abnormal changes to cervical tissues.

The connection between excess estrogen and certain cancers is clear. Harmful estrogens are difficult to detoxify in the liver (especially if the liver is already over worked) and are stored in fat. In a recent study published in the Journal of the National Cancer Institute, researchers linked obesity to breast cancer. Women with a higher body mass index (BMI), which equated to higher levels of body fat, also had elevated levels of hormones, particularly estradiol, which is the most potent of the estrogens produced in the body. The researchers concluded, "The increase in breast cancer risk with increasing BMI among postmenopausal women is largely the result of the associated increase in estrogens, particularly bioavailable estradiol."

According to a study featured in Cancer Causes and Control, women can reduce their breast cancer risk by maintaining a normal weight, because that reduces the amount of hormones stored in fat.