On Easter Sunday, 35-year-old Pensacola, Fla.-based Caleb Carmichael finished his seventh marathon in as many days, an effort to fundraise for two covid-19-related charities. Noble? Absolutely. Healthy? Perhaps not.

During the pandemic, social media has encouraged people to dive into fitness challenges. With extra time on their hands, exercise buffs are taking up running and streaming high-intensity interval training, with the goal of reaching their fittest level ever. But there’s a case to be made for keeping things in check. Moderate levels of exercise benefit the immune system. Going beyond that, however, could weaken it, which is not a good thing with the coronavirus sickening and killing people around the world.  

Intense exercise can harm the immune system

Years of research into the relationship between hard and/or long efforts and their impact on immunity reveal that once you’ve crossed a certain line, you become more susceptible to illness. One of the seminal studies on the topic  came from David Nieman, director of the human performance laboratory at Appalachian State University. By following nearly 2,400 runners at the 1987 Los Angeles Marathon before and after the race, “we were able to identify thresholds of exercise where your immunity begins to decrease,” Nieman says.

That study touched specifically on duration of exercise and found that, for the most part, up until about the 90-minute mark, immunity remains strong. After that point, however, it weakens. Runners in Nieman’s study who completed the marathon — an effort much longer than 90 minutes — became sick in the week following the race at a rate of 5.9 percent higher than those who trained but did not complete the race.

Although not 100 percent certain about why immunity decreases as duration increases, Nieman says that draining the body of its main energy source, glycogen, is probably to blame. “Your brain registers the lack of glycogen, and then you get the negative immune response,” he says.

More evidence that intense physical and mental stress can suppress the immune system came from a study of the Finnish team at the 2018 Winter Olympics, which comprised both endurance and shorter-duration event athletes. It found that 45 percent of the athletes suffered from the common cold during a 21-day observation period surrounding the competition.

Amelia Boone, a 36-year-old attorney and world-champion obstacle course racer from Golden, Colo., has taken this knowledge to heart and is keeping her training in check at the moment. “I have extra time on my hands because I’m not commuting, so it’s tempting to do more,” she says. “More training would be good stress management, too, but it’s just not smart right now.”

Not everyone is an endurance athlete, however, so what about other forms of exercise and their impact on immunity? That’s an ongoing debate, and the most recent research could not form a solid conclusion. However, the key point of agreement is that many factors go into making us susceptible to infection when we exercise intensely. Those factors include anxiety, sleep, nutritional deficits, exposure and travel. The CDC warns that the coronavirus outbreak can be stressful to many people.

Why moderate exercise is the best right now

“Regular exercise boosts immunity, very hard efforts temporarily lower immunity, and chronic overtraining lowers immunity in more enduring ways,” says Brad Stulberg, a California-based performance coach and author of “The Passion Paradox.” “Right now, you want to keep your exercise program in the first two buckets, and ideally, the first.”

Another reason to prioritize moderate exercise, Stulberg says, is that it will lessen the risk of injury at a time when physical therapists, massage therapists and other practitioners are unavailable for hands-on visits.

Nieman sees exercise along a risk-benefit continuum. “It’s important to find the sweet spot,” he says about the pandemic. “I recommend getting out every day and engaging in a moderate level of activity, but don’t take it to a point where you are chronically tired.”

Also, he says, “you may have the virus and not know it. If you exercise hard, that’s not a good strategy.” And if you have any symptoms at all, he warns: “Do not exercise.” 

Finding that sweet spot

Nick Bracciante, a physical therapist and CrossFit coach in West Chester, Pa., continues to train clients through remote programming. “I have an ongoing conversation with clients about tracking their intensity,” he says. “I like them to use a perceived level of exertion, and then we monitor that over time to recognize trends.”

If he sees too many high-level workouts in a client’s log, Bracciante reels them back in. “With newer clients, in particular, I try to have them follow a three, two, one rule,” he explains. “In every three-day cycle, two of them should be moderately intense workouts, followed by a day of rest.”

To ensure that rest is adequate, Bracciante also has his clients keep sleep journals. “If I notice someone is only getting six hours of sleep a night, that’s not enough to offset the exercise,” he says.  

Stulberg suggests “sticking with simple movements, like squats, push-ups, lunges and plank” to avoid injury. “It’s important to remember that every bit counts right now. A brisk walk in an uncrowded space will get you 99 percent of the way there.”

For Boone, moderation means monitoring her heart rate to ensure she’s in an easy range and adding in low-level exercise such as hiking, walking and riding her Elliptigo — a mobile version of an elliptical machine — to round things out. “This is where I am for the duration of the pandemic,” she says. “I’m trying to impress upon people that this situation has impact on our bodies and stress levels, so getting out to exercise at an easy pace is what’s important.”

For seven-marathon-runner Carmichael, the next few days to a week will look like a lot of sleep and very short, low-intensity exercise. His efforts have raised somewhere in the range of $6,000 for a local food pantry, which he says is worth his calculated risk. “I needed to do something that would be crazy enough to get people’s attention,” he says. “I ran at a very reserved pace throughout the effort, and I was very conscious of getting enough sleep and the right nutrition.” But, he adds, “I’ll definitely be taking care of myself from here on out.”

Amanda Loudin is a Maryland-based health and fitness writer.