Caring for someone else adds extra stress, which can depress a person's immune system. Harvard researchers thought that weakening of the body's defenses might be one cause of breast cancer, and studied long-term caregivers risk.

What the reserachers wanted to know: Does long-term caring for another person increase a woman's risk for breast cancer?

What they did: The doctors used a study of nurses begun in 1976 that asks nurses a variety of health and lifestyle questions every two years. Picking up data from1992, 1996, and 2000, the researchers examined the answers of nearly 70,000 women from all over the United States who were asked whom they cared for (outside of their work), how many hours each week they looked after that person, and how stressed out they felt by it. They excluded women who had breast cancer in 1992 and correlated women who were diagnosed later with the amount and type of care they gave to another person. They also looked at data on hormone samples for some of the women, taken between 1989 and 1990, to check for levels of estradiol, a type of the hormone estrogen, which has been correlated with an increased risk of cancer.

What they found: Caregiving, even when it caused a lot of stress, was not associated with an increased risk of breast cancer. Women who spent more than 15 hours a week caring for an adult, such as a spouse or parent, reported higher levels of stress and depression than other women, but that did not put them at higher risk for breast cancer. In fact, the researchers wrote that women who were very stressed out because of adult care tended to have slightly lower incidences of breast cancer. They also had lower levels of estradiol compared with other postmenopausal women.

What it means to you: Caring for loved ones can be hard in numerous ways, but one of the researchers said, in a press release, that she hoped this study would "be reassuring to women" who are in position of looking after someone.

Caveats: The scientists took samples of the hormone levels before the women reported that they were caring for someone else, so it is hard to tell if lower estradiol levels (associated with increased cancer risk) are related to caregiving. In order to make that connection, the scientists would have needed to look at the estradiol levels both before and after the women had been in the role of caregiver. In addition, breast cancer sometimes does not appear until 15 to 20 years after the first malignancies form If long-term care does cause breast cancer, it may not materialize for decades, and the researchers looked only at data for eight years, from 1992 to 2000.

Find out more: The Susan G. Komen Breast Cancer Foundation (the promoter of the famous pink ribbons) has a site with information on all aspects of breast cancer.

The National Breast Cancer Coalition also has a good website.

Read the article: Kroenke, C.H., Hankinson, S.E., Schernhammer, E.S., Colditz, G.A., Kawachi, I., Holmes, M.D. "Caregiving Stress, Endogenous Sex Steroid Hormone Levels, and Breast Cancer Incidence." American Journal of Epidemiology. June 1, 2004, Vol. 159, No. 11, pp. 1019-1027.

Abstract online: http://aje.oupjournals.org