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  [22]Covid-19
  How staying indoors affects your immune system
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  A man on his phone in front of a sunset (Credit: Getty Images)
  By Linda Geddes 22nd May 2020
  While staying inside our homes is helping to shield us from the
  pandemic, missing out on time outdoors might alter our risk of catching
  diseases in other ways.
  F

  For the past two months, a sizable chunk of the world’s population has
  been shuttered inside their homes, only stepping out for essential
  supplies. Although this may have reduced our chances of being exposed
  to coronavirus, it may have had a less obvious effect on our immune
  systems by leaving us more vulnerable to other infections.

  Humans evolved on a planet with a 24-hour cycle of light and dark, and
  our bodies are set up to work in partnership with sunlight. One of the
  most obvious examples of this is the production of vitamin D in the
  skin in response to UVB exposure. This daily dose of vitamin D can help
  to strengthen our bones and teeth, but it also has an effect on our
  immune cells.

  Vitamin D enables the macrophages in our lungs – a first line of
  defence against respiratory infections – to spew out an antimicrobial
  peptide called cathelicidin, killing bacteria and viruses directly. It
  also tweaks the [26]activity of other immune cells, such as B and T
  cells, which orchestrate longer-term responses. People with low levels
  of vitamin D are at greater risk of viral respiratory tract infections
  such as influenza.

  You might also like:

  • [27]Why we should all be wearing masks
  • [28]How lockdown is helping bees
  • [29]The ingredients for a longer life

  Researchers are now investigating whether vitamin D supplements could
  even reduce the risk of some of the severe complications associated
  with Covid-19. Earlier this month, Rose Kenny, a gerontologist at
  Trinity College Dublin, and her colleagues published data suggesting
  that European populations with the highest death rates from Covid-19,
  including Spain and Italy, [30]have the lowest levels of vitamin D.
  This may sound counterintuitive, given their sunny climates, but it is
  thought that changes in lifestyle have [31]led people to spend more
  time indoors, which combined with [32]greater use of sunscreen in these
  countries, may be responsible for the lower levels of vitamin D.
  For those without access to a garden it has been hard to get enough
  natural sunlight during the pandemic lockdown (Credit: Getty Images)

  For those without access to a garden it has been hard to get enough
  natural sunlight during the pandemic lockdown (Credit: Getty Images)

  Although other factors may also help to explain the high death rates
  from Covid-19 in these countries, “there’s strong circumstantial
  evidence for an association between vitamin D and the immune pathways
  that we know are implicated in Covid and particularly the severe Covid
  response,” says Kenny. First, vitamin D appears to reduce levels of a
  biochemical that causes inflammation called interleukin-6, which is
  associated with the severe breathing difficulties seen in the disease.
  Vitamin D also changes the availability of the same ACE2 receptor on
  lung cells that Sars-CoV-2 – the virus that causes the disease Covid-19
  – uses to gain entry to these cells and establish an infection. If
  vitamin D has already altered these receptors, then it may make it
  harder for the virus to gain a foothold in the body.

  Although randomised controlled trials are needed to confirm this
  protective effect, Kenny suggests all adults should consider vitamin D
  supplements during the current crisis. But there is a strong argument
  for getting a dose of vitamin D by spending more time outdoors as some
  of the restriction measures in many countries ease – particularly as it
  can bring other benefits.

  Although there is no scientific data proving that regular exercise
  makes us less susceptible to catching Covid-19, various studies have
  suggested that it [33]bolsters our defences against other viral
  infections including influenza and the common cold, as well as
  increasing the immune response to vaccination.

Several studies have suggested that spending a few days in a forest results
in an increase in the number and activity of our natural killer cells

  One explanation for these benefits is stress reduction. “We know that
  people use exercise as a buffer for stress, and it’s very clear that
  high levels of chronic stress are not good for the immune system,” says
  Neil Walsh, who studies the impact of exercise on the immune system at
  Liverpool John Moores University in the UK. “So, if you can reduce your
  stress levels by being active, then that will have a positive impact on
  your health.”

