The results are reported in the current issue of Menopause, the journal of the North American Menopause Society.
"The results from this study tell us that we need to look much more closely at the effects of stress and socioeconomic status on risk for endometrial and breast cancer in women," says Carol Shively, Ph.D., professor of comparative medicine at Wake Forest Baptist.
"The outcome of this study is a precautionary tale," said Kathleen Grant, Ph.D., (co-author), in an editorial about the research. "Social stress, perhaps caused by increases in social isolation and hostile social experiences, or lack of control over social interactions, may place postmenopausal women at risk for breast and endometrial cancer."
Shively and colleagues studied the effects of stress and moderate alcohol consumption on breast and endometrial tissue, which is the lining of the uterus. They evaluated type and quantity of cells, density of tissue, number of dividing cells, and number of progesterone and estrogen receptors. Levels of sex steroids, such as estrogen, and adrenal steroids, such as cortisol, were also measured. All of these may be markers for cancer risk.
For the study, postmenopausal female monkeys were placed in groups so they would naturally establish a social hierarchy from dominant to subordinate. Previous research has shown that subordinate monkeys have increased heart rates, more of the stress hormone cortisol and more cardiovascular disease. The current study showed that compared to dominant monkeys, the socially stressed subordinate monkeys were at increased risk for endometrial cancer, which affects 1 percent to 2 percent of women and is most common in older women.
"We know that lower social status is stressful for both humans and monkeys," said Shively. "This study shows that in monkeys, social stress was associated with cellular changes that may increase endometrial cancer risk."
The subordinate monkeys also had changes in their breast tissue, but these were not as significant as the uterine changes. "There may be an effect, but it's not as strong as in the uterus," said Shively.
The researchers also looked at the effects of moderate alcohol consumption on risk for breast and endometrial cancer. In humans, several large studies have found that alcohol consumption, even in moderate doses, appears to increase the risk of breast cancer. However, the studies relied on women's self-reports of how much they drank. Research shows that most people don't accurately report their alcohol consumption.
The monkey study was designed to directly compare postmenopausal monkeys who drank a moderate and controlled amount of alcohol with those who didn't drink alcohol. Half of the monkeys were trained to voluntarily consume two drinks of alcohol every weekday for 26 months. There was no difference in the cancer markers between the two groups.
"The research suggests that moderate alcohol consumption in postmenopausal women not taking hormone therapy may not be harmful to health," said Shively.
She pointed out that the results might not apply to women who undergo hormone therapy, or to premenopausal women. Researchers believe that alcohol may increase estrogen levels in women whose bodies still produce estrogen. Increased levels of estrogen are associated with higher risk of breast cancer.
Shively also noted that researchers don't know how alcohol consumption affects other types of postmenopausal therapies, such as raloxifene, tamoxifen or soy.
Other researchers involved in the project were Thomas Register, Ph.D., Jami Johnson, and J. Mark Cline, D.V.M., Ph.D., all from Wake Forest Baptist.
About Wake Forest University Baptist Medical Center: Wake Forest Baptist is an academic health system comprised of North Carolina Baptist Hospital and Wake Forest University Health Sciences, which operates the university's School of Medicine. The system comprises 1,282 acute care, psychiatric, rehabilitation and long-term care beds and is consistently ranked as one of "America's Best Hospitals" by U.S. News & World Report.