Study that links drinking milk with breast cancer flawed, says prof

A nutrition professor in Halifax says a new study that associates drinking milk to a higher risk of getting breast cancer fails to paint the full picture, and shouldn't stop anyone from picking up a glass of milk.

Researchers at Loma Linda University Adventist Health Sciences Center in California tracked the dairy intake of 52,975 women from the U.S. and Canada for nearly eight years.

The women were all cancer-free at the beginning, and by the end of the study, researchers had recorded 1,057 new breast cancer cases.

The study, published in the International Journal of Epidemiology, concludes that women who drink a cup of milk a day could have up to a 50 per cent higher chance of getting breast cancer. The study says the risk could be much higher if women consume two or three cups of milk a day.

Those findings led to some dramatic news headlines this week that are "blowing this study way out of proportion and really misrepresenting the actual results of this study," Jennifer Brady, an assistant professor at Mount Saint Vincent University, told CBC's Information Morning.

Brady said while the study shows there's a correlation or potential association between milk drinking and breast cancer risk, that's not the same thing as showing there's causation.

She added that the study itself "is not strong enough" because it doesn't take into account what else the women ate over the eight years.

Overall diet should be considered

There are many factors that impact whether someone is at risk of breast cancer, Brady said.

For example, some of the women who weren't drinking milk were also vegan and therefore likely eating more vegetables and fruit, and less processed meat than some of the women who drank milk.

"This is one of the issues in general with nutrition research is that when you take something away, you're generally also adding something else back into the diet," Brady said. "So if you're not looking at overall diet quality, then that is a problem for the study design or for the strength of the study itself."

The study also assessed the participant's diets at the beginning of the study, and then measured how many women got breast cancer eight years later. But Brady said the women's diets likely changed a great deal over those years.

Brady also takes issue with how people have interpreted the study's conclusion that one cup of milk a day was associated with up to 50 per cent increase in risk. She said it's important to put that into perspective.

If you took 100 women who never drank milk, you would still expect about two per cent of them to be diagnosed with breast cancer, she said. If those 100 women started drinking milk, you'd expect 50 per cent more of them to be diagnosed with breast cancer.

That means the risk would move from two to three per cent, Brady said, and that's only if "the study results are absolutely rock solid, which they're not."

Really that 50 per cent stat is also a little bit of a red herring.- Jennifer Brady, MSVU

"That 50 per cent stat is also a little bit of a red herring there because what that is reporting is something called relative risk. That's very different than someone's absolute risk," she said.

She said it's helpful to have research in this area, but cautioned that it requires far more context.

"It's just one perspective and it's one type of study that doesn't show causation," she said. "We need multiple studies. More research is needed as always to really unpack all of the complexity that's going on here."

Brady said there are many proven ways women can lower their risk of breast cancer, including eating lots of fruits and vegetables, limiting their alcohol intake, eating whole grains and exercising.

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