Berlin - Observational studies in Europe and the United States have found that Covid-19 patients were more likely to have vitamin D deficiency. The finding sparked a discussion on whether taking the "sun vitamin" as a precaution could reduce the risk of corona infection. Some experts believe this is the case and recommend taking a supplement containting a moderate dose of the vitamin, especially in winter. Other studies are looking into whether vitamin D supplements can mitigate the effects of Covid-19. The data on this is much less clear. What is clear, however, is that taking high doses of vitamin D as a prophylaxis is probably not a good idea. Here, we try to answer common questions about vitamin D and corona:

Where does the body get the vitamin D it needs?

Vitamin D is a hormone precursor. Unlike other vitamins, the body can produce vitamin D itself - through sunlight or, more precisely, UV radiation of certain wavelengths (UVB). It is important to spend time outdoors. Vitamin D is indispensable for healthy bones. About 10 per cent of the required vitamin D can be taken in through diet - for example with eel, mackerel, herring, salmon, offal, mushrooms or eggs.

Why do deficiencies often occur in winter?

Because of Germany's geographic location, sufficient UV-B radiation is only available from April to September between 10am and 3pm, provided that the head and forearms are exposed to the sun without sunscreen for about 10 to 15 minutes a day. According to the Robert Koch Institute (RKI), during this time the body is not only able to cover acute needs, but can also build up vitamin D reserves in fat and muscle tissue for the winter.

What diseases are associated with vitamin D?

According to the RKI, links between vitamin D levels and various chronic diseases are suspected but have yet to be proven. There are indications that vitamin D may prevent respiratory diseases such as pneumonia and is also important for cardiovascular health.

Similarly, some studies suggest that it may reduce the risk of diabetes and some cancers. However, these studies did not find overwelming evidence that people taking the vitamin fared better than control groups.

"Since the data situation is not clear, no clear recommendations can be made so far with regard to vitamin D treatment for these areas of application," says Matthias Weber, head of the endocrinology and metabolic diseases department at Mainz University Medical Centre and spokesperson for the German Society for Endocrinology. In addition, the metabolic pathway of vitamin D in the body varies greatly from person to person: this might explain the sometimes contradictory study results.

Vitamin D plays a protective role in infections because it has a modulating function in the immune system. It enters the body's cells via a binding site, where it can attach itself to the DNA and act as a switch for certain genes, thus enabling the formation of antibodies, for example, but also regulating excessive immune reactions.

Can taking vitamin D protect against Covid-19?

Several observational studies in hospitals have shown that Covid-19 patients had low vitamin D levels. It has also been reported that people with low vitamin D levels tend to die from the disease more often than others.

The German Society for Nutrition says a link between vitamin D deficiency and increased corona risks is probable. However, the results of previous studies were not sufficient to prove a clear cause and effect, hence taking vitamin D supplements cannot be recommended across the board. The Federal Institute for Risk Assessment (BfR) is also currently not aware of any studies that prove that taking vitamin D supplements protects against corona infection or disease.

In a recent preprint study, a research team led by Xavier Nogués writes that administering vitamin D can reduce Covid-19 death rates by 60 per cent. Yet Martin Smollich, a pharmacologist and professor at the Institute of Nutritional Medicine at the University Hospital of Schleswig-Holstein in Lübeck, has made several posts on Twitter pointing out flaws in the statistical calculations of the study, which has not yet been peer-reviewed.

In the study, 551 randomly selected corona patients received either calcifediol, a hormone precursor in vitamin D metabolism, or no vitamin D (control group). The result: 36 patients (6.5 per cent) in the vitamin D group died, while 57 (15 per cent) in the control group died. However, Smollich points out that more patients with pre-existing conditions such as high blood pressure and diabetes and a correspondingly poor prognosis were highly significantly overrepresented in the second group. The patients were not randomly distributed. Smollich concedes that the direct risks associated with ineffective vitamin D intake are low. "The main risk is indirect: those who believe in vitamin D may neglect effective protective measures," Smollich concludes.

