Resist infection with moderate exercise (but know when to quit)
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[A new study] suggests that being physically active makes you less vulnerable to getting sick. “Our data show that physically active people have a 40-50% reduction in the number of days they’re ill with acute respiratory infections,” says David Nieman, director of the Appalachian State University Human Performance Lab.
But more is not better. Last year, Nieman published a review that he says indicates “beyond a doubt” that engaging in sustained and prolonged high-intensity exercise (think a long, hard run or multihour endurance event) provokes an increase in stress hormones, inflammation, and oxidative stress. Usually this effect is transient, and you can “snap back and carry on,” he says. But some people are more susceptible to that stress than others, and if you’re under mental or emotional stress, eating poorly, or not sleeping enough, you will become even more vulnerable.
Exercise in the Goldilocks zone is probably genuinely protective against infection. What the most active people often don’t realize is that intense exercise might actually undermine your health in the short term — it’s stressful, and monopolizes biological resources while you recover.
When there’s a novel virus like Covid-19 and no vaccine or cure available, athletes—yes, athletes—tend to be in the vulnerable category as well.
So moderate exercise, but with a caveat: any significant workout during the incubation period of a virus may be equivalent to overdoing it, and eliminate the benefit of similar workouts in the weeks before. This is during a phase of the infection when symptoms may be nonexistent or very subtle, just a vague feeling of being run-down. Most active people will push through that feeling, knowing full well that it often doesn’t amount to anything. But the stakes are higher during this pandemic: if you go out for a run and you just aren’t feeling it, don’t try very hard to push through. If it still feels like a slog, just turn around and go home. If it's a false alarm, great — you'll be ready for another try soon enough. But if you actually are incubating SARS-CoV-2 (or any other virus), then giving up on the run was probably wise.
Who disagrees and why? Is it really “beyond a doubt” that extreme exercise puts us at risk? Dr. Derek Griffin points out that “one says ‘beyond doubt’ and others say different.” There’s a good scientific concensus that extremes of exertion can compromise immunity, the topic is actively debated, and Dr. Nieman is on one side of that debate (see Campbell, Simpson).