Estrogens are steroid compounds that function as primary sex hormones.

They readily diffuse across the cell membrane, and inside the cells, they interact with estrogen receptors.

They affect multiple organ systems in men and women, including cardiovascular, respiratory, excretory, reproductive and skeletal systems.

Estrogens play an important role in the appearance of breast cancer, cancer of the reproductive organs, cardiovascular disease and osteoporosis (which turns the bone prone to fracture). These hormones are used as part of some oral contraceptives and also in hormonal replacement therapy of postmenopausal women.

A recent complex Danish research supports the common idea that differences in an estrogen gene (ESR1) change the risk of heart attack and stroke due to hormone replacement therapy. But the research also found that these changes were also connected to a higher risk of breast cancer.

The two variants of the ESR1 gene are named the C allele and the T allele and, as any individual possesses two copies of any gene; there are three possible combinations: identical copies of the C allele (CC) or the T allele (TT), or their combination (CT). 21 % of the people were found to be CC, 50 % CT, and 29 % TT.

In the most complex study of this type, involving a large pool of subjects (2,495 with ischemic heart disease, 856 with ischemic cerebrovascular disease (including stroke) and 1,256 with breast cancer for up to 25 years) that gave an increased statistical power to the research, the researchers could assess the connection between the risk of cardiovascular disease and ESR1 allele.

The team also searched for more information on 9,244 people from the Danish general population.

The researchers compared rates of cardiovascular diseases (heart attack, angina, stroke, venous thromboembolism), breast cancer, cancer of reproductive organs (ovaries, uterus and prostate), and hip fracture (linked to osteoporosis) among the different individuals, depending on their genetic pool.

The team discovered that the two alleles did not bias high density lipoprotein cholesterol reaction to hormone replacement therapy or risk of cardiovascular disease, cancers of reproductive organs or hip fracture. But women with TT formula in their genetic pool presented a 40 % higher risk of developing breast cancer than women with CC genetic complex.