Dairy milk associated with greater risk of breast cancer

Consuming dairy milk is associated with a greater risk of breast cancer in women, according to a new study by researchers at Loma Linda University Health in California and published in the International Journal of Epidemiology.

For the observational study, researchers evaluated dietary intakes of nearly 53,000 North American women, all of whom were initially free of cancer and were followed for nearly eight years. Dietary intakes were estimated from food frequency questionnaires (FFQ), also repeated 24 hour recalls, and a baseline questionnaire had questions about demographics, family history of breast cancer, physical activity, alcohol consumption, hormonal and other medication use, breast cancer screening, and reproductive and gynecological history.

By the end of the study period, there were 1,057 new breast cancer cases during follow-up. No clear associations were found between soy products and breast cancer, independent of dairy. But, when compared to low or no milk consumption, higher intakes of dairy calories and dairy milk were associated with greater risk of breast cancer, independent of soy intake.

The researchers noted that the results had minimal variation when comparing intake of full fat versus reduced or nonfat milks, and there were no important associations noted with cheese and yogurt.

The evidence suggests that consistently drinking as little as one cup of dairy milk per day may increase breast cancer rates up to 50 percent. Current U.S. Dietary guidelines recommend three cups of milk per day, the researchers said.

A hazardous effect of dairy is consistent with the recent Adventist Health Study-2  report, a long-term health study exploring the links between lifestyle, diet, and disease among members of the Seventh-day Adventist church, suggesting that vegans but not lacto-ovo-vegetarians experienced less breast cancer than non-vegetarians.

Gary Fraser, MBChB, PhD, first author of the paper, said the possible reasons for these associations between breast cancer and dairy milk may be the sex hormone content of dairy milk, as the cows are of course lactating, and often about 75 percent of the dairy herd is pregnant. Breast cancer in women is a hormone-responsive cancer. Further, intake of dairy and other animal proteins in some reports is also associated with higher blood levels of a hormone, insulin-like growth factor-1 (IGF-1), which is thought to promote certain cancers.

“Dairy milk does have some positive nutritional qualities, but these need to be balanced against other possible, less helpful effects,” Fraser said in a statement. “This work suggests the urgent need for further research.”