CNN  — 

Is your inbox all a flutter over the latest Covid-19 miracle cure – vitamin D? Just like hydroxychloroquine, the antimalarial drug that US President Donald Trump championed, vitamin D is having its day in the sun on the internet and social media.

And just like hydroxychloroquine, it’s leading some to ask, “What can it hurt if I take vitamin D supplements?”

Actually, it can hurt a lot. In the case of hydroxychloroquine, the World Health Organization stopped a clinical trial after finding that seriously ill Covid-19 patients who were treated with hydroxychloroquine and chloroquine were more likely to die.

And taking too much vitamin D can lead to a toxic buildup of calcium in your blood, causing confusion, disorientation and problems with heart rhythm, as well as bone pain, kidney damage and painful kidney stones.

According to the Institute of Medicine of The National Academies’ Food and Nutrition Board, the recommended daily dose of vitamin D for anyone over age 4 is 600 IU/day in the US. For anyone over 70 years of age in the US, the dose goes up to 800 IU/day. In the UK, the recommended daily amount is 400 IU/day.

Levels in other parts of the world are country-specific to reflect environmental and dietary differences, but typically also range between 400 and 800 IU/day.

For ages nine and up, the maximum upper limit that can be tolerated is 4,000 IU/day in both in the US and UK, with research suggesting that long term use might be associated with increases in all-cause mortality, greater risk of cancer, cardiovascular events and more falls and fractures among the elderly.

Yet recent Google searches find people asking about the intake of much higher levels, even up to 60,000 IU per week.

“To date, there is no evidence that very high vitamin D levels are protective against COVID-19 and consequently medical guidance is that people should not be supplementing their vitamin D levels beyond those which are currently recommended by published medical advice,” said Robin May, who directs the Institute of Microbiology and Infection at the University of Birmingham in the UK, via email.

To address the surge of interest, the UK’s National Health Service added a coronavirus update to their informational page on vitamin D: “There have been some news reports about vitamin D reducing the risk of coronavirus. However, there is no evidence that this is the case.”

That warning was echoed by another group of scientists from the UK, Europe and the US. They published a vitamin D consensus paper warning against high doses of vitamin D supplementation and debunking the connection to the novel coronavirus.

“The continued spread of … the disease COVID-19 that is caused by SARS-CoV-2, has led to calls for widespread high-dose vitamin D supplementation,” the group wrote. “These calls are without support from pertinent studies in humans at this time, but rather based on speculations about presumed mechanisms.”

How did vitamin D become an internet sensation?

The fuss began when researchers in the US and UK began comparing the vitamin D levels of various countries to their coronavirus death rates and found an association: The countries who reported lower levels of vitamin D also had higher death rates from Covid-19.

Was that association caused by a lack of vitamin D or by other factors at play during the pandemic?

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After all, people around the world who are at highest risk of vitamin D deficiency – those with underlying chronic disease, an older age or darker skin color – are also the same people at highest risk of dying from Covid-19.

Having a darker skin makes it more difficult for the skin to absorb enough sun to manufacture vitamin D. Our bodies are also less efficient at creating vitamin D as we age, thus many of the elderly are typically vitamin D deficient. And a lack of sufficient vitamin D is closely associated with common chronic diseases such as cardiovascular disease and diabetes, among others.

The coronavirus has unfairly ravished those same groups: Both socially-disadvantaged communities of color and those with underlying health conditions, many of whom are elderly, have been hardest hit.

In the US, for example, data collated by the Kaiser Family Foundation from 35 states found nursing home residents accounted for at least 34.6%​ of the more than 87,000 coronavirus deaths ​recorded as of May 15. In the UK, 38% of all deaths from Covid-19 by May 1 were elderly care home residents, according to the latest estimates from the Office of National Statistics.

In the US, a study found that while black Americans represent 13.4% of the population, counties with higher black populations made up more than half of all Covid-19 cases and almost 60% of deaths. In the UK, recent data found blacks were four times as likely to die of Covid-19 as whites, while people of Bangladeshi, Pakistani, Indian and mixed ethnicities also had a “statistically significant raised risk of death.”

“This can be partly explained because people from ethnic minorities suffer from a number of social and economic disadvantages. For example, they are more likely to live in poor and overcrowded housing, making social distancing more challenging,” said William Henley, a professor of medical statistics at the University of Exeter in the UK, in email.

However, Henley added, not all of that increased risk can be explained by known factors, so investigations into vitamin D are “urgently” needed to see if it too may play a role.

“It is too early to draw firm conclusions about the importance vitamin D in explaining Covid-19 susceptibility,” Henley said. “However, given the current lack of therapeutic options for tackling Covid-19, playing close attention to population levels of vitamin D merits attention.”

The basics of vitamin D

Like all vitamins, D is an essential micronutrient, required in small quantities for normal cell function, growth and development. The primary function of vitamin D is to help the body absorb calcium and phosphate, thus keeping muscles and teeth healthy and bones strong and less likely to break.

The body makes vitamin D when skin is directly exposed to the sun. Indirect exposure, such as sun shining through a window, will not create vitamin D. Cloudy days, shade, living in high altitudes and farther from the equator also affect our body’s production.

For countries like the UK where the skies are often overcast, additional supplementation may be necessary during the months of October through March, according to the UK National Health Service.

A severe lack of D can result in soft, thin, brittle bones during childhood, a condition called rickets predominately seen today in malnourished children in developing countries. Rickets is extremely rare in developed countries, as many foods such as milk, infant formulas, bread, orange juice, yogurt, margarine and cereal are fortified with vitamin D by manufacturers.

According to the US National Institutes of Health, “almost all of the U.S. milk supply is fortified with 400 IU of vitamin D per quart, and so are many of the plant-based alternatives such as soy milk, almond milk, and oat milk.”

The best food source for vitamin D is fatty fish such as salmon, tuna and mackerel; plant sources are minimal, requiring vegans and vegetarians to look at supplementation.

But vitamin D does much more than protect bone and muscle development. It helps brain nerve cells carry messages, and helps the immune system fight off invading bacteria and viruses.

Studies have shown that vitamin D deficiency impairs the immune system, and some older studies found that vitamin D supplements may reduce the risk of respiratory virus infections and immune system overreaction.

Former CDC Director Dr. Tom Friedan pointed to some of those studies in an op-ed he wrote for Fox News, suggesting that additional supplement might be good for the vulnerable in society, such as the elderly and those with chronic conditions.

“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,” Friedan wrote.

But those infections are not Covid-19, and there is no known benefit at this time to taking higher than recommended doses of vitamin D, some experts say.

“Globally, there is mixed evidence around the role of vitamin D and respiratory tract infections,” said Michael Head, a senior research fellow in global health at the University of Southampton in the UK.

“There is a certain amount of correlation, but huge uncertainty about causation. It is certainly a topic that warrants further research, but with necessary caution applied to overstating any new evidence that emerges in this pandemic,” Head said.

Studies are underway to see if supplementation with D will have an effect, but until then, the experts who wrote the vitamin D consensus paper urge caution:

“As a key micronutrient, vitamin D should be given particular focus – not as a ‘magic bullet’ to beat Covid-19, as the scientific evidence base is severely lacking at this time — but rather as part of a healthy lifestyle strategy to ensure that populations are nutritionally in the best possible place.”

CNN’s Emma Reynolds, Shelby Lin Erdman and Rob Picheta contributed to this story.