For millennia, cow’s milk has been a dietary staple for many. Throughout modern advertising history, marketing efforts have pointed excitedly at the myriad benefits of drinking milk, deploying stars like Beyonce, Harrison Ford and even Kermit the Frog to convince us all that we aren’t getting enough of it.

But new evidence seems to suggest quite the opposite: daily consumption of dairy milk has been linked with a greater risk of breast cancer.

A new study, published in the International Journal of Epidemiology, has found “fairly strong evidence” that drinking even one cup of dairy milk a day is linked with a sharp increase in a woman’s risk of developing breast cancer — in some cases, by up to 80 per cent.

“Consuming as little as one-quarter to one-third of a cup of dairy milk per day was associated with an increased risk of breast cancer of 30 per cent,” Dr. Gary E. Fraser, the first author of the research paper, said in a news release.

“By drinking up to one cup per day, the associated risk went up to 50 per cent, and for those drinking two to three cups per day, the risk increased further to 70 percent to 80 per cent.”

yacobchuk via Getty Images Even one glass of milk a day can increase the risk of developing breast cancer.

Over the course of nearly eight years, the observational study tracked the dietary intake of some 53,000 North American women, all of whom were aged 30 or older and all of whom were initially cancer-free.

By the end of the study period, 1,057 breast cancer diagnoses had cropped up.

The study’s authors believe this might be a result of the sex hormone content of the milk, since about 75 per cent of the dairy herd is pregnant. Intake of dairy and other animal proteins has also been previously associated with higher blood levels of growth factor-1(IGF-1), which has been linked with certain cancers.

Researchers did not seem to find an association between soy milk and an increased risk of breast cancer.

“However, dairy foods, especially milk, were associated with increased risk, and the data predicted a marked reduction in risk associated with substituting soy milk for dairy milk,” Fraser said. “This raises the possibility that dairy-alternate milks may be an optimal choice.” (It’s no wonder recent years have seen advances in synthetic biology, in the quest for cow-free dairy.)

It should be noted, though, that the research located a link between dairy consumption and breast cancer, but not a definitive cause-effect relationship.

Wendy Demark-Wahnefried, a professor of nutrition sciences and senior scientist with the O’Neal Comprehensive Cancer Centre at the University of Alabama Birmingham, told Global News that, while the study is “very strong,” this distinction should be made.

“It’s a very large sample size [and] the sample is racially and ethnically diverse,” she said. “[However], this is one study and it’s observational. There’s no cause and effect … only associations.”

Mizina via Getty Images The study's authors suggested dairy-alternate milks could be a better option.

What do the official dietary guidelines suggest?

Perhaps most problematic is that current U.S. dietary guidelines still recommend the average person drink three cups of milk a day, which they associate with “improved bone health” among other things.

Fraser says this recommendation should be viewed “with caution.”

In Canada, on the other hand, commercial milk sales have continued to decline as Canadian shoppers pivot away from dairy.

In 2015, the Globe and Mail reported that, between an aging population, concerns about animal welfare, and the rise of veganism, Canadians seem to be slowly falling out of love with dairy, and that sales have been slowly decreasing over the last three decades.

The whole Canadian dairy industry, in fact, is reaching a crisis. According to dairy farmers in Newfoundland and Labrador, sales are continuing to slide toward a nadir.

“We have these super cows producing a lot of milk, and a lot of consumers don’t see anything natural in that,” Sylvain Charlebois, a business professor at University of Guelph, told the Globe and Mail. “And that’s why some Canadians just reject that.”

In 2019, the Canadian food guide was updated to de-emphasize the role of dairy in a healthy diet, exchanging milk for water as a healthy beverage of choice.