A Further Link Between Vitamin D Levels and Covid Severity

Source: Susanne Pälmer /Pixabay

A previous post on the “striking link” between vitamin D levels and the severity of lung infections, including those by COVD–19 and especially the Omicron variant, was surprisingly popular, with more than 1 million views in a day.

The main study cited in that post found that half of vitamin-deficient people developed severe, life-threatening illness with Covid, compared to fewer than 10 percent of those who had normal levels before becoming sick.

How to explain these results? First, consider the difference between innate and acquired immunity. The first is inborn, written into our genes. It confers lifelong protection against specific pathogens that over eons of evolution we have been most likely to encounter. When exposed to them the immune system is ready to fight. Acquired immunity, by contrast, is adaptive and develops after exposure to novel antigens. Some are common, others rare, yet others unique. All varieties of COVID are instances of acquired antigens.

But why the popular interest now in vitamin D and infection? Broadly, because it affects multiple organs and the immune system. It is synthesized in the skin in response to sun exposure, specifically ultraviolet-B wavelengths. Artificial light is inadequate. It is therefore easy to see how the pandemic kept many people indoors for over two years, causing a considerable number to fall below the threshold for adequate vitamin levels, which conventionally has been regarded as 20 nanograms per milliliter of blood.

New data, however, advise a minimum of 50 nanograms per milliliter, with levels below that associated with weakened immunity in patients hospitalized with COVID. The vitamin D receptor is expressed in almost all cell types, where it activates between 200 and 500 genes, many related to immune function.

While the vitamin’s synthesis takes place in the skin, excessive sun exposure increases aging and one’s risk of skin cancer. Likewise, one should never take large amounts in the mistaken belief that “if a little is good, then more is better": Vitamin D is toxic when taken in excess.

Which brings me to the main question readers have asked: "If I do take supplements, then what is the optimal daily dose?" After 20 years of intense study, the consensus is 800 international units (IU) a day. Those at risk of influenza, COVID–19, or both might consider taking 10,000 IU daily for two weeks to rapidly raise their concentration, followed by 5000 IU daily until achieving blood levels of 40–60 nanograms/ml. Avoid guessing, though: Have your levels checked by your primary physician.

Confounding and frequently overlooked factors are geographic location and time of year, both of which determine how much sunlight one receives at a given latitude. Studies have compared infection rates in Minnesota, the United Kingdom, and Finland—all northern countries. In each, respiratory infections are more common during the winter months that coincide with fewer hours of sunlight and thus lower intrinsic vitamin D levels in the population.

Immunity depends on D metabolites to work properly; the more sun one gets, the more Vitamin D is produced. Related to this, the seasonality of the pandemic is pronounced, most severe when the sun is low in the sky. Cold weather keeps people indoors and heavily clothed. When they do venture outdoors their sun exposure is reduced relative to what they receive during warmer months.

After the late-winter 2020 outbreak in Minnesota, hospitalizations and deaths declined. As residents transitioned to a warm and sunny summer, they spent more time outdoors in the sunshine wearing fewer clothes. Then, as winter approached, they got less sun exposure. Vitamin D levels in the population diminished, with a consequent weakening of innate immunity—and an increase in illness.

These kinds of analyses suggest that the pandemic might have been substantially mitigated if officials had recognized the importance of maintaining Vitamin D levels during the winter. This is not idle speculation: Worldometers.info publishes a deaths-per-million-of-population statistic that yields interesting insights. As of November 2021, countries in higher latitudes had higher death-rates-per-million due to less sun exposure during the winter:

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Poland: 2,128

UK: 2,102

France: 1,809

Minnesota: 1,645

Source: Sumankley / Pixabay

Yet there is a paradox: Finland, one of the most northern countries in the world, has a dramatically low death rate of 225 per million. How could they achieve this? If the innate immunity/vitamin D model is sound, then Finland should have experienced high levels of COVID-19, especially in the winter. But the low death rate implies that Finns are relatively immune.

Besides Finland’s program of supplementation, residents of Scandinavian countries generally consume a lot of fatty fish such as herring, salmon, and trout, which are natural sources of dietary vitamin D. A four-ounce serving of salmon on average contains 1,000 IU. From my own experience, I know that cooked, smoked, and pickled fish are served at nearly every meal in Norway.