I’m often asked by patients facing cancer in my psychiatry practice: “If I’m feeling negative or depressed, will that mean a more negative outcome? Will it affect my cancer?”
The belief that ‘staying positive’ impacts peoples’ cancer outcomes is not an uncommon one. Surveys show that the majority of patients and even oncologists subscribe to this belief! But scientific research suggests a very different picture. Studies have not found that depression causes cancer or worsens cancer outcomes, and similarly, research has failed to show that treating depression improves cancer outcomes. So why the confusion?
First off, let’s clarify. I often hear people confuse ‘feeling depressed’ with ‘being in a depression’. These are not the same. Feeling depressed (aka, feeling sad, feeling down, feeling low, etc.), is a normal part of the human experience. We have all felt sad at times, and feeling sad or depressed is one of the many emotions we can experience as part of the broad range of normal human emotions. Being in a depression (the medical terminology is Major Depressive Disorder) is an actual medical diagnosis which is much less common and which affects people beyond how they feel. It affects their thinking, their behaviour, and impairs their overall functioning. Many of the folks I encounter confuse these terms, and it’s not uncommon for someone to tell me that they believe they are suffering from depression when, in fact, what they are experiencing is not depression (the disorder) but sadness (the emotion). There is, indeed, a correlation between mood disorders and healthcare outcomes, including cancer, (More on that below. Keep reading.) but what is less likely is a correlation between feeling sad and cancer outcomes.
Secondly, we have to remember that correlation (i.e., association) is not the same as causation.
To better understand this concept of correlation versus causation, consider this: in the summer months, the amount of ice cream sales increases. So does the number of car accidents. Ice cream and car accidents are, therefore, correlated with one another. But we all know that ice cream does not cause car accidents. So while it’s true that ice cream is correlated with car accidents, it would be misguided to advise people to avoid ice cream as a way of reducing the number of car accidents!
Back to cancer: depression (the disorder) is correlated with worse healthcare outcomes, as has been shown in many studies in cancer and other illnesses. But this is probably best understood by remembering the ice cream example. What we know about many diseases, including cancer, is that we often find higher rates of depression among people who are sicker or suffering more, physically. It might be tempting to therefore infer that depression leads to people being sicker, but that would be inaccurate. The best we can say is that there are higher rates of depression in groups of people who are sicker.
Putting it all together, there is strong evidence for a correlation between major depressive disorder and a number of health outcomes, both in cancer and in other illnesses. But this does not mean that major depressive disorder causes cancer. In fact, there is no evidence to support this claim, and studies have found that treating depression does not lead to improvements in cancer. What’s more is that most people will never be diagnosed with major depressive disorder, and feeling down, depressed, worried, anxious, scared, upset, among many others, is part of the normal human experience. These emotions, in their own right, have not been shown to impact cancer outcomes.