I’m currently getting over a cold. In fact, as I write this I have a warm drink and a box of tissues right beside my laptop. It isn’t a bad cold. it’s just enough to annoy me, interrupt my sleep, and cause me to miss a few workouts. It’s my off-season (I’m not training for any events or races), so it isn’t a big deal. But when it happens in the spring or mid-summer, I am not so cavalier about missing training sessions. In fact, I can get downright ornery.

It doesn’t seem to matter how many times I get sick during the race season or how many times the athletes I coach fall prey to the seasonal flu; I still do a lot of hand-wringing over whether or not I should exercise while sick. Should I be jumping on my bike or doing a heavy lifting session?

What is the immune system?

Your immune system is comprised of six components that do their best to protect you from foreign invaders.

  1. Lymph nodes and lymphatic system, which recognize and fight invading pathogens
  2. Respiratory system, which creates mucus, coughs and sneezes to trap and remove contaminants
  3. Skin, a relatively thin but very effective barrier against invading pathogens
  4. White blood cells, which attack pathogens in your blood and other tissues of your body
  5. Your spleen, a major organ that helps protect you from bacterial infections
  6. Your stomach, which contains acid that kills harmful bacteria and also contains good bacteria that help to fight pathogens and absorb nutrients. (Antibodies secreted by your intestinal cells also help to fight off foreign invaders.)

What does immune health mean?

Every day we come in contact with thousands of different viruses and bacteria. We touch things like a seat on a bus or a cart at the grocery store and then we touch our face. The bugs can then get access to our bodies through our mucosal surfaces (eyes, nose, mouth, or a break in our skin).

The majority of the time the invading foe will be thwarted by our mighty white blood cells, which capture and kill the bugs before they can replicate and enter our bloodstream.

As gross as it sounds, we actually swallow a surprisingly high number of bacteria and pathogens every single day, but most of them die in our saliva or in the acid and healthy bacterial environment of the stomach. Unfortunately, some bugs are stronger than others or have mutated in ways to evade our immune response, and then we are susceptible until our immune system adapts and finds a way to kill the new version of the invader.

Can exercise weaken your immune system?

Well, sure. Like all good things, you can certainly overdo it. Quite often when one of the athletes I coach finishes an Ironman triathlon or a marathon, they get sick a few days later (usually within a 72-hour window). Research has shown that as few as 90 minutes of high-intensity endurance exercise can make you more susceptible to illness for up to three days after the session.

The reason for this is a release of certain hormones that can cause a temporary decrease in the proper function of the immune system. When you perform a hard exercise session, you significantly increase the release of cortisol and adrenaline, your body’s “fight or flight” stress hormones, which can suppress the immune system.

So, if you are already sick with a cold or respiratory infection, high-intensity exercise such as heavy weightlifting or very long exercise such as marathon training can further weaken the immune system.

To help people decide whether to hit the gym or stay in bed, the ACSM offers the following recommendations:

Do exercise moderately if your cold symptoms are confined to your head. If you’re dealing with a runny nose or sore throat, moderate exercise is permissible.

Don’t “sweat out” your illness. This is a potentially dangerous myth, and there is no data to support that exercise during illness helps cure it.

Do stay in bed if your illness is “systemic”—that is, spread beyond your head. Respiratory infections, fever, swollen glands, and extreme aches and pains all indicate that you should rest up, not work out.

In a nutshell (and we’ll cover this again later) if your symptoms are from the neck up, go easy. But if you have a fever or general aches and pains, rest up and let your body get over the illness.


Brock Armstrong’s functional approach to fitness and movement comes from his extensive experience in the fitness, movement and wellness industry working with many leading personal trainers, coaches, and wellness consultants including: Monica Reinagel, Mark Sisson, Katy Bowman, Dave Asprey, Ben Greenfield, and Mark Divine.

Brock is a writer and podcaster for Quick and Dirty Tips and Scientific American. He also co-hosts the podcast Change Academy with Monica Reinagel and has appeared as a guest and co-host on many health and wellness podcasts, such as: Ben Greenfield Fitness, Primal Blueprint, Primal Endurance, Endurance Planet, and The Whole Athlete.

Brock is the founder and owner of Brock Armstrong Coaching, a company that has helped hundreds of people cross the finish line of marathons and triathlons since 2011.


Photo: Unsplash – Coen Van De Broek