According to lead investigator Candyce H. Kroenke from BWH and Harvard Medical School (HMS), "To date, most research examining the role stress plays in breast cancer incidence has evaluated short-term, acute stressors. Though there is little empirical evidence to suggest that this type of stress promotes the development of breast cancer, the possible influence of stress on breast cancer remains a compelling question.

"Fortunately, this new research provides evidence that high levels of stress sustained over a longer period of time do not appear to increase breast cancer risk. This [research] will hopefully be reassuring to women who are concerned about this question. In fact, the correlation between chronic stress from caregiving and lower endogenous hormone levels is interesting and suggests a possible lowering of risk with time. This [possibility] then raises the question, what impact do chronic stressors early in life have on breast cancer risk?"

Using data from the BWH Nurses' Health Study (NHS), researchers tracked 69,886 women ages 46 to 71 years old over a period of eight years between 1992 and 2000, in which 1,700 of these women developed breast cancer. After analyzing information from these women, the researchers found that providing high numbers of hours of informal caregiving and self-reported stress did not predict a higher incidence of breast cancer. However, compared with women providing no adult care, women providing more than 15 hours per week of adult care (a median 54 hours care per week) had lower levels of estradiol and bioavailable estradiol - hormones that at higher levels are related to increased breast cancer risk. This indicates that chronic stress is unlikely to have an adverse influence on breast cancer and that the effects of stress on health may be more complex than originally considered.

Each year, 182,000 women are diagnosed with breast cancer and 43,300 die. One woman in eight either has or will develop breast cancer in her lifetime.