Research funded by the National Cancer Institute found consuming one cup of dairy milk a day could increase breast cancer risk by up to 50 percent.

Scientists have studied the potential effects of soy and dairy foods on breast cancer risk for decades, but the research is somewhat inconsistent. Researchers from Loma Linda University, in partnership with the National Cancer Institute and World Cancer Research Fund, sought to determine if either (or both) of these foods could actually impact one's cancer risk.

The study, published in the International Journal of Epidemiology, analyzed nearly 53,000 women over the age of 30 for eight years. Participants filled out an initial health questionnaire and proceeded to complete food frequency questionnaires and 24-hour recalls for the duration of the study. None of the participants had a history of cancer at the start of the study, but 1,057 women were diagnosed with breast cancer after eight years.

While the study found no clear associations between soy and increased breast cancer risk (soy actually reduced the risk), researchers found significant associations between dairy milk and breast cancer risk. Those who consumed a cup of milk per day—regardless of milk fat percentage—experienced a 50% increased risk of breast cancer, while those who consumed two or more cups saw a 70-80% increase in breast cancer risk. Again, these numbers are just based on correlation, not causation.

The authors say this phenomenon is likely because dairy milk has a high concentration of sex hormones that could increase breast cancer risk, as the cows are lactating and approximately 75% of dairy farm cows are pregnant at a given time. Milk also contains Insulin-Like Growth Factor 1 ( IGF-1), a hormone that is a probable risk factor for breast cancer that's not destroyed during the pasteurization process.

Interestingly enough, these strong associations were found in milk, but were much lower in yogurt and cheese. Both yogurt and cheese have significantly lower hormone levels per gram. The authors say this is likely due to the aging process cheese undergoes and add that yogurt is typically consumed in smaller amounts than milk.

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

hands holding a glass of milk
Credit: skaman306/Getty Images

The Bottom Line

This is a large study that makes strong associations and certainly poses a need for further studies on dairy products and breast cancer risk. However, there is still plenty of other conflicting research out there showing that milk and milk products have little to no impact on breast cancer risk. We reached out to Susan B. Komen Breast Cancer Foundation to see what a breast cancer expert thought about the study's findings.

"The relationship between dairy consumption and breast cancer risk is currently under study as a factor that may increase breast cancer risk as well as decrease breast cancer risk," says Erica Kuhn, M.P.H., manager of education and patient support at Susan G. Komen. Most studies have found no link between consuming dairy products and breast cancer risk before menopause, and it appears unlikely that dairy products are related to breast cancer after menopause."

Kuhn says some researchers suggest that the high fat content in many dairy products may increase one's risk for breast cancer, while others have studied whether the calcium and vitamin D content in dairy products could protect against breast cancer. She says the study's findings regarding soy are consistent with other research, with no link to an increased or decreased risk.

If you like milk, it's safe to keep drinking it—and certainly to continue to eat yogurt and cheese—even if you're worried about breast cancer. While the researchers found an association between drinking milk and breast cancer the overall risk is still low. About 2% of women developed breast cancer over the course of that study. Alcohol is one beverage that we know is associated with breast cancer, so you're better off cutting out your glass of wine than your glass of milk. Following a Mediterranean-style diet and limiting added sugars may help keep you healthy too.