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Studies do not support unhealthful relation between animal foods and breast cancer

August 17, 2009

Breast cancer is the 7th leading cause of mortality in the United States and results in approximately 41,000 deaths each year. Although genetic factors are important, there is considerable evidence that breast cancer risk is related to modifiable lifestyle factors, such as physical activity, body weight, alcohol intake, and dietary choices. The September 2009 issue of The American Journal of Clinical Nutrition reports the results of 3 human studies designed to better delineate the relation between animal foods and breast cancer risk.

"These studies highlight two very important points," said American Society for Nutrition Spokesperson Shelley McGuire, PhD. "First we all need to remember that there are really no such things as 'bad' foods. Second, observational studies that show associations between diet and health need to be considered with a proverbial grain of salt. These studies clearly provide additional and strong evidence that consumption of meat and dairy products by women does not, by itself, increase breast cancer risk. Further, moderate and mindful consumption of these foods can be very important in attaining optimal nutrition for most women who often do not consume sufficient iron and calcium."

In the first study, which was a controlled dietary intervention trial conducted in the United States, 35 obese postmenopausal women with type 2 diabetes received conjugated linoleic acid (CLA) supplements or a control supplement (safflower oil) each for 36 wk; adiposity was assessed. In another study, researchers examined the association between CLA intake from natural sources and breast cancer incidence in a large cohort of initially cancer-free Swedish women for 17.4 y. The third study assessed whether the consumption of meat, eggs, and dairy products was associated with breast cancer risk in a very large group of healthy European women followed for 8.8 y.

These studies provide no evidence that animal-food consumption increases (or decreases) risk of breast cancer, although CLA supplementation may decrease adiposity (a major risk factor for this disease). In an editorial, Linos and Willett remind us that these studies did not assess the relation between animal-food intake during early life and later breast cancer, a likely important piece of the puzzle. Nonetheless, they conclude, "These data are sufficient to exclude any major effect of consuming these foods during midlife or later on risk of breast cancer." Perhaps we finally have the answer to this long-standing question.

American Society for Nutrition