In an interview with dpa news agency, he said a causal relationship between vitamin D deficiency and Covid-19 could not be proven. Rather, low levels of vitamin D measured upon admission to hospital could be the consequence (and not the cause) of the Covid-19 disease. During an acute, severe infection, vitamin D levels drop drastically for a short time. In addition, a vitamin D deficiency "occurs more frequently than average in diseases and living conditions that increase the risk of Covid 19, i.e. old age, obesity or type 2 diabetes".

This is also the view of Matthias Weber. Clinical studies on the influence of vitamin D on Covid-19 are mainly observational studies, he says. "They only show that two events occur together, but not that one is the cause of the other," Weber says. For example, patients suffering from other diseases also often have low vitamin D levels. "There are many plausible explanations that a serious illness can cause low vitamin D levels." This also explains why administering vitamin D does not bring success in treating this disease, he says, adding that in order to formulate a recommendation for vitamin D administration, further large placebo-controlled clinical studies are needed.

Who is vulnerable to vitamin D deficiency?

The risk of vitamin D deficiency generally increases in people who rarely spend time outdoors. This applies, for example, to chronically immobilised children and adolescents, the chronically ill or those in need of care, who may also not be able to consume a balanced diet. People with darker skin also produce comparatively little vitamin D because of the high melanin content of their skin and could be affected by an undersupply. Obese people are also at risk of vitamin D deficiency. This is because the vitamin D is stored in their abundant fatty tissue, meaning less of it is able to enter the blood and have a beneficial impact.

How much vitamin D should an adult take?

Generally speaking, a daily supplement of between 800 and 2,000 International Units (I.U.) is sufficient. For this purpose, supplements from the drugstore are just as good as those from the pharmacy.

The German Society for Endocrinology advises corona risk groups to take 400-1,000 I.U. per day. These include the very elderly, bedridden patients, residents of nursing homes or chronically ill people who rarely if ever spend time outdoors. With this approach, one takes advantage of possible, but so far not definitively proven, benefits without risking potential disadvantages of an overdose. In individual cases, higher intake of vitamin D intake can make sense - but this should be done under medical supervision and after an appropriate diagnosis.

Vitamin D can only be absorbed into cells with fats and should always be taken together with a meal containing at least moderate amounts of fat.

Should children take vitamin D in winter?

In a joint statement put out by the Nutrition Commission of the German Society for Paediatrics and Adolescent Medicine and the German Society for Paediatric Endocrinology and Diabetology, oral supplements with 400-500 I.U. vitamin D per day is recommended for infants in the first 12 to 18 months of life in addition to vitamin D intake with breast milk or infant formula. Vitamin D should be given in combination with fluoride prophylaxis.

In the case of premature babies or chronically ill children, the paediatrician will provide individual advice. An increased risk of vitamin D deficiency exists for children from socially disadvantaged backgrounds, obese children, children with an immigrant background and infants fed exclusively on breast milk and not receiving an additional vitamin D supplement.

According to the statement, the suitable total vitamin D intake - from sunlight-dependent, endogenous synthesis as well as from food and supplements - for children from the age of one, adolescents and adults is 600-800 I.U. per day.

Exposure to the sun twice a week between 10am and 3pm from April to September for 5 to 30 minutes with head, arms and legs uncovered is sufficient for adequate vitamin D production in childhood and adolescence.

Can too much vitamin D be harmful?

As a fat-soluble vitamin, vitamin D is not excreted in the urine when too much is present in the body. "D vitamins accumulate in the body," explains Helmut Schatz, board member of the DGE. An overdose of vitamin D over a longer period of time can lead to nausea, loss of appetite, abdominal cramps, vomiting or serious side effects such as kidney stones, kidney damage as well as disorders of the cardiovascular system. A recent study in the journal Annals of Internal Medicine suggests that vitamin D doses of more than 1,000 I.U. per day may have a negative effect with increased risk of falls in older, frail people.

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