Should you exercise if you're sick?

You've set a goal to hit the gym five days a week. But on day five, you wake up feeling under the weather. Should you work out anyway or stay in bed and recover? 

That depends on the illness, according to Dr. Michael Jonesco, a sports and internal medicine specialist at The Ohio State University Wexner Medical Center.

"I rely on the golden rule of the 'neck check,'" Jonesco told Live Science. "That means any [illness] at the neck and above is usually a general guideline that it's probably safe to push through."

For example, if you have a sore throat, a runny nose, a headache or an ear infection — but no fever — you're probably fine to do your workout in most cases. 

Related: Why do you sometimes gain weight after exercising?

But for an illness with symptoms below the neck, you'll probably want to stay home. 

These include "things like bad gastrointestinal symptoms, significant myalgias [muscle pain] or body aches, even a fever, which is a sign of systemic illness — you know, that your whole body is mounting this inflammatory response to try to heal a systemic bug," Jonesco said. "These are all things we say, look, probably [take] 48 hours of rest and reassess."

The reason this rule of thumb works is that most problems above the neck don't involve the heart and lungs. "That's really what we're trying to protect," Jonesco said. 

Even digestive distress can tax these organs because it can lead to dehydration, which stresses the heart. (Besides, let's face it: If you're vomiting and running to the bathroom every five minutes, your workout is probably not your first priority.)

Of course, the neck check is more of a guideline than a hard-and-fast rule. If you have nasal congestion that makes it hard to breathe, for example, that still might be a reason to skip your workout.

So what's the harm in exercising when you're sick? Besides the fact that you might feel miserable, it can make you sicker. Studies have shown that running a marathon, for example, suppresses the immune system and puts people at higher risk of infection for up to three days afterward. 

Although your regular workout probably isn't a full marathon, exercising while you're sick can put similar stress on your system. "Your body is so busy putting energy in those systems required to exercise that it has to take that energy from other systems," Jonesco said.

Even if you don't make yourself sicker, you're likely not getting the exercise benefits you're after in the first place. The calories you burn are likely to come from a breakdown of your muscles, for one thing. Plus, the main rewards of exercise come from recovery, and it's harder to recover from a workout when you're sick. 

"You're really not seeing a benefit, because all you're really trying to do is survive that individual session," Jonesco said. "You're not really training the body to a point where it can recuperate adequately.

"When in doubt, probably sit it out," he added. "If you're really unsure if you should exercise, your body is probably telling you it's not ready."

But if you've listened to your body for a few days and you find you're feeling better, that's still not the time to go all out on your gym session. Jonesco recommended taking it day by day. 

"Don't expect to go back to your prior level," Jonesco said. "It's going to take time to rehydrate, kind of refuel the body, the glycogen stores, the energy stores." (Glycogen is the sugary substance that your muscles use to store energy.)

"So, on your first day back, I usually suggest starting at about half of the intensity of your typical workout, knowing that it's going to take you a day or two to kind of get back into the flow of things," Jonesco said. "And then, on the next day back, you can increase that to about 75% and then go as tolerated."

Originally published on Live Science.