But high stress may put women at risk of other serious illnesses warn the researchers, a team from Denmark.
The findings follow an eighteen year study of over 6,500 women in Copenhagen. At the start of the study researchers asked the women what levels of stress they experienced routinely in their lives, and classified the results into low, medium and high levels. Stress was defined as tension, nervousness, impatience, anxiety, or sleeplessness. (Stress levels were not measured throughout the study.) In calculating the effects of stress, researchers also adjusted the results for other factors, such as whether they had children or whether they were menopausal, which would have an influence on developing breast cancer. They did not account for risk factors such as family history of the disease however.
Of the 251 women diagnosed with first-time breast cancer over the study period, researchers found that women reporting high levels of stress were 40% less likely to develop breast cancer than women reporting low levels of stress.
The study further found that, for every increased level of stress on a six-level scale, women were 8% less likely to develop breast cancer.
One explanation for the findings may be that sustained levels of high stress may affect oestrogen levels - which, over time, may have an influence on developing breast cancer. But this theory has not been tested, and research in this area so far has mainly been restricted to animals, caution the authors.
Despite the findings, the authors warn that stress-induced changes in hormonal balances are not a healthy response, and continued stress may play a damaging part in other illnesses - particularly heart disease.