It’s now two weeks since the lockdown started in the UK, on March 23. One of the only reasons we can leave home is for a daily dose of exercise.

But before you lace up your trainers to join the newly enthused hordes of runners now pounding the pavements, it’s worth considering just how much exercise we should be doing during this pandemic, to maximise our chances of fighting the virus.

Why? Because multiple studies show that exercise can influence the body’s immune system. Regular bouts of moderate-intensity exercise can have a positive effect by reducing our susceptibility to infection. By contrast, too much exercise can have the opposite effect, leaving us more vulnerable.

The ‘immunity-exercise paradox’

“Regular moderate exercise ensures that our lymph, which is the circulatory system of the immune cells, keeps flowing,” explains Dr Jenna Macciochi, an immunologist at the University of Sussex. “This helps move immune cells around the body, to carry out their surveillance function, looking out for anything untoward. That being said, the relationship between exercise and immunity is a complicated one. It’s a double-edged sword, dubbed ‘the immunity–exercise paradox’. There is a moment when a ‘healthy’ level of exercise shifts into ‘too much’ and can backfire on our health.”

After a marathon, for example, a 1990 study showed 13 per cent of finishers developed an upper respiratory infection, compared to only two per cent for runners who trained for the marathon but didn’t run it.

Too much high-intensity exercise, or exercising again before we’ve sufficiently recovered, can trigger stress chemicals, like cortisol, says Dr Macciochi, which suppress the immune system, just like psychological stress. On the other hand, a sensible amount of exercise can improve our sleep and help control our weight, both of which can help immunity.

Exercise will also help prevent muscle loss, which can creep up on us from middle-age. This is important for our immunity because our thymus gland – which produces immune system master controllers called T cells – starts shrinking from our thirties. “This weakens the immune system as we age,” says Dr Macciochi. “But moving our muscles produces chemicals that can help mitigate this and keep our thymus gland rejuvenated.”

Exercise and Covid-19

While there isn’t, as yet, any research on exercise and Covid-19, sports scientists Tamara Hew-Butler, Associate Professor of Exercise and Sports Science, and Professor Marian Fahlman at Wayne State University in Detroit, have reviewed the research on how exercise affects the immune response to the flu, to offer some practical tips on the ‘just right’ amount.

Writing on The Conversation, the scientists cite a large study looking at exercise frequency, in 24,656 Chinese adults, who died during the 1998 Hong Kong flu epidemic.

“This study showed that people who did no exercise at all or too much exercise – over five days of exercise per week – were at greatest risk of dying of flu (as opposed to other causes) compared with people who exercised moderately,” say the researchers.

No one should exercise if they have flu-like or Covid-19 symptoms, but what if we’re already infected, but not yet showing symptoms? Will exercise be helpful, or harmful at this point? “Since you’re in the incubation period, you want to strengthen your immune system as much as possible,” says Professor Fahlman. “Assuming the coronavirus acts like other viruses, moderate-intensity exercise is one way to do that. However, it is not a panacea and it does not operate in isolation. You also need to eat right, sleep well and handle stress.”

So what is the ideal exercise ‘sweet spot’ that we should be aiming for, when it comes to getting our immune system fired up and ready to fight?

Based on their analysis of flu research, Hew-Butler and Prof Fahlman suggest 20-45 minutes of mild to moderate exercise up to three times a week. For exercise to be considered moderate, you should be aiming for a perceived exertion of seven, on a scale of one to ten, and should still be able to talk, even if you are running, says Hew-Butler. However, the number of exercise sessions you can safely do will likely depend on how much exercise you are used to. If you’re already a dedicated exerciser you can probably do more and stay healthy. However, we should be striving to maintain (not gain) strength or fitness during this quarantine period.

“It’s not cut and dried,” says Hew-Butler. “The important thing is to take time for recovery.” Though you can still do something light, like walking on rest days.

You should certainly avoid exercising to exhaustion, whether that’s through long exercise sessions, or through extremely high-intensity bursts, as Prof Fahlman’s research shows just three 30 seconds all-out sprints are enough to decrease secretory IgA whose job it is to neutralise viruses.

But what if we are new to exercise, is it a good idea to start a programme now? It’s certainly good idea to get moving, says Hew-Butler. Just don’t overdo it or ramp up too quickly.

The type of exercise you do, whether that’s an online Barrecore class, joining Joe Wicks on YouTube or a fast walk around the block, matters less than simply moving, says Hew-Butler. “The important thing is to do something,” she says. Just don’t start training for a marathon.

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