Eunyoung Cho, Sc.D., Brigham and Women's Hospital and Harvard Medical School, Boston, and colleagues studied the association between red meat consumption and breast cancer in 90,659 women who were part of the Nurses' Health Study II, a large ongoing study of nurses who responded to an initial questionnaire in 1989. Dr. Cho and colleagues followed the women from 1991 (when they had an average age of 36) through 2003. The participants filled out food questionnaires in 1991, 1995 and 1999, on which they recorded how often they regularly consumed more than 130 different foods and beverages. Every two years, they reported whether or not they had developed breast cancer; reported cases were confirmed through hospital records and pathology reports. Only women who were premenopausal and had not previously had cancer were included in this analysis, and those who went through natural menopause or had their ovaries removed during the 12 years of the study were excluded after that date.
By the end of the study, 1,021 women had developed breast cancer, including 512 cases that were estrogen and progesterone receptor positive, 167 that were estrogen and progesterone receptor negative, 110 with mixed status and 232 with unknown status. The highest intake of red meat was not significantly associated with the risk for breast cancer overall or for hormone receptor--negative cancers, but was associated with an increased risk for hormone receptor--positive cancer. Women who ate more than one and one-half servings of red meat per day had almost double the risk of hormone receptor--positive breast cancer compared with those who ate three or fewer servings per week. The associations remained similar when the researchers calculated red meat intake in grams instead of servings, and also when they split the women into five groups based on how much red meat they ate.
"Several biological mechanisms may explain the positive association between red meat intake and hormone receptor--positive breast cancer risk," the authors write. Known cancer-causing compounds in cooked or processed red meat increase mammary tumors in animals and have been suspected of causing breast cancer in humans. In addition, cattle in the United States are treated with hormones to promote growth, which could also influence breast cancer risk. The type of iron available in red meat also may enhance tumor formation.
"Given that most of the risk factors for breast cancer are not easily modifiable, these findings have potential public health implications in preventing breast cancer and should be evaluated further," the authors conclude.