Several listeners have asked me to offer some perspective a new study that’s been in the news. Tess wrote:
“A few days ago, I heard about a study finding a 70-80% chance of breast cancer in women who consume 2-3 cups of milk a day. That seems pretty high. I feel like everyone would have breast cancer, since the dietary guidelines recommended 2-3 cups per day. What are your thoughts regarding this study? Do you think we should stop consuming dairy?”
At first, I wondered if Tess had misunderstood or mis-stated the findings. But then I found a press release from Loma Linda University, in which they quoted the lead researcher, Gary Fraser, as saying that his study provides “fairly strong evidence that either dairy milk or some other factor closely related to drinking dairy milk is a cause of breast cancer in women.” He goes on to say “for those drinking two to three cups per day, the risk increased… to 70% to 80%.”
In other words, 8 out every 10 women who drink 3 cups of milk a day will get breast cancer. As Tess points out, this doesn’t seem to square with reality.
And, indeed, that’s not even remotely what the study found.
Before I continue, let me just reiterate something I have said many times in the past. Dairy is not essential to a healthy diet. You—and your kids—can get all the calcium, protein, and other nutrients you need without consuming milk or dairy. If you don’t want to consume dairy products, for any reason whatsoever, you have my complete support.
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On the other hand, milk is a convenient and affordable source of high-quality protein, highly absorbable calcium and other nutrients. Assuming that you don’t have an allergy or other intolerance, dairy can supply valuable nutrition without significant risk.
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Should you stop drinking milk?
For those of you who do drink milk and are wondering whether you should stop, let’s take a closer look at what the study actually found.
But first, a bit of good news: Fraser and his colleagues found absolutely no association between the consumption of cheese or yogurt and breast cancer risk.
The researchers did, however, find an association between milk consumption and breast cancer risk. Specifically, those who reported drinking more milk were more likely to develop breast cancer during the study.
Correlation is not causation
In his remarks, the head researcher suggests that drinking milk causes breast cancer. But, of course, correlation is not causation. Just because two things occur together does not mean that one of them caused the other. Given the nature of epidemiological data, it’s quite possible that the cases of breast cancer that occurred had nothing to do with drinking milk.
Given the nature of epidemiological data, it’s quite possible that the cases of breast cancer that occurred had nothing to do with drinking milk.
In fact, other much larger analyses have found no relationship between dairy consumption and breast cancer risk. A 2015 meta-analysis of 22 studies involving more than a million and a half women from both Western and Asian countries found that dairy consumption was inversely related to breast cancer. In other words, the more dairy people reported consuming, the lower their risk of breast cancer.
So, there’s that.
Relative versus absolute risk
In his statement, Fraser also seems to have confused absolute risk with relative risk. He’s quoted as saying that drinking milk increased breast cancer risk to 80% percent. Surely, he meant to say that it increased the risk by 80%. And this one little word makes a huge difference.
As Tess pointed out, if the risk were increased to 80%, that would mean that eight out of 10 women in that group would get breast cancer. In fact, only about 2% of all the women in this study were diagnosed with breast cancer. One in 50. That’s the absolute risk.
Fraser seems to have confused absolute risk with relative risk.
The analysis then found that those drinking 3 cups of milk were about 80% more likely to be diagnosed than women who drank no milk at all. That’s the relative risk.
For example, let’s say that the risk of getting breast cancer for non-milk drinkers was 1.4%. An 80% increase in the relative risk would mean that those drinking 3 cups per day the most milk would have a 2.5% risk. That’s about 1 in 40 women, not 8 out of every 10.
And how many women are drinking 3 cups of milk per day? Very, very few.
According to National Health and Nutrition Examination Survey, the average American adult woman consumes less than ¾ of a cup of milk per day. The number of women potentially being exposed to the higher risk associated with consuming 3 cups of milk a day is extremely small. And that’s assuming that the results of this study apply to the larger American population—and there are several good reasons to question whether they would.
Participants in the Adventist Health Study tend to be healthier than the average American. Specifically, they are less likely to smoke and drink, more likely to exercise and maintain a healthy weight and to follow a mostly vegetarian diet. Not surprisingly, they tend to stay healthier and live longer than the average American. They also drink even less milk on average than the average American woman—meaning that those drinking 3 cups of milk a day are truly outliers. And that might explain why the results of this study contradict the results of most other similar studies.
Risk is only a statistic
Risk is merely a statistic. It doesn’t allow you to tell the future. And everyone’s tolerance for risk is different. If you are so nervous about breast cancer that you’re willing to change your behavior based on a statistical increase in your risk, you still have to decide which statistics to believe. According to this study, not drinking milk could make you slightly less likely to get breast cancer. According to the meta-analysis I cited earlier, drinking more milk could make you slightly less likely to get breast cancer. But, once again, neither option offers a guarantee.
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Here are few other things you might want to take into consideration: Do you enjoy milk? How much of it are you currently drinking? Is it a major source of protein or calcium for you? If you currently drink milk and you’re thinking of stopping, what would you replace it with? What nutrition advantages or disadvantages do those other foods offer
In the end, the overall quality of your diet will have a greater impact on your health than the presence or absence of any single food. And in the case of cancer specifically, your dietary choices are only one of many factors that contribute to your risk.
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To discuss this research and it’s implications further, join me on the Nutrition Diva Facebook page where I frequently host live discussions and Q&A sessions.