Can vitamin D supplements prevent or cure COVID-19?

By Ignacio J. Molina Pineda de las Infantas, University of Granada

Social media has widely echoed the possibility that vitamin D administration could be a fundamental factor in the treatment and prevention of COVID-19. These messages have been promoted by some Italian and Spanish professionals, based on insufficiently contrasted personal observations. But what is the truth in it?

What is vitamin D?

Vitamins are essential substances for the body's functions, but we are not able to synthesize them and therefore they must be obtained from the diet. Vitamin D belongs to the group of fat-soluble vitamins (soluble in fats) and, consequently, its absorption will be similar to that of other fats.

The precursor to vitamin D is present in oily fish, shellfish, eggs and dairy, although it can also be produced on the skin after sun exposure. This (inactive) molecule must be transformed in the liver to 25-hydroxy-vitamin D, which is also inactive. That implies that you still have to undergo a second transformation in the kidney to 1,25-dihydroxy-vitamin D, which is the one that is finally active. Vitamin D is essential for calcium absorption mechanisms to function properly and for this mineral to bind to the bones.

Vitamin D and the immune system

Beyond its important role in bone metabolism, vitamin D is highly relevant in the immune system. Vitamin D deficiency, especially during the childhood stage, predisposes to the development of autoimmune diseases, such as multiple sclerosis, rheumatoid arthritis or disseminated lupus erythematosus.

Why? On the one hand, because this vitamin decreases the function of B and T lymphocytes, which are responsible for specific immunity, including the decrease of proinflammatory mediators of interest in the disease. But, simultaneously, it enhances the activity of nonspecific immunity. Therefore, maintaining correct levels of vitamin D contributes to the correct balance and functioning of the immune system.

Despite our sunny weather, vitamin D levels in Spaniards are often low, especially in winter. Does that mean that we should supplement our diet with vitamin D to protect ourselves from respiratory diseases?

Vitamin D supplements protect moderately from respiratory infections

Several clinical trials have been conducted to answer this question. The overall conclusion is that vitamin D supplementation protects moderately from respiratory infections. With an important nuance: it does it only in those people who started from a deep deficiency of vitamin D and who received this supplement at low doses and continuously.

That means that not all people would benefit from this protective factor. Therefore, a general supplementation with vitamin D would not be justified. Furthermore, if high levels of the vitamin were reached, they could have adverse effects.

Vitamin D deficiency in critically ill patients

Critically ill patients frequently have very low levels of vitamin D, so some time ago it was proposed to administer it to patients admitted to Intensive Care Units, since this deficiency was associated with cases of worse prognosis and higher mortality. The results of the clinical trials that were carried out were disappointing, since they showed that supplementation did not alter the evolution of these patients. Nor does its mortality rate.

Another very recent study explains why this happens. It turns out that patients supplemented with vitamin D raise the blood concentration of 25-hydroxy-vitamin D (which, remember, is inactive). But they are unable to increase the levels of 1,25-dihydroxy-vitamin D, which is actually active. Consequently, neither bone metabolism nor immunity are altered.

The conclusion is that it is useless to administer the inactive vitamin to ICU patients if it cannot be transformed in the kidney, for reasons that we do not yet know but that may have to do with an adaptive response to the general state of critical gravity. Bad news, then.

Vitamin D and COVID-19

Despite the fact that it has been assumed that vitamin D is decreased in patients with COVID-19, the reality is that this fact has not been reliably included in any scientific publication to date. But, even if it were confirmed and indeed it was, it is to be expected that the patients with COVID-19 would have the same characteristics as the critically-ill patients previously mentioned. In other words, its massive supplementation probably would not improve clinical evolution either.

And what about its consumption to prevent infection? We must remember that its protective effect would be linked to specific cases in which there was a significant prior deficiency. Those patients would need continued administration at low doses. Therefore, the best prevention is to maintain our optimal levels naturally, through a varied diet and moderate exposure to the sun, especially in winter.

The danger of social networks

The COVID-19 pandemic is the first we have experienced with social networks in full swing. This has made isolated clinical observations have had a profound impact on the population, who are anxiously searching for effective treatments, including clearly fraudulent miracle drugs.

The dissemination of new therapeutic approaches must be examined, contrasted and validated by the scientific community, and not by social networks in three-minute videos. Even when they are promoted by well-meaning professionals. The potential unwanted effects of self-medication, including vitamins, put the population at unnecessary risk.

Ignacio J. Molina Pineda de las Infantas, Professor of Immunology, Center for Biomedical Research, University of Granada

This article was originally published in The Conversation. Read the original.