High Intake of Dairy Milk Associated with Greater Risk of Breast Cancer

In a study published in the International Journal of Epidemiology, higher intakes of dairy milk were associated with greater risk of breast cancer, when adjusted for soy intake.¹

Given this finding, researchers recommended that the current US Dietary guidelines of a recommended 3 cups of milk a day should be viewed with some caution.

“My recommendation to people would be to, particularly if you have a family history of breast cancer, for a woman in that situation, to be cautious about dairy milk,” said first author Gary E. Fraser, MBChB, PhD. “And if you have an easy way, until it’s clarified, use some of the non-dairy milks that are around.”

Using a study cohort of 52,795 North American women, initially free of cancer, followed for 7.9 years, the researchers assessed dietary intakes and incidence of invasive breast cancers. Overall, the participants experienced 1,057 new breast cancer cases during follow-up.

There was no clear association found between soy products and breast cancer, independently of dairy. However, higher intakes of dairy calories and dairy milk were associated with hazard ratios (HRs) of 1.22 (95% confidence interval [CI], 1.05-1.40) and 1.50 (95% CI, 1.22-1.84), respectively, comparing the 90th and 10th percentiles of intakes. Moreover, full fat and reduced fat milks saw similar results and no important associations were noted with cheese and yogurt.

“It actually turned out that we started off primarily to look at soy products in breast cancer, but along the way of course had to put in the dairy, and when we did that we found that the apparent protective effect of soy went away,” Fraser explained, “and the signals seemed to reside within the dairy products, indicating that, in our data at least, the initial appearance of protection from soy was actually, in fact, largely due to the absence of dairy.”

Substituting median intakes of dairy milk users by those of soymilk consumers was associated with HR of 0.68 (95% CI, 0.55-0.85). Additionally, similar-sized associations were found among pre- and post- menopausal cases, with CIs also excluding the null in estrogen receptor (ER) and progesterone receptor (PR) cancers. Less biased calibrated measurement-error adjusted regressions demonstrated yet stronger, but less precise, HRs and CIs that still excluded the null.

“It’s a complex situation, the question of dairy and cancer, as you probably know,” Fraser said. “We are also finding, and have published already, that, consistent with other studies, dairy consumption appears to be protective for colorectal cancer. So, you can see, it’s a complex kind of situation.”

Fraser suggested that a potential reason for these associations between breast cancer and dairy milk could be the sex hormone content of dairy milk, given that the cows are lactating and often about 75% of a dairy herd is pregnant, because breast cancer in women is a hormone-responsive cancer. And though the data is not yet published, Fraser also suggested that the same results were shown for prostate cancer, another hormone-responsive cancer. Furthermore, the intake of dairy and other animal proteins in some reports has also been correlated with higher blood levels of a hormone, insulin-like growth factor-1 (IGF-1), which could promote certain cancers. 

“Milk contains bovine IGF-1 which is absorbed and is not destroyed by pasteurization,” the authors wrote. “Moreover, intake of milk has also been associated with higher levels of endogenous IGF-1, a proliferative hormone that is a probable causal factor in breast cancer.”

The research presented is consistent with previous findings of a hazardous effect of dairy, including the AHS-2 report which suggested that vegans but not lacto-ovo-vegetarians experienced less breast cancer than non-vegetarians. Regardless though, causality specifically attributable to dairy products was not proven by this research and deserves further consideration. 

“I think that what we need to do is to do some careful feeding studies of dairy and look at the metabolic consequences,” Fraser said. “Let’s make some careful measurements as to what are the metabolic consequences of dairy consumption, what are the impacts on gene expression for instance, in other words to dig much more carefully into mechanisms, that’s something that I would like to do.” 

References:

Fraser GE, Jaceldo-Siegl K, Orlich M, Mashchak A, Sirirat R, Knutsen S. Dairy, soy, and risk of breast cancer: those confounded milks. International Journal of Epidemiology. doi:10.1093/ije/dyaa007.