Different studies showed that people with vitamin D deficiency had more severe COVID-19 symptoms and that vitamin D treatment to hospitalized COVID-19 patients reduced the severity of the disease. (Example, thanks Daniel for the link!)
Sunlight is a major source of vitamin D. But season, time of day, cloud cover, skin tone, and clothing affect sun exposure and therefore vitamin D synthesis. (Source) For instance, sunlight is usually enough during summer: long and sunny days, more time spent outside, wearing skin-revealing clothes. But according to this study done in Manchester, UK, during winter:
Sun exposure levels prevent winter vitamin D deficiency in 95% of healthy white adults and 83% of adolescents, while 32% of the photodermatoses group and >90% of the healthy South Asians were deficient.
So in winter, sunlight is normally enough to avoid vitamin D deficiency (but not “insufficiency”) for white people, whereas people with dark skin require increased oral vitamin D. So could lockdowns increase vitamin D deficiency among white people, and therefore COVID-19 deaths? Especially in countries such as France where it’s forbidden to go out more than 1 hour and more than 1 kilometer away from your home. Provided distances are respected, governments should actually encourage people to go out during the day!
And how come health authorities don’t recommend to take oral vitamin D, especially to dark-skinned people? Vitamin D is cheap and without known side effect, and even in normal times people lack it in winter.
That’s also the opinion of Danish economist Lars Christensen. According to him, 4 factors explain 60% of national variations in COVID deaths, independently from local government responses:
Number of elderly (esp. men) as proportion of population,
Severely overweight proportion of population
Proportion of population with dark skin in the northern hemisphere
Season and climate
And all these factors are linked to vitamin D deficiency!
So make sure you have enough vitamin D this winter :) Here are the best sources of vitamin D:
oily fish – such as salmon, sardines, herring and mackerel,
fortified foods – such as some fat spreads and breakfast cereals