High levels of sex hormones in older women can double their risk of breast cancer, Cancer Research UK scientists report in the Journal of the National Cancer Institute1.
Postmenopausal women with high amounts of oestrogen and testosterone in their bloodstream are at over twice the risk of the disease as those with low levels of the hormones, according to the study.
Testing for sex hormones may ultimately lead to useful ways of predicting a woman’s risk, while learning more about the causes of breast cancer may bring new ways of preventing the disease.
And since obesity is the biggest known reason for high levels of sex hormones among postmenopausal women, scientists believe that maintaining a healthy weight could help women bring down their breast cancer risk.
Researchers from Cancer Research UK’s Epidemiology Unit in Oxford pooled together data from nine separate studies, carried out in the UK, US, Italy and Japan, and including a total of 1765 healthy women and 663 with breast cancer.
In each study, blood samples were taken from postmenopausal women, levels of sex hormones were measured and women were followed for a number of years to see who developed breast cancer. By combining the results from all nine studies, scientists were able for the first time to reliably estimate the effect of postmenopausal hormone levels on breast cancer risk.
For each of the hormones tested, researchers compared one fifth of women with the highest blood levels with the fifth with the lowest. High levels of free estradiol – an active form of oestrogen – more than doubled the risk of breast cancer compared with women who had low levels of the hormone. Those with high levels of testosterone were also at more than twice the risk.
The study also found that women with raised levels of a molecule called sex hormone binding globulin, which reduces the activity of sex hormones, were at lower than usual risk. Interestingly, women who are obese have low levels of this molecule, which probably also helps to raise their risk of breast cancer.
Team leader Dr Tim Key comments: “We’ve known for some time that reproductive and hormonal factors were involved in the development of breast cancer. But until now, we haven’t had strong evidence for a direct link between the blood levels of ‘natural’ hormones and risk of the disease. Our study confirms that high levels of sex hormones can raise risk.”
“In the future, our research may allow testing to predict a woman’s risk of breast cancer and could provide leads for improved drugs to help prevent the disease. But meanwhile it’s important that women try to maintain a healthy weight, since this will reduce their blood oestrogen levels and breast cancer risk.”
Sir Paul Nurse, Director General of Cancer Research UK, says: “Obesity is responsible for about 7 per cent of the UK’s cancer deaths. This important study establishes one of the reasons for this link, highlighting the importance of maintaining a healthy weight. And in the future, it may be possible to reduce the risk of cancer in women with high levels of sex hormones.”
- Journal of the National Cancer Institute94 (8)