There’s an ongoing effort in the media that defines a truly impressive amount of health reporting. While the details are finicky, the basic idea is simple — describing every food product as either causing or curing cancer. Whether it’s the miracle vegetable broccoli stopping colorectal cancer in its tracks, or sugar causing basically every cancer in the book, we love to hear stories about foods and the big C. If you believe the headlines, anyone who so much as glances at a Coke is going to morph into a giant tumor overnight.
This week saw the newest victim of the cancer/no cancer wars — dairy. According to news sources from around the world, dairy isn’t only a staple of Western cuisine, it’s also giving women everywhere breast cancer. And not only large amounts either — apparently, just a glass of milk a day will raise your risk of the dreaded disease by 50%!
Fortunately, while the risk sounds big, it may not be that meaningful to your life at all.
Chances are that dairy isn’t giving you breast cancer.
The study that has everyone up in arms against cow’s milk was a large epidemiological cohort study looking at women in Canada. Basically, the scientists took a large group of women, divided them up based on how much milk they reported drinking each day, and followed them up for about a decade. They then compared the rates of breast cancer in each group, and found that women who reported consuming more milk dairy at the start of the study had higher rates of breast cancer than those who drank less milk.
The study used a very large number of people — more than 50,000 women from across the US and Canada — and some fancy statistical techniques to try and make sure that this relationship was actually causal. After adjusting for a range of things, including diet and lifestyle factors, the association remained, indicating that perhaps dairy was indeed making people sick.
And it was by many measures quite a well-done study. The statistical analysis was robust, the outcome assessment fine, which all makes the findings a bit more reliable.
However, there are a few caveats right off the bat that raise some red flags. As soon as you read a bit more closely, it seems that dairy might in fact not be causing breast cancer.
The first thing to note is that this study was done using a dataset of Canadian Adventist women, most of whom were white. That immediately means that the applicability of this study to most of the world’s population is a bit limited — it may be entirely meaningless if you were born in South Korea, for example. Another thing to note is that the association between breast cancer and dairy was only for milk, so if you’re eating cheese, yoghurt, or really any other dairy product you don’t need to worry at all, based on these results.
Another important point is that there was no risk increase for premenopausal women. This may be because breast cancer is much rarer in this group, but based on this study there’s no reason to change your dairy habits if you haven’t gone through menopause yet.
It’s also worth noting that, while the relative risk increase was scary — 50% to 80%! — the absolute risk increase was much smaller. While it’s hard to back-calculate the exact figures from the study*, the absolute risk increase appears to be somewhere between 1–2%. In other words, over a decade about 2 in 100 women who drank no milk at all got breast cancer, compared to 3–4 in 100 for women who drank a glass or two of milk per day.
On top of all of this, the study also had a fatal flaw — it was observational. Now, observational research is extremely worthwhile and important, but it makes it hard to draw direct causal conclusions. In other words, we can say that milk was associated with an increased risk of breast cancer, but we don’t know if milk caused the cancer or if it was something else entirely.
This is especially true when you consider that the study only measured dairy intake once — they asked women at the start how much milk they drank, then assumed that didn’t change over a decade. That’s a huge assumption — I know my milk drinking habits have varied wildly over the last 10 years, and I think that’s true for many people!
It’s possible that something that the study didn’t measure — like wealth, or vegetable intake — caused both the increased risk of cancer and the difference in milk drinking. We know that breast cancer rates differ in rich and poor women, but this study didn’t control for income at all, which is a big issue.
This is supported by the fact that women who drank more milk were in many ways less healthy in this study. They drank more, had higher BMIs, ate more processed food, exercised less, and were generally just a bit worse off. It’s quite likely that there are other things that the study simply couldn’t measure that make the relationship between milk and cancer a bit less robust than the headlines suggest.
In other words, dairy probably isn’t quite as deadly as you might’ve heard.
Ultimately, this is another nutritional epidemiological study that may have very little meaning to your life at all. At best, we can say that women who drink more milk have a small increase in breast cancer risk, but even that’s not certain.
What’s much more likely is, as I’ve said before, that food and dietary patterns are enormously complex, and teasing apart the relationships between individual foods and health concerns is incredibly difficult. It’s unlikely that we’ll ever know if dairy causes an increased risk of breast cancer for sure, and this study doesn’t get us much closer to an answer. This is especially true considering that there’s been a huge amount of research on this question in the past, and in fact a number of studies have shown that dairy (including milk) decreases the risk of breast cancer.
In reality, the truth is probably that dairy is neither the hero nor the villain of this story. If you like milk, drink it. If you don’t, don’t. If you’re worried about your health, speak to a health professional like a doctor or dietitian, they’re much more reliable than scary stories on the internet.
Just don’t stress too much about dairy.
It probably isn’t giving you breast cancer after all.
You can now listen to Gid on the Sensationalist Science podcast for your weekly dose of scientific shenanigans and media muddling:
*For the nerds, this is because the authors didn’t give any exact numbers on breast cancer cases by group, they only gave hazard ratios and relative risks. The estimates in this piece are based on some rough calculations using the total proportion of breast cancer in the study and the reported relative risks.