Should you work out when you're sick?

What fit bodies need to know during flu season

Exercise strengthens your immune system and helps you ward off wintertime colds and flu. However, certain types of activity can spike levels of cortisol, a stress hormone that suppresses your ability to fight off invading germs. The key is to be strategic with your training to strike the perfect balance.

When those around you are sick

If you’re about to board a germ-ridden airplane or share an office space with a sneezing coworker, consider temporarily limiting your workout duration. A 2014 study found that running at a moderate pace for two hours resulted in a temporary dip in immune function.

Instead, go with a shorter, more intense session. In the same study, a 30-minute hard run had a negligible effect on subjects’ immunity. This may mean that other types of HIIT, whether it's burpees or cycling, similarly do not invite illness.

When you're already sick

When you're not feeling 100 percent, “there’s nothing wrong with doing moderate work,” says Denver-based personal trainer, Jamie Atlas. That could be a relaxed spin or easy strength session, for example. Or, go ahead and take that yoga or barre class you've been wanting to try. “This can be a good time to challenge yourself with something new, rather than something intense.”

In Atlas’s experience, getting active with the sniffles doesn’t worsen symptoms or open the door to additional infections. Science bears him out: According to Thomas Weidner, PhD, a Ball State University kinesiologist who has studied the relationship between exercise and illness, relatively short, easier workouts don’t increase the duration or severity of a basic head cold (his subjects trained for 40 minutes, every other day).

When you should really stay in bed

Atlas draws the line at conditions affecting the lungs and gut which lodge themselves in your core and GI tract and don’t respond well to the additional load of activity. Nausea, diarrhea, cramps, and coughing up phlegm signal that you’re already battling a major invader, and that working out could tank your immune system even further.

Ultimately, your overall energy levels provide the best insight into whether going to the gym will sabotage your ability to return to healthy. “Take stock of your mental state and whether you slept well the night before,” says Atlas. Sometimes, it's better to take the day off to focus on recovery.