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 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 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.
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, have the lowest levels of vitamin D.
This may sound counterintuitive, given their sunny climates, but people with darker skin need more sun exposure to make the same amount of vitamin D as people with paler skin.
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.
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 bolsters our defences against other viral infections including influenza and the common cold, as well as increasing the immune response to vaccination.
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.”
Numerous studies have found that getting outdoors in nature lowers people’s 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 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 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.
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.
Getting outdoors can also improve the quality of our sleep. Our time shut inside could disrupt 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, find it easier to fall asleep at night, and experience less disrupted sleep, compared to those who are exposed to dimmer light.