The well-established body of literature demonstrating the harmful effects of non-bioidentical hormones might lead some women to fear taking bio-identical hormones as well. A review of the published scientific literature indicates those fears are misunderstood and unfounded. For example, thirteen studies document that non-bioidentical progestin significantly increases estrogen-stimulated breast cell replication and growth.5-17 In stark contrast; seven studies have shown that bioidentical progesterone does not induce estrogen-stimulated breast cell proliferation.18-24
Numerous studies have demonstrated an increased risk of breast cancer with the use of non-bioidentical progestins.1, 5, 25-48 However, the use of bioidentical progesterone has not been associated with an increased risk of breast cancer. Quite the contrary, research has revealed that bioidentical progesterone decreases the risk of breast cancer. In a study published in the journal Breast Cancer Research and Treatment, 80,000 postmenopausal women using various forms of HRT were followed for more than 8 years. Women who used estrogen in combination with non-bioidentical progestins had a 69% increased risk of breast cancer, compared to women who had never used HRT. However, for women who used bioidentical progesterone in combination with estrogen, the increased risk of breast cancer was completely eliminated with a significant reduction in breast cancer risk compared with non-bioidentical progestin use.49
In another investigation, these same researchers found a 40% increased risk of breast cancer for women who used estrogen with non-bioidentical progestin. Interestingly, in women who used estrogen combined with bioidentical progesterone, there was a promising trend toward a reduced risk of breast cancer, compared to women who had never used HRT.25 In essence, bioidentical progesterone appeared to protect women against the development of breast cancer. These findings confirm work done six years earlier that found a trend toward a reduced risk of breast cancer in 1,150 women using bioidentical progesterone, compared to non-users of progesterone.50
Compelling research offers further insight into natural progesterone’s ability to defend against breast cancer. In a fascinating study, scientists administered estrogen alone, bioidentical progesterone alone, estrogen plus bioidentical progesterone, or placebo to 40 women prior to surgery to remove a breast lump. The hormones were applied topically to the breast for about 12 days before surgery. As expected, when given alone, estrogen caused a 62% increase in breast cell proliferation rates compared to placebo. Conversely, the addition of bioidentical progesterone to estrogen resulted in a significant decrease in the estrogen-induced increase in breast cell proliferation rates. Even more impressive was the finding that the group receiving bioidentical progesterone alone had a 66% lower breast cell proliferation rate compared to the placebo group.18
A growing body of literature has documented a strong connection between a woman’s progesterone levels and her subsequent risk for breast cancer. A trial reported in the International Journal of Cancer in 2004 measured blood levels of progesterone in 5,963 premenopausal women. Incredibly, the analysis of the data revealed that those women with the highest blood levels of progesterone levels who had regular menses experienced an 88% decreased risk of breast cancer.51 These findings corroborate another study in which 1,083 women treated for infertility were followed for upwards of 33 years to determine their subsequent breast cancer risk. Compared to women with normal progesterone levels, those deficient in progesterone had a 540% increased risk of premenopausal breast cancer, and were 10 times as likely to die from any cancer.52
Similarly, researchers at the University of North Carolina School of Public Health measured progesterone levels in pregnant women, who were then followed for upwards of 32 years. The researchers discovered that those women with the highest blood levels of progesterone during pregnancy had a promising trend toward a lower risk of breast cancer, compared to women with the lowest levels of progesterone during pregnancy. When the researchers analyzed the risk of breast cancer in women under age 51, those with the highest progesterone levels had a staggering 70% decreased risk compared to the group with the lowest progesterone levels.53
Findings from two other investigations revealed that survival rates for breast cancer are strongly correlated with the patient’s progesterone levels at the time of surgery.54,55 One study noted that 65% of women with a progesterone level of 4.0 ng/mL or more on the day of their surgical treatment of node-positive cancer were alive 18 years later, while only 35% of women with low progesterone levels on the day of surgery were still living after 18 years. The scientists noted that progesterone lowers the expression of vascular endothelial growth factor, which promotes the increase in new blood vessels (angiogenesis) that is essential for tumor growth. These scientists concluded: “This study has confirmed that a raised level of progesterone at the time of tumor excision is associated with an improvement in prognosis for women with operable breast cancer.”55
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