Just what is “immunity”? Is it a property that gives rise to the immune reaction in the body? Or is it the reaction of the body, when a foreign substance enters, which we know as immunity?

While science is at work to create a vaccine or protocols to control Sars-CoV-2, a marketplace has arisen of substances and foods that claim to promote “resistance” and “immunity”.

The journal, Nutrition, of the Swiss research publishing group, MDPI, carries a paper by Nipith Charoenngam and Michael F Holick, of the Boston University School of Medicine and the department of medicine, Mahidol University, Bangkok, which goes into the mechanism of the immune reaction and how it could be influenced. And, apart from several physiological benefits of adequate Vitamin D, the paper reports a positive indication that the vitamin could protect people from Covid -19.

An important point the paper makes is that with current lifestyles, there is widespread Vitamin D deficiency, the world over. Over 40 per cent of Americans, with 82 per cent Black Americans and 70 per cent Hispanics, the paper says, are Vitamin D deficient. And, apart from the poor bone health that is well known, Vitamin D deficiency is directly associated with a host of conditions, related to the immune system — psoriasis, or eruptions on the skin, Type 1 diabetes, paralysis, rheumatoid arthritis, tuberculosis, sepsis, or blood poisoning, respiratory infection and significantly, Covid-19.

The immune reaction of the body is brought about mainly by white blood cells, or lymphocytes, which arise in the bone marrow. Cells called monocytes lead to bodies called macrophages, which destroy particles or organisms that do not match cells that are native to the body, and this is the innate immune system. Monocytes also give rise to the adaptive immune system, through the dendritic cells, or Antigen Presenting Cells, that identify and hold out the active features of the virus to other cells.

The T helper cells carry this information to B cells, which give rise to antibodies, or agents that can attach to the features of the viruses and deactivate them. In short, there is a well-oiled system of identifying invaders and generating agents that can keep them under control.

The template of antibodies against an infection can stay, as a memory, for very quick action if the invader should come again — what we call acquired immunity. The interesting thing about these different cells of the immune system is that they all have surface features, or receptors, which enable them to attach to Vitamin D.

And then, there is recorded evidence that low levels of Vitamin D are associated with conditions that are the result of tepid immune response to infection, or excessive response, to the body’s own cells, as in “auto-immune” diseases. Vitamin D is thus found both to enable immune cells to act against invaders as well as to restrain their action against the body’s own cells – making it an immunoregulatory hormone.

Vitamin D, in its useful form, is called a hormone because it is the kidneys that convert Vitamin D that the skin generates when exposed to sunlight, or we take as supplements, into the active form that regulates body functions. As for sources of Vitamin D, very few foods are sources — eggs, oily fish do contain the vitamin, but not in great quantities.

The practical source is only the action of UV light from the sun, on the skin. When UV light falls on the skin, there is instantaneous generation of Vitamin D, which gets transported by the bloodstream to the kidneys, where it changes to the active form. Ninety percent of our vitamin D comes from exposure to sunlight, the paper says. A well-known function of Vitamin D is to help absorption of calcium and phosphorus, for healthy bones, and for over a century, Vitamin D supplements have been prescribed to prevent Rickets and bandy legs in children.

But now we understand the mechanism of active Vitamin D and the value of the hormone in other areas of human health. There are statistics that connect several diseases with low Vitamin D levels. The fact that most of the Vitamin D we have comes from sunlight explains different seasonal diseases.

Influenza in the northern hemisphere, for instance, is known to peak during the winter months, when nights are long and daytime sunlight is feeble. It is also known with more waking hours spent indoors, the incidence of bone related ailments has increased.

The paper says that the highest incidence of Type 1 diabetes in the world is in Finland. A likely explanation for this, and the high incidence in all countries that have low sunshine for a long part of the year, is that people do not get the sunshine to form Vitamin D. The explanation is supported by the fact that administering supplements does help control the incidence of T2 diabetes.

The same is the case, the paper says, with multiple sclerosis, a condition that causes paralysis. Patients of inflammatory bowel diseases are found to be consistently Vitamin D deficient. While this could be because these patients cannot retain dietary inputs, a reverse relationship, of low Vitamin D leading to bowel disease, has been found.

As for tuberculosis, the paper mentions Niels Ryberg Finsen, who won the Nobel Prize in 1903 for showing that sunbathing was a remedy. Other instances described are rheumatoid arthritis, sepsis and respiratory illness, influenza, whose incidence increases with Vitamin D deficiency.

In the case of Covid-19, it is still early to establish a connection. But what we do know is that in the US, there are disproportionately more cases of Covid-19 among black Americans and among those who are obese. Now, the incidence of Vitamin D deficiency is the highest, in the US, among black Americans, at 82 per cent. In this category at any rate, there is a clear correlation with Covid-19 and the Vitamin D level.

It is well known that the efficiency of Vitamin D generation from sunlight is the highest in fair skinned or white populations. In fact, it is considered that populations at the higher latitudes, who migrated after homo sapiens first appeared in Africa, have evolved to be fair skinned precisely to get more Vitamin D from the scant sunlight in regions closer to the Poles.

This is then the reason, in addition to low sunlight exposure because of geography and the modern style of working and living in covered spaces, that black Americans are low in Vitamin D. In the case of those who are obese, the explanation is that Vitamin D is fat soluble.

Hence, when the vitamin is generated in the skin or ingested with food, it gets absorbed by fat cells before it reaches the kidneys, to get to the active state. Hence the deficiency — and the correlation with high incidence of Covid-19 again shows a relationship. It would appear that getting adequate, direct sunshine on bare skin, is the way to be free of a large number of illnesses, and one of them is very likely Covid-19.