Current U.S. Dietary guidelines recommend that Americans should drink three cups of milk per day. But women who consume even a cup of dairy milk daily on average may be at greater risk for breast cancer, suggest new findings from a study published in the International Journal of Epidemiology by researchers at Loma Linda University Health in Loma Linda, California.

Researchers evaluated the dietary intakes and cancer risk of nearly 53,000 North American women. They were cancer-free at baseline and followed for about eight years. Dietary intakes were estimated using food frequency questionnaires, repeated 24-hour recalls and a baseline questionnaire that asked about demographics, family history of breast cancer, physical activity and more. By the study’s end, 1,057 new breast cancer cases were identified.

“Consuming as little as 1/4 to 1/3 cup of dairy milk per day was associated with an increased risk of breast cancer of 30%,” said Gary E. Fraser, PhD, the first author of the study. “By drinking up to one cup per day, the associated risk went up to 50%, and for those drinking two to three cups per day, the risk increased further to 70% to 80%.” It didn’t matter whether participants consumed full-fat milk, reduced-fat or nonfat (skim) milk; the risks were similar.  

But there’s some good news. Fermented dairy foods such as cheese and yogurt were not associated with any increased breast cancer risk. Eating yogurt regularly has also been associated with reduced risk for lung cancer and, in men, colorectal cancer. Fermented foods are believed to have positive effects on the microbiome, which may reduce cancer risk.

Nor did soy foods appear to increase breast cancer risk. In fact, researchers saw a reduced risk in those who drank soy milk instead of dairy milk.

According to Fraser, a possible reason for the association between breast cancer and dairy milk could be the sex hormone content of milk (about 75% of dairy herds are pregnant). How does that relate to breast cancer? Most breast cancer is hormone-responsive. In addition, according to some reports, consumption of dairy and other animal proteins has been linked to higher blood levels of the insulin-like growth factor-1 hormone, thought to promote certain cancers.

While observational studies such as this one can’t establish cause and effect, it is consistent with the largest randomized study to date establishing that a low-fat diet rich in fruits and vegetables—which often means less meat and dairy—may be protective. See "A Low-Fat, Plant-Based Diet Cuts the Risk of Dying of Breast Cancer."

Wondering what to drink instead of dairy milk? Although further research is needed, Fraser suggests that milk alternatives may be the best choice for milk lovers who want to protect against breast cancer while keeping up with their nutritional intake.   

For related coverage, read “Could Your Daily Orange Juice Be Increasing Your Cancer Risk?” and “Could Time-Restricted Eating Prevent Breast Cancer?”