  If you can do that by exercising in a park, woodland or other green
  space, then so much the better.  Numerous studies have found that
  getting outdoors in nature – even to an urban park – lowers people’s
  [34]heart rates and blood pressure, as well as normalising secretion of
  the stress hormone, cortisol. Longer-term, living close to, and
  engaging with, nature is linked to a [35]reduced risk of cardiovascular
  disease, type 2 diabetes and early death.

  Besides higher levels of physical exercise and vitamin D, various
  theories have been put forward to explain these findings. One is that
  spending time outdoors may help to counter stress and loneliness, by
  bringing us into contact with other people. Then there’s the
  [36]Attention Restoration Theory, which suggests that natural patterns
  and movement effortlessly engage our attention, providing our
  overstretched brains with an opportunity to rest and recover.
  Millions of people have not been able to get the exercise and vitamin D
  they would normally get while walking to work or school (Credit: Getty
  Images)

  Millions of people have not been able to get the exercise and vitamin D
  they would normally get while walking to work or school (Credit: Getty
  Images)

  It’s also possible, however, that trees are affecting our immune
  systems more directly – several studies have suggested that spending a
  few days in a forest results in an increase in the number and activity
  of our natural killer cells – immune cells that help to detect and
  destroy viruses and cancer cells – in our blood. Scientists in Japan
  have proposed that the inhalation of substances called phytoncides,
  which are released by trees, may be a contributing factor. These have
  been shown to [37]alter the activity of human natural killer cells when
  they’re grown outside the body, although further work is needed to
  confirm whether inhaling them has a similar effect.

  “In practice, I think these various pathways probably work in synergy,”
  says Catharine Ward Thompson, director of the OPENSpace research centre
  at the University of Edinburgh, and co-author of a World Health
  Organization [38]report on urban green spaces and health. “Phytoncides
  may be important, but you probably need to be fully immersed in the
  natural environment for some time to get these benefits, whereas
  psychological benefits such as relaxation and lowering of stress might
  be easier to obtain.”

Office workers who are exposed to more bright light during the morning by
walking to work find it easier to fall asleep at night

  Getting outdoors can also improve the quality of our sleep. Our time
  shut inside during lockdown could have disrupted our circadian rhythms
  – internally generated, close-to-24-hour cycles in the activity of
  numerous biological processes, including sleep. Our circadian rhythms
  are usually kept synchronised, or entrained, to the time of day when we
  are outdoors through the action of bright light hitting a set of
  light-sensitive cells at the back of the eye. These eye cells
  communicate with a patch of brain tissue called the suprachiasmatic
  nucleus, which functions as the body’s master clock.

  “Indoor light is typically too low to promote entrainment, so if one
  does not go outdoors all week, these rhythms may become disrupted,
  resulting in disturbed sleep,” says Mariana Figueiro at the Lighting
  Research Center in Troy, New York. Her research has shown that office
  workers who are exposed to more bright light during the morning, by
  walking to work, for example, [39]find it easier to fall asleep at
  night, and experience less disrupted sleep, compared to those who are
  exposed to dimmer light. (Read more about [40]why natural light is so
  important for our sleep.)
  While stuck indoors during the pandemic, many people have had to get
  creative with how they get their exercise but getting outside brings
  other benefits too (Credit: Reuters)

  While stuck indoors during the pandemic, many people have had to get
  creative with how they get their exercise but getting outside brings
  other benefits too (Credit: Reuters)

  “Circadian disruption and sleep curtailment have been linked to a
  reduced immune system response,” Figueiro says. “So, while light may
  not have a direct impact on immune function, it can have a strong
  indirect impact via its ability to entrain the circadian system and
  improve sleep.” Exposure to bright light during the morning also has a
  positive impact on people’s mood and may help to guard against
  depression.

  As for how much time you need to spend outdoors to reap these benefits,
  it is difficult to say.  Although morning light is particularly
  important for keeping our circadian rhythms synchronised, optimal
  vitamin D synthesis occurs around noon, when the UVB rays in sunlight
  are at their peak.

  So, if lockdown conditions allow, you should strive to get outdoors at
  least once a day, whilst taking steps to maintain social distancing and
  sunburn. Sunlight and nature are great healers, and they also come for
  free.

  Linda Geddes is the author of Chasing The Sun: The new science of
  sunlight and how it shapes our bodies and minds.

  --

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