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If newspaper headlines are to be believed, there is a simple way to protect ourselves from the coronavirus: take a daily vitamin D tablet. Some say the benefits are so great that people should take mega-doses. But the scientific evidence doesn鈥檛 back up the claims. What should we believe?

Vitamin D is a fat-soluble compound made in the skin through the action of sunlight, and is also found in foods such as oily fish. It is chiefly known for building strong bones, but also plays a role in the immune system, including helping to kill virus-infected cells.

Some studies had previously suggested that taking vitamin D tablets wards off respiratory infections like coughs and colds, but in June, the UK鈥檚 Scientific Advisory Committee on Nutrition (SACN) concluded that the evidence isn鈥檛 strong enough to support such an effect. However, it does , as do many other health bodies.

What about for covid-19? Several researchers have said that vitamin D has potential to protect people from being infected with covid-19, or if they are infected to help to reduce their illness, and some supporting evidence has started to accrue.

However, most of the evidence is in the form of observational studies, which can be misleading because they don鈥檛 prove that low vitamin D causes infections, just that it correlates with them. For instance, one study to make headlines last week found that people admitted to hospital with covid-19 had , was of this type.

A third factor could be causing both low vitamin D and susceptibility to the virus, for instance obesity, because the vitamin is stored in fat tissue.

The best way to assess the benefits of vitamin D supplements would be trials giving people with covid-19 the vitamin to see if it can help treat their symptoms. One recent such study from Spain found that people in hospital with covid-19 who received high doses of vitamin D were than those who didn鈥檛 receive it.

However, the study involved just 76 people, and there happened to be more cases of high blood pressure in the group that got no vitamin D, which would have made their prognosis worse. But even when the results are corrected for that difference, there was still a large effect, says Jos茅 Manuel Quesada G贸mez at the Queen Sof铆a University Hospital in C贸rdoba, Spain, who was involved in the study.

The trial also only tells us that vitamin D seems to work as a treatment for hospital patients, not that it works as a prevention method. It is promising, but a larger study is needed, says Susan Lanham-New, who is a member of SACN.

Regardless of vitamin D鈥檚 impact on covid-19, there is one thing that all experts seem to agree on: most people in temperate climates like in the UK become vitamin D deficient over winter, when there isn鈥檛 enough sunlight to prompt production of the vitamin in the skin. And so, if only to support strong bones, most people should take a standard vitamin D supplement every day, from October onwards in the northern hemisphere.

If people are concerned about coronavirus and want to take double the daily recommended dose, it is unlikely to do harm, says Lanham-New. 鈥淭he most important thing is to avoid vitamin D deficiency.鈥