Given the current world climate, boosting the immune system is a top priority for many people. In fact, immunity depends on many factors. Some factors are within your control, while others are beyond your control. However, exercise does play a role in immunity, so it’s worth understanding how your exercise efforts affect your ability to fight infection. Here, experts explain when exercise can be helpful and when it can be harmful, plus how to exercise safely if you’re worried about immunity.

“Exercise, including cardiovascular as well as resistance training, helps strengthen the immune system and counteract the effects of immune decline, the gradual deterioration of the immune system,” explains Tom Holland, an exercise physiologist, sports nutritionist and author of the Micro Workout Program.

But how exactly does this work? Holland explains that in the long term, exercise mobilizes T-cells, a type of white blood cell that helps protect the body from infection.” This holds true for both moderate-intensity and high-intensity cardiovascular exercise,” he added.

Evidence also suggests that obesity may lead to immune suppression and increased susceptibility to disease, says Aaron Brown, a certified personal trainer and research assistant in Ultimate Performance. But research suggests that regular exercise can help reverse some of these effects, even without weight loss. So it’s likely that everyone’s immune system could benefit from some exercise, regardless of their weight.

While positive changes caused by exercise have been observed at the cellular level, there is also strong epidemiological evidence, or evidence observed at the population level, that exercise is a good thing for our immune system.

“As humans, our susceptibility to disease follows a J-shaped curve,” explains Dr. Todd Buckingham, an exercise physiologist at the Mary’s Free Bed Sports Rehabilitation Performance Laboratory. This means that people with no (or very little) physical activity have a moderate risk of upper respiratory infections, which is how you classify COVID-19. Buckingham explains that people with moderate physical activity have a low risk of upper respiratory infections, about 40 percent less than those with low/no activity.

The end of the aforementioned J-curve involves those with “exhaustive” levels of physical activity, Buckingham says, for whom the risk of upper respiratory infection is high – about 50 percent higher than for those with low levels of physical activity.” Because COVID-19 is a respiratory disease, it’s especially important for athletes (and non-athletes) to focus on this issue now.”

Holland agrees, noting that really high-intensity and especially prolonged aerobic exercise – the activities marathoners and triathletes perform while training – can adversely affect the immune system.” These athletes often suffer from a mild cold or sore throat during the progressive phase, which is usually due to the long workouts and training volume,” he adds.

This may be due in part to something known as the hormesis effect.” The hormesis theory suggests that there is an appropriate dose of stress that our physiology actually adapts to in the medium to long term to protect us from future bouts of stress,” explains Brown.” Too little stress and we won’t be able to adapt and progress from the sport; too much training stress and we may end up not being able to recover, get injured, suffer pain, or even see our performance suffer.” So, there is a range of “just enough” exercise, and we don’t want to go too much over or under that range.

There’s also some evidence, Brown says, that individual sessions of high-intensity exercise suppress the immune system.” The production of immune cells called lymphocytes is down-regulated, and other hormone and protein messengers in our bodies seem to be negatively affected.” In the 1980s and 1990s, he says, it was thought that this temporary suppression of the immune system created a window of opportunity for increased susceptibility to disease.

Newer research suggests that this may not be the case, though. Instead of lymphocytes being down-regulated, it may just be that they’ve moved to a location where, for example, to the lungs are expected and ready to “do battle” in the face of infection, notes Dr. Tracey Evans, a science writer and medical researcher. But this research is nascent, so it’s hard to confidently make recommendations based on it.

In any case, being unlikely to exercise too vigorously is something most people need to worry about. Holland explains that even if an at-home HIIT program is popular, the vast majority of people don’t really exercise at these higher intensities.” That said, for those who do push their cardiovascular workouts into the anaerobic ‘red zone’, it’s best to endure a few weeks of these unique times while continuing to exercise.”

If you’re not exercising, now is a good time to start.
“With many states enacting stay-at-home orders, it gives us the opportunity to spend more time exercising,” says Buckingham.” Don’t worry about the gyms, pools and fitness centers closing. Just look around you, anything can be used for exercise.”

If you’re already exercising, relax.
“Now is the time to err on the side of caution,” says Holland. When in doubt, he suggests it’s easier to focus on cardiovascular endurance – but don’t overdo it.LISS workouts are a great option, he adds.

If you’re training strenuously, back off the intensity.
Buckingham agrees, pointing out that if you’re a regular exerciser who’s pushing the limits every day, now is the time to back off your training.” With all races and events through May (and possibly further) postponed or cancelled, there’s no need to do extremely hard training and dig your immune system into a hole right now. Continue your daily workout routine, but lower the intensity,” he advises.

“Exercise 3-5 days a week for 30-60 minutes a day while keeping your heart rate less than 75% of your maximum (approximate your maximum heart rate by subtracting 220 from your age if you don’t know what it is) and you’ll get the most benefit,” says Buckingham.” That way, you’re doing enough to boost your immune function and reduce your risk of infection without doing too much to put yourself at increased risk.”

Evans says, “If you feel like you’re battling something, the safest route is to sit and exercise.” This can do more harm than good, and it can have serious consequences.”