You are familiar with that moment when you awaken the morning following a truly demanding workout and realize that while you were asleep, someone exchanged your typically functional body with one that is as inflexible as timber and causes pain with even the slightest movement? Yes, this refers to that bittersweet excruciating experience known as DOMS — delayed onset muscle soreness — that you have likely encountered after an exceptionally grueling workout.
However, if you have ever developed a cold or flu shortly after one of these particularly agonizing recovery periods, you are aware that the uncomfortable sensation of “dying from the inside out” appears to spread directly from your muscles to your nose, lungs, sinuses, and throat. It’s as if your body is poisoning itself to penalize you for subjecting it to such a demanding workout in the first place. But is this phenomenon genuine? Can you truly be in so much pain that you make yourself ill?
As it turns out, there is a widely accepted theory that prolonged, intense exercise leads to a brief period of weakened immune function, as stated in an article published in the Journal of Applied Physiology. This theory originated in the early 1990s with a study conducted by David Nieman, Ph.D., who introduced the concept of the “J-shaped curve.” This suggests that regular moderate exercise may decrease the risk of upper respiratory infections (commonly known as the common cold), while regular intense exercise may increase the risk of these infections. Since several components of your immune system undergo immediate changes after intense physical exertion, this “open window” of altered immunity (which can last between three hours and three days) may provide bacteria and viruses with an opportunity to attack, according to a 1999 study published in Sports Medicine.
Furthermore, more recent studies continue to support the notion that an extremely challenging workout can burden your immune system’s ability to keep you healthy. A study involving 10 elite male cyclists discovered that a lengthy session of intense exercise (in this case, two hours of rigorous cycling) does temporarily enhance certain aspects of the immune system response (such as certain white blood cell counts), but also temporarily decreases other variables (such as phagocytic activity – the process by which your body defends itself against infectious and noninfectious particles in the environment and eliminates unwanted cells), according to a study published in Exercise Immunology Review. A review of pertinent studies published in 2010 also revealed that moderate exercise may result in an improved immune system and anti-inflammatory response, which aids in recovering from respiratory viral infections, while intense exercise may alter the immune response in a manner that favors the proliferation of pathogens.
And if you work out intensively for two consecutive days, you may observe a similar outcome; CrossFitters who engaged in back-to-back days of vigorous CrossFit exercises effectively inhibited their typical immune response, as stated in a research study published in Frontiers in Physiology.
Exercise in the extended term is highly beneficial for you: It diminishes inflammation throughout your body and puts you in much better condition from a cardiovascular perspective, a lung perspective, and an inflammation perspective,” says Purvi Parikh, M.D., an allergy and immunology specialist at NYU Langone Health. “But in the short term, immediately after intense exercise, it will exert pressure on your body, and you’ll experience a lot of inflammation in your muscles, your chest, and all over because it’s truly arduous labor, she explains.
The point is, while the theory is widely accepted and makes a lot of sense, there still needs to be more research to prove exactly what’s happening. After all, you can’t precisely subject people to a grueling workout and then compel them to exchange saliva with someone infested with germs in the name of science. “It would be challenging (and unethical) to conduct a study in which individuals are exposed to infectious agents after exercise,” says Jonathan Peake, co-author of the article published in the Journal of Applied Physiology.
So while your super tough HIIT workout might be responsible for your repulsive cold, take it with a grain of salt. You’re still going to receive plenty of advantages from HIIT-style exercise, so you shouldn’t completely abandon it in the pursuit of a germ-free existence.
Your greatest option is to intensify your emphasis on recovery to equalize your risk: “Even without exercise, lack of sleep and stress weaken your immune system and predispose you to becoming ill, and if you add a strenuous workout on top of that, you’re even more vulnerable,” says Dr. Parikh. (Here are just a few of the advantages of recovery, plus how to maximize it.)
In fact, obtaining ample sleep, minimizing psychological stress, consuming a well-rounded diet, avoiding deficiencies of micronutrients (particularly iron, zinc, and vitamins A, D, E, B6, and B12), and consuming carbohydrates during prolonged training sessions should all aid in reducing the adverse effects of intense exercise on your immune system, according to a 2013 study published in Limits of Human Endurance. So make sure you’re caring for your body (in addition to conquering your tough workouts) and you’ll be just fine.
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