You know the moment when you wake up the morning after a really tough workout and realize that while you were sleeping, someone switched your normally functioning body with one that's as stiff as wood and hurts to move an inch? (Thanks, leg day.) Yeah, we're talking about that bittersweet hurts-like-hell experience of DOMS—delayed onset muscle soreness—that you've probably experienced after a particularly grueling workout.
But if you've ever come down with a cold or flu shortly after one of these especially painful recovery periods, you know that the uncomfortable "I'm dying from the inside out" feeling seems to spread directly from your muscles to your nose, lungs, sinuses, and throat. It's like your body is poisoning itself to punish you for putting it through such a tough workout in the first place. (Related: 14 Stages of Being Sore After a Workout)
But is that a real thing? Can you really be so sore that you make yourself sick?
Turns out, there's a well-accepted theory that prolonged, intense exercise results in a short period of weakened immune function, according to a new article published in the Journal of Applied Physiology. It started in the early 1990s with a study by David Nieman, Ph.D., who introduced the "J-shaped curve" suggesting that regular moderate exercise may decrease the risk of upper respiratory infections (aka the common cold), while regular intense exercise may increase the risk of these infections. Because many parts of your immune system change immediately after heavy physical exertion, this "open window" of altered immunity (which may last between three hours and three days) may give bacteria and viruses a chance to strike, according to a 1999 study published in Sports Medicine.
And more recent studies continue to support this idea that a super-tough workout will bog down your stay-healthy system. A study of 10 elite male cyclists found that a long session of intense exercise (in this case, two hours of hard cycling) does temporarily boost some aspects of the immune system response (like certain white blood cell counts), but also temporarily decreases some other variables (like phagocytic activity, the process your body uses to protect itself from infectious and noninfectious environmental particles and to remove unwanted cells), according to a 2010 study published in Exercise Immunology Review. A review of relevant studies published in 2010 also found that moderate exercise may lead to an enhanced immune system and anti-inflammatory response, which improves recovery from respiratory viral infections, while intense exercise may shift the immune response in a way that gives pathogens a better foothold. And if you exercise hard two days in a row, you might see the same type of effect; a study on CrossFitters found that two consecutive days of high-intensity CrossFit workouts actually suppressed normal immune function, according to a 2016 study published in Frontiers in Physiology.
"Exercise in the long term is very good for you: It reduces inflammation throughout your body and makes you in much better shape from a cardiovascular standpoint, a lung standpoint, and an inflammation standpoint," says Purvi Parikh, M.D., an allergist/immunologist with the Allergy & Asthma Network. "But in the short term, right after intense exercise, it will put strain on your body, and you'll have a lot of inflammation in your muscles, your chest, and all over, because it's really strenuous work."
The thing is, while the theory is well-accepted and makes a lot of sense, we still need more research to prove exactly what's going on. After all, you can't exactly put people through a grueling workout and then force them to swap spit with someone crawling with germs in the name of science. "It would be difficult (and unethical) to conduct a study in which people are exposed to infectious agents after exercise," says Jonathan Peake, co-author of the article recently published in the Journal of Applied Physiology.
So while your crazy-tough HIIT workout might be to blame for your gross cold, take it with a grain of salt. You're still going to get tons of benefits from HIIT-style exercise, so you shouldn't ditch it during cold and flu season in the name of staying germ-free. (Plus, those hard workouts are actually more fun.)
Your best bet is to amp up your focus on recovery to even out your risk: "Even without exercise, lack of sleep and stress weaken your immune system and pre-dispose you to getting sick, and if you add a heavy workout on top of that, you're even more vulnerable," says Parikh.
In fact, getting adequate sleep, minimizing psychological stress, consuming a well-balanced diet, avoiding deficiencies of micronutrients (particularly iron, zinc, and vitamins A, D, E, B6 and B12), and eating carbs during prolonged training sessions should all help decrease the negative effects of intense exercise on your immune system, according to a 2013 study published in Limits of Human Endurance. So make sure you're taking care of your body (in addition to crushing your tough workouts) and you'll be just fine.