Nutrition

No, Drinking Milk Does Not Cause Breast Cancer

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Oh my.

The press are at it again. 

Scaring the bejesus out of everyone for clickbait. 

In case you missed it, a new study published in the International Journal of Epidemiology (IJE), concluded that “high intakes of dairy milk were associated [italics added for emphasis] with greater risk of breast cancer, when adjusted for soy intake”.  This study involved over 52,000 women from the Adventist Health Study, a large cohort study that aims to measure the effect of lifestyle and diet on disease and mortality among Seventh-day Adventists. The IJE study compared the consumption of soy and dairy and rates of breast cancer among Adventists from North America over an 8 year follow-up period.

Unfortunately, the press have honed in on one of the many data points presented in the study – a 50 percent relative risk in breast cancer with drinking one cup of milk per day, and led with this as the headline.

Does this mean drinking milk will give you breast cancer?

Um, nope.

Let’s break this down, shall we?

Association Does Not Equal Causation

An observational study, such as this, cannot demonstrate causation.  In this study, the researchers analyzed food intake data to determine if dairy consumption was associated with increased incidence of breast cancer after the follow-up period of 7.9 years, for this particular population.  Once again, the key word being associated

With this type of study design, researchers can only observe participants and look at outcomes. There is no treatment given, no control group, and no way to assess whether or not the outcome is the result of a specific treatment. Cause and effect cannot be determined in an observational study.  Even though the researchers in this study found an association between increased milk consumption and breast cancer risk, they cannot say the milk causes breast cancer. The authors state as much in the original paper:

“causality specifically attributable to dairy products is not proven by this work”

The study authors found no association with breast cancer risk and yogurt or cheese, and they also found a decreased risk of breast cancer with soy consumption.

The Data Collection Method was Flawed

The researchers used food frequency questionnaires (FFQ) at baseline to collect data about participants’ dietary intake. FFQs contain a list of foods and beverages, and participants report how often they consume these foods, and in what serving size. These types of questionnaires are meant to provide an estimate of usual intake. 

The problem is that food frequency questionnaires are notoriously poor at capturing accurate diet histories.  This is for a variety of reasons. People tend to under-report the amount of food they eat and often incorrectly calculate serving size. And a lot of people won’t admit to eating foods that are perceived as “bad”.

Also a problem: the researchers never checked consumption of dairy and soy again over the course of the 8 year follow-up period. I don’t know about you but my eating pattern has changed quite a bit over the past 8 years. With the exception of coffee, that’s been a constant. Although, I did have that matcha latte phase…

Not All Confounding Variables were Accounted For

A confounding variable is something other than what is being measured, that can have an effect on the outcome of a study.

In this study, known risk factors for breast cancer, such as family history of breast cancer, alcohol consumption, and BMI would be considered confounding variables – we know they effect breast cancer rates. The Canadian Cancer Society lists 17 established risk factors for breast cancer on their site (note the absence of dairy or any other food, for that matter). In this study, the researchers adjusted for some of these risk factors (like BMI, alcohol consumption, and family history of breast cancer), but not all. So it’s possible that there were other variables that contributed to the outcomes.

Additionally, the data used for these confounding variables was self-reported, and like dietary intake, was only measured once at the beginning of the study. So any change in BMI, alcohol consumption, or whether or not a family member received a breast cancer diagnosis over the course of the 8 year study period was not considered.

Also not considered: other aspects of the participants’ diets. Fruits and vegetables? Not included. Whole grains? Not included. We know these foods can be protective against cancer, so it’s possible the intake of these foods effected the results, but we’ll never know.

Relative Risk is Not the same as Your Individual Risk

This is what the media almost always gets wrong, or at the very least, misrepresents. 

Remember that 50% increase in cancer risk that newspaper headline was reporting?  That is the relative risk of a glass of milk contributing to you getting breast cancer, according to the study.

You might be wondering if your risk would increase by the same amount if you drink milk. Well, even if it did, it might not be a significant risk to you as an individual. As an example, your individual risk of getting breast cancer might be 2%. If the relative risk of breast cancer increases by 50% from drinking one cup of milk (and I’m not saying that it does), then your risk is now 3%. Would that stop you from drinking milk?

A person’s individual risk of breast cancer is based on a variety of factors. If you are already at high risk for breast cancer, or perhaps have survived a diagnosis in the past, reading headlines about studies like this can be scary. Remember, dairy consumption isn’t considered an established or even emerging risk factor for breast cancer, based on all research to date. If you truly are concerned, it’s best to discuss this with your care team to determine if the results of an individual study, such as this one, are relevant to you.

It’s Time to End the Fear Mongering

Dear Journalists,

Can you please stop making people scared to eat food?

Reporting nutrition research this way increases fear, and fuels food shaming and morality associated with individual food choices.  This causes confusion and stress, which is not good for people’s health.

Thanks,

Concerned Dietitians Everywhere.

P.S. Keep reporting on nutrition studies, so we all know about them. Just do so in a less harmful way.

P.P.S #notalljournalists


Should I Drink Cow’s Milk?

Well, that’s entirely up to you. 

There’s a lot of great nutrients in cow milk – it’s a convenient source of protein, calcium, vitamin D (if fortified), and phosphorus. It’s also not the only place to get these nutrients.

There are environmental and animal welfare considerations to make with cow’s milk, if you’re in a privileged position to consider such things.

Health conditions and allergies come into play too.  A person with a lactose intolerance might choose to steer clear of fluid milk, and if you live with chronic kidney disease, I’d suggest discussing it with your kidney dietitian to get a recommendation on the total amount to include in your diet.

Maybe you just like the way it tastes on your breakfast cereal.

There’s lots to consider – but please don’t base your decision on fear.

Should I Stop Reading Sensationalized Headlines About Nutrition?

Yes.

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