Dec. 14, 2005 -- Postmenopausal women who eat lots of dairy products may be less likely to develop breast cancer.
Don't skip over the word "may" in that sentence. It's too soon to declare dairy as protector against breast cancer, the researchers caution.
The study comes from the American Cancer Society's Marjorie McCullough, ScD, RD, and colleagues. It appears in Cancer Epidemiology, Biomarkers, and Prevention.
Women's Diets Studied
McCullough's study included more than 68,000 women, most of whom were white and middle class.
All of the women had completed menopause and didn't have breast cancer when the study started in the early 1990s.
They completed lengthy surveys about their diets and lifestyles. The surveys covered consumption of 68 foods including milk, cheese, yogurt, ice cream, and other dairy products. Most of the dairy products consumed were low fat.
More Dairy Foods, Less Breast Cancer
The women were followed through August 2001. By then, they had had 2,855 cases of breast cancer.
Calcium isn't only found in dairy products. However, dairy products were the biggest calcium source for women in McCullough's study.
No Link Seen With Supplements
Vitamin D, which is also present in dairy foods, wasn't linked with an overall lower risk of breast cancer.
Blood vitamin D levels in people are made up from dietary sources and also from exposure to sunshine. Since levels in the blood were not measured in this study, they only looked at the relationship of breast cancer to the dietary intake of the vitamin.
Other studies cited in this article have suggested that higher levels of vitamin D may lower the risk of breast cancer.
Small Drop in Risk
McCullough is a senior epidemiologist in the American Cancer Society's Epidemiology & Surveillance Research department.
"Our findings suggests that dairy products, composed mainly of low-fat sources, or some component within these foods are associated with a small but significantly lower risk of breast cancer in postmenopausal women," McCullough says, in a news release.
Her study states that dairy products contain elements that may have anticancer effects, such as calcium and vitamin D. On the other hand, the study cites that hormones and growth factors found in dairy products may raise the risk of some cancers.
"Thus, the net effect of dairy product consumption on breast cancer risk may reflect a balance of beneficial and detrimental factors," the researchers write.
No Recommendations Made
"More study is needed before we can make concrete recommendations," McCullough says. She notes that the reason for the lower risk of breast cancer isn't clear.
The study shows that women who had the highest intake of dietary calcium also tended to smoke less, be more physically active, take menopausal hormone therapy and multivitamins, be leaner, have gained less weight since age 18, and consume less fat and alcohol. Those women, though, did not have a different menstrual or pregnancy history compared with the group with the lower intake of calcium.
"While we controlled to the best of our ability for other possible explanations, it's certainly possible women who consume low-fat dairy products have other health-related behaviors that could also lower the risk," McCullough says.
The study doesn't show whether the women had favored or avoided calcium throughout their lives.