A new observational study just released and published in the International Journal of Epidemiology, states that drinking cow’s milk could increase the risk of breast cancer by up to 84%. Except that this statement is based on a flawed study. Here’s why.
The consumption of dairy products is not related to any cancer except for prostate cancer, for which there are only very modest associations.
What does the study really tell us?
If we wanted to give a coherent answer to this question, we would have to say the following: after statistical adjustment of soy consumption, moderate to high consumption of cow’s milk increases the relative risk of developing cancer by 22% to 84% in menopausal or perimenopausal women who attend Baptist churches. It’s not quite the same as “drinking cow’s milk would increase the risk of breast cancer,” is it?
Secondly, this is an observational study that complements the evidence already available. If you want to process the information correctly, you should be familiar with this evidence. However, the meta-analyses available in the scientific literature on the consumption of cow’s milk (skim, semi-skim and whole milk) and the risk of breast cancer in the general population suggest that there is no correlation between these two variables.
Furthermore, the authors make it clear that the correlation between cow’s milk and breast cancer only becomes positive if it is adjusted for the consumption of soy in their cohort. It is not positive by itself. But the most surprising thing is this: Of this cohort of 52,795 women, only 1,057 developed breast cancer during follow-up, although the prevalence for the region is one in eight women. One in eight women in a cohort of this size would statistically expect no less than 6,599 cases of cancer. But if you have followed this, the baseline sample is distorted, so you may conclude that there appear to be fewer cases of cancer in this subcategory of the American population, perhaps due to lifestyle components.
On the other hand, it is also questionable whether these cancers would not have occurred anyway, with or without the (excessive or otherwise) consumption of cow’s milk or soy. This would require knowledge of the basic cancer incidence rate in this particular population. Finally, the authors are bogged down in causal explanations to justify their findings. Remember that a fact must be highlighted before the cause can be sought.
What can be done?
Unless you consume a lot of soy or cow’s milk, you should not change your eating habits for these two foods based on these results. Eat a balanced diet rich in fruits and vegetables, exercise regularly and avoid toxic substances such as alcohol and tobacco if you want to take care of your health and reduce the risk of developing cancer later on. Remember that the consumption of dairy products is not absolutely necessary. You can eat them for pleasure if you like them, up to 3 servings a day, according to current dietary recommendations.
Things to Retain
In the meta-analyses available on this subject, the consumption of cow’s milk is not associated with an increased incidence of breast cancer.
This new study is an observational study that identifies an increased risk in a very specific cohort, and only if the incidence is adjusted for soy consumption.
There is no reason to change your eating habits (except for large excesses), as the results of this study give no reason for this.