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As we race to develop effective treatments and a vaccine against COVID-19, people are looking to reduce their risk of getting sick. One thing that might help is as obvious as the sun in the sky and as close as your medicine cabinet – Vitamin D.

Higher COVID-19 mortality rates among older people and those with chronic conditions suggest that a weakened immune system contributes to poor outcomes. There are many crackpot claims about miracle cures floating around, but the science supports the possibility – although not the proof – that Vitamin D may strengthen the immune system, particularly of people whose Vitamin D levels are low.

Vitamin D supplementation reduces the risk of respiratory infection, regulates cytokine production and can limit the risk of other viruses such as influenza. A respiratory infection can result in cytokine storms – a vicious cycle in which our inflammatory cells damage organs throughout the body – which increase mortality for those with COVID-19. Adequate Vitamin D may potentially provide some modest protection for vulnerable populations.

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This is especially important for people who are Vitamin D deficient – and, surprisingly, that might include more than 40 percent of US adults. People who live in the northern part of the U.S. are at greater risk of deficiency.

There is evidence of seasonality in some respiratory illnesses, including influenza and tuberculosis. A leading hypothesis is that seasonality is due to the reduction in Vitamin D because of decreased exposure to sunlight in winter months. There is no seasonality of influenza or tuberculosis in some tropical climates (such as south India), where weather – and sunlight exposure – remains more constant throughout the year.

When I worked in India, from 1996-2002, I requested that Centers for Disease Control and Prevention (CDC) send an epidemic intelligence officer in to investigate, and Dr. Lorna Thorpe, the lead author on the resulting study, found that there was more seasonality in the northern climates, which have a cool or cold winter season, and little or none in the southern areas of the country, which are hot all year around.

Right now, we don’t know if Vitamin D deficiency plays any role in the severity of COVID-19. But given the high prevalence of Vitamin D deficiency in this country, it is safe to recommend that people get the proper daily dosage of Vitamin D.

Most people’s bodies manufacture Vitamin D in the skin when exposed to the sun. About 15 minutes a day of direct sunlight is sufficient for many people’s bodies to manufacture enough Vitamin D; people with darker skin need longer exposure to sunlight to manufacture the same amount. In winter, people in northern latitudes may not be able to make any Vitamin D from sunlight. Sunscreen lengthens the exposure time needed.

We can do lots of things to improve our resistance to infection. Taking a multivitamin that includes Vitamin D, or a Vitamin D supplement, probably can’t hurt, and it might help.

Many people, then, need Vitamin D supplementation. Few foods are naturally rich in vitamin D (egg yolks and fatty fish such as salmon are two), making fortified foods and vitamin supplements important. We have been adding Vitamin D to milk for nearly a century, originally to reduce rickets, which is why most children are not Vitamin D deficient.

As we get older and drink less milk, we need other sources for Vitamin D. Most daily multivitamin supplements contain enough Vitamin D – though it’s possible to take too much, so it’s important not to overdose. How much is enough? Doses between 800 IU and 2000 IU are probably safe; a reasonable and commonly used dose is 1000 IU a day. Too much Vitamin D can cause nausea and vomiting, weakness, and frequent urination, and lead to bone pain and kidney stones. Vitamin D can also interact with certain medications, so you should check for interactions before taking supplements.

For now, go outside and get some sun – but make sure you follow social distancing guidelines to avoid close contact with other people, and don’t go out if you’re ill or can’t go out safely. Taking a walk will also help you get physical activity and alleviate cabin fever. Eat healthy foods that contain or are supplemented with Vitamin D. Take a daily multivitamin supplement (but don’t double the daily dose just because you’re worried).

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We can do lots of things to improve our resistance to infection. These include getting regular physical activity, getting enough sleep, stopping smoking and other tobacco use, and, for people living with diabetes, getting it under control. Taking a multivitamin that includes Vitamin D, or a Vitamin D supplement, probably can’t hurt, and it might help.

As we continue to work to mitigate the impact of COVID-19, anything we can do to strengthen our resistance is a step in the right direction.

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I thank Professor David Gartner of Arizona State University for suggestions and research on this topic.