No, Milk Is Not Going To Give You Breast Cancer. The Latest Study, Decoded.
Dairy has been in the news a lot lately. Whether it’s the environmental impact of the dairy industry, supposed horrible treatment of dairy cattle, Starbucks widening their non-dairy milk offerings, or a new diet yet again claiming that milk is ‘inflammatory,’ dairy just can’t catch a break. Sales of dairy have dropped over the past decade, likely due at least in part to all the negative press milk gets.
As if milk needed more problems in its life, there’s a new study claiming that a cup of milk a day increases risk of breast cancer by 50%.
So predictably, the media and everyone on social are all freaking out. Because CANCER.
A news article from CTV Canada with the ominous headline “One cup of milk per day associated with up to 50 per cent increase in breast cancer risk: study” goes on to say this:
“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 per cent to 80 per cent.”
More alarmingly…these figures correspond with the current U.S. dietary guidelines, which recommend three cups of milk per day for adults.
If you’ve seen the headlines, you might be reconsidering your dairy habit.
But if there’s one thing I’ve taught you all, it’s to take a step back when you see this sort of thing in the news. Take a breath, and get more information. Most likely, all is not how it appears.
I secured a venti Starbucks and sat down to read the full study this morning.
I admit that this study was dense as hell, and I am not a statistician. So, I called in help from Gideon M-K, Epidemiologist-At-Large (his description), who helped me take the data apart and interpret the results. Without him, I wouldn’t have been able to write an accurate takedown of this study.
Here’s what this study is all about:
Researchers collected data on 52,795 women over eight years, looking to compare the effect of soy intake vs milk intake on breast cancer risk.
The average age of the participants was 57 years old.
Both soy and dairy milk have been controversial over the years in terms on breast cancer: there are plenty of studies (and bloggers and nutrition gurus) trying to draw an association between soy and milk and cancer, but none have really gotten very far, since the evidence refuses to conclusively point in their favor.
Participants took a food frequency recall once, at the beginning of the study period. BMI was taken once, at the beginning of the study period. (hint: RED FLAG)
Researchers controlled for various known breast cancer risk factors, including demographics, family history of breast cancer, anthropometrics (like weight), alcohol use, activity level, hormonal and other medication use, breast cancer screening, and reproductive history.
At the end of the eight years, 1057 participants (2%) had breast cancer. 86% of them were post-menopausal.
Cheese and yogurt didn’t seem to have the same effect on cancer risk that fluid milk did.
Soy appeared to lower cancer risk when consumed in place of milk.
All of this seems really scary, doesn’t it! And the headlines…it’s enough to make you stop putting milk in your coffee! Hey Barista, soy milk on the bar, please!
I want to start this section with a special shout-out to the media and the study author for being incredibly irresponsible by sensationalizing the results and making them into a big deal, when in fact they’re not.
I also want to take this opportunity to remind you that correlation does not equal causation, meaning that just because a result seems to be linked to a behavior, doesn’t mean that the behavior is the CAUSE of the result. Especially in the case of nutrition, there are so many confounders that could creep in. Here are a few:
This study didn’t control for diet over 8 years. Participants completed one diet assessment, and results were collected 8 years later. What if their diets changed? Has your diet changed over the past 8 years?
There were no fruit or vegetable intakes measured. Those who consumed more soy likely consumed more fruits and vegetables overall. That’s my guess, but unfortunately the study didn’t bother reporting that metric (if they measured it at all). Diet quality wasn’t measured, either.
The study did show that the women who consumed milk were also heavier, ate more processed red meat, and were less active.
Obesity is a suspected risk factor for breast cancer.
The researches said they controlled for weight (BMI), but participants were weighed once, again at the beginning of the study. How many of them put on weight over the 8 years? The majority of the women were post-menopausal, making weight gain likely in that 8-year span.
Add that to the fact that post-menopausal women are more likely to develop breast cancer, and you’ve got more confounders.
And what’s with milk increasing risk of breast cancer in participants, but cheese and yogurt not affecting risk at all? Although the authors tried to explain that it may be the processing of milk into those other products which could affect its cancer-causing properties, eh. I’m not buying it.
The researchers didn’t give actual numbers for soy versus milk consumers who ended up with breast cancer. So we don’t really know how many of the 1057 who were diagnosed by the end of the study, were in each group. That’s not critical, but it’s annoying and careless.
I think the biggest and most important thing to take away though, is the relative versus absolute risk. This is the most common trick that researchers and the media use to get us to click on a headline or give a study more airtime, and it’s dirty AF. It’s the bait-and-switch of the ’50%’ number when in real life, the risk is not actually even close to that.
Can you imagine if drinking 3 cups of milk a day increased your risk of breast cancer to by 80%, like the study author said? Something is obviously very wrong here.
Here’s what that is:
The relative risk of one cup of milk causing you to get breast cancer is 50%, according to the study. This means that if you drink a lot of milk, versus a little milk, your lifetime risk of getting breast cancer goes up by 50% (or higher).
But the absolute risk, which means the risk to you if you drink milk versus not, is only around 2%.
So in a group of 100 women who never drank milk, about 2 would get breast cancer over a decade. If they all decided to drink an extra glass of milk a day, you’d expect to see one extra case of breast cancer.
So even if this study was solid as a rock, which it’s NOT, that’s not exactly the screaming ‘YOU’RE GONNA GET CANCER FROM MILK’ headline that’s freaking everyone out.
(Relative versus absolute risk is like the biggest thorn in my side ever, and Gideon’s blog post about it is the best explainer I’ve ever seen. After reading it, you’ll never be fooled by another nutrition media clickbait headline again!)
You know what I also hate about this study? The fact that the researchers tell us that their crap study says that milk gives women cancer, then turn around and point fingers at the US Dietary Guidelines that recommend 3 cups of milk a day.
Think what you want about the Guidelines, but this piss-poor behavior increases distrust in science and our food system.
This study was correlational, and didn’t prove causation. Was it the milk (but not cheese or yogurt) that caused cancer? Or, was it something completely unrelated?
The research didn’t measure diet quality, fruit, vegetable, or whole grain intake.
The women in the study were weighed once and gave one dietary recall in 8 years. Diet and weight can change in 8 years, which can also contribute to cancer risk. Plus, most of the women were post-menopausal – and this increases risk for cancer as well. The women who were diagnosed with cancer had other risk factors beyond ‘milk,’ such as being less active, having higher BMI, and eating more processed meat. What else were they or where they not eating that may have contributed to the outcome? We’ll never know.
The headlines are incredibly stupid. Sure, they’ve captivated people and made them feel anxious. But they’re also extremely misleading. Headlines like these erode trust in science.
If you’ve had breast cancer, you didn’t cause it by drinking milk.
If you drink milk, there is no compelling evidence – still – that it increases cancer risk.
And if you jump at every nutrition headline you read, remember this article.