Insight | Exercise and immunity: moderate helps, extreme hurts

Exercise and immunity follow a curve, not a straight line. Doing nothing leaves you worse off, regular moderate exercise lowers your risk of getting sick, and very hard or very long efforts push the risk back up for a while.

Dr. Roger Seheult calls this the J-curve. He points to a randomized trial at McMaster University that followed students for six weeks before exams. Students who did not exercise saw depression scores rise sharply. Those who raised their heart rate to about 140 beats per minute for about 20 minutes, three times a week, saw depression, anxiety and inflammatory markers all fall. A high-intensity group, at around 180 beats per minute, improved less on depression, reported the same stress and had higher interferon. He adds that Olympic athletes are more susceptible to getting sick during high-intensity training.

Andrew Huberman reaches the same conclusion from immunology. Sessions of 60 minutes or less at a hard but not all-out effort raise baseline innate immune activity for many hours, perhaps up to 24. Brisk walking for about 60 minutes a day increases T cell, natural killer cell and macrophage activity. But, he says, "it is absolutely not the case that more is better." People who have just run a marathon, or are deep in marathon training, are severely immune compromised. He is not discouraging marathons, only urging people to understand the trade-off.

They differ in emphasis. Seheult's evidence is about intensity, while Huberman's is mostly about duration and piled-up load, and Huberman endorses a 12-minute very hard session. The article's fair reading is that short, occasional hard efforts can fit, while frequent, prolonged or stacked ones cause trouble. They also read inflammation differently: Seheult treats the interferon rise as a cost, while Huberman says a mild inflammatory response is how the immune system gets primed. The article notes neither source presents a trial that counted infections, and the heart rates and time markers are guides, not universal cut-offs.

What to do with it: exercise most days at a moderate level and keep most sessions to an hour or less. Huberman warms up for 5 to 10 minutes, trains for about 50 to 60, is careful beyond 75, avoids two hard sessions in one day and takes one day a week off. Use high intensity sparingly. Never sacrifice sleep for exercise; after a bad night, train early at 25 to 50% less intensity and duration. After long fasted workouts, eat complex carbohydrates and some fruit within about an hour.

Do not train through illness. Whole-body malaise, unexplained fatigue or a throat tickle mean go home, take a hot shower or bath and get to bed early. With symptoms, skip exercise entirely.

Exercise is one layer, not a shield. As Seheult puts it, "It's not a diet. It's a lifestyle. And part of that lifestyle is exercise." Anyone who is immune-compromised, pregnant, has a heart condition or a chronic condition, is returning after illness, or is unsure what intensity is safe should talk with a doctor before changing their training.