Breast cancer is a hormone-dependent disease, and high levels of the female hormone estrogen sustained over long period of time have been associated with an increased risks of breast cancer. For this reason, women taking hormone replacement therapy (which contains estrogen), those who begin menstruating at an earlier age, and those who go through menopause at a later age are at a modestly increased risk of developing breast cancer due to their increased exposure to estrogen.

Since psychological stress activates the body’s hormone-producing endocrine system, it has been proposed that stress levels may affect breast cancer risk. Some research has shown that major stress events may increase a woman’s risk of developing breast cancer, perhaps due to hormonal effects on the immune system. But less attention has been paid to the effects of prolonged everyday stress. According to one theory, certain stress hormones (e.g., cortisol) activated by chronic stress, decrease estrogen levels over the long term, which in turn, may actually decrease the risk of breast cancer.

A new study in the September 10, 2005 issue of the British Medical Journal found that women with higher levels of self-reported stress had a significantly decreased risk of developing breast cancer, compared with women with low stress levels.

About the Study

This study included 6,689 women from the Copenhagen City heart study, which followed a group of Danish men and women from 1976 to 1999. The women in this study had no history of breast cancer.

The participants filled out questionnaires that measured the intensity and frequency of their stress, based on their reported sensations of tension, nervousness, impatience, anxiety , and sleeplessness. The women were categorized as having low, medium, or high stress levels, based on scores from the questionnaires.

The researchers followed the women for up to 18 years and tracked which participants developed breast cancer.

In their analyses, the researchers controlled for factors that might influence the risk of breast cancer, including oral contraceptive use, hormone therapy, menopause status, body mass index (BMI, a measure of weight in relation to height), number of children, physical activity, alcohol consumption, and level of education.

The Findings

Ten percent of the women in the study reported high levels of stress. Over the course of this study, 251 women developed breast cancer.

The higher a woman’s stress questionnaire score (indicating higher levels of stress), the less likely she was to develop breast cancer. Women reporting high levels of stress were 40% less likely to develop breast cancer, compared with those with low stress levels.

Sixteen percent of the women were taking hormone replacement therapy when the study began, and the effect of stress on breast cancer risk was particularly pronounced in those women.

These findings have several important limitations. First, many factors contribute to breast cancer risk, and the researchers did not collect information on a number of important factors (most notably, family history) which may or may not have influenced the findings. Furthermore, stress is difficult to measure, and the researchers relied on the participants’ self-reports to quantify stress level, which may not have accurately reflected their physical stress response. Finally, more women in the high stress group died during follow-up, and the cause of their death may have affected their risk for breast cancer.

How Does This Affect You?

Despite its many drawbacks, this study suggests that women with higher chronic stress levels are less likely to develop breast cancer. While the findings cannot be used to explain why this is the case, the researchers propose that chronic stress may be associated with lower levels of estrogen, leading to a reduced risk of breast cancer. More study into the relationship between stress and breast cancer risk may lead scientists to develop new medications that manipulate hormonal responses, and help prevent or treat breast cancer.

Viewed in isolation, this study’s surprising and somewhat paradoxical results may lead you to conclude that high levels of self-perceived stress actually good for your health. Don’t forget, though, that 39% of the women in the high stress group died during the study, compared with 30% in the medium stress group and 35% in the low stress group. While we cannot assume that stress directly contributing to this slightly higher mortality, other studies support the view that prolonged stress does have harmful health effects, including an increased risk of cardiovascular disease.