Advancing health through science, education and medicine


James MacDonald, M.D., FACSM

Q: I mostly exercise to keep my weight down, but I am intrigued by some things I read about boosting my immune system by working out.  What’s the evidence for this? In short, ‘what’s the skinny’ on working out to fight off colds?
“Exercise is medicine” is a concept near and dear to the heart of ACSM, but even we want to note that, in the real world, there is no panacea.  Exercise comes close to being that magic “cure all,” but it remains important to look skeptically at any intervention you might take up for your health.  And so, we appreciate your question. 

Indeed, is there evidence that exercise can boost the immune system to help fight off infections?

Yes is the brief answer, especially if you are exercising moderately.

If you are hoping to avoid infections, it appears that there may be a ‘J-shaped curve’ where we see people with both low and very high exercise levels having increased risks. That is, the proverbial “couch potato” will be more at risk for catching a cold than the regular exerciser.  However, this may also be the case for someone who is in the middle of an intense, strenuous build-up for a competition, though the evidence for the heavy exerciser is somewhat mixed, according to a joint consensus statement on overtraining from the European College of Sport Science and the American College of Sports Medicine.

I would like to emphasize that most of us will not approach the levels of exercise that may result in ‘overtraining’ and the associated theoretical risk of a depressed immune system. Most of us can usually stand a bit more physical activity, and the regular, moderate exerciser can definitely expect to enjoy a reduced rate of infections, with a wealth of evidence to support that claim.

So, get outside or head to the gym for a workout, and you may not need that box of Kleenex.

Q: That sounds like good news.  But what about conditions that pose bigger risks to me, such as cancers. I’ve read a healthy immune system can prevent cancers.  Can physical activity help me fight off cancers the way it helps me fight off colds?

If you are wondering about the fitness of your immune system to ward off worse conditions than a cold or flu – cancers for instance – we have, possibly, even better news for you. 

There are many factors involved in the development of a cancer. Poor immune function is thought to be a part of the development of many cancers. Physical activity has been suggested to enhance immune function and, thus, to be involved in cancer prevention. For instance, regular moderate exercise can improve the number and function of natural killer cells that are able to attack most types of cancer, and this action has been thought to aid in tumor suppression.

To give you a less theoretical flavor of the benefits of exercise in cancer prevention, I want to share two meta-analyses with you. Meta-analyses are a type of modern research study that looks at all the evidence and comes up with an overall, science-based conclusion about a question. A meta-analysis that looked at physical activity and lung cancer prevention was just published in 2016, for instance. It reported some intriguing results: if you were a current or former smoker, being physically active reduced your risk of lung cancer by 21 percent, and the more physically active you were the better.  On the other hand, the researchers found no protective effect for exercise in those who had never smoked.

And so the results of this study are nuanced. If you have never smoked, your overall risk for lung cancer is very low, though it is not zero. This study would suggest that exercise won’t bring your already low risk any lower. However if you, or anyone you know, ever did smoke, then by all means increasing the physical activity level will reduce the risk. And if you are a current smoker, then definitely try to stop. But also try to get more physically active, as both decisions will help lower your risk of lung cancer.

The second meta-analysis looked at the question of preventing colon cancer. The authors of this 2009 study found an inverse relationship between levels of physical activity and risk of developing colon cancer. They found the overall risk of developing colon cancer in those with moderate levels of physical activity as compared to those who lived sedentary lives was 24 percent lower. They found that both men and women benefitted. Unlike the story with lung cancer, there was no group in this study that did not benefit. Exercise was found to help everyone lower their risk of developing colon cancer.

There are many more studies looking at physical activity and prevention of different cancers, and most of these tell a similar story: exercise is not a ‘panacea’ for reducing the cancer risk for everyone, but it’s close.

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