TORONTO -- Two recent studies have found an association between vitamin D and COVID-19 suggesting that people who are deficient in the micronutrient are more likely to experience serious health complications if infected, but the authors say more research is needed.

In one study, Northwestern University researchers in Illinois found that patients from countries with high COVID-19 mortality rates, such as Italy, Spain and the U.K., had lower levels of vitamin D compared to patients in countries that were not as severely affected.

According to Health Canada, vitamin D is produced in the skin from UVB sunlight exposure and can increase calcium and phosphorus absorption from food to help maintain strong bones and teeth. Low levels of vitamin D can speed up bone loss and increase the risk of fractures. The vitamin can also stimulate immune responses.

The study, which has not been peer-reviewed, was pre-printed April 30 in the online journal medRxiv. Researchers looked at data from hospitals and clinics across China, France, Germany, Italy, Iran, South Korea, Spain, Switzerland, the U.K. and the U.S. They compared this with data on the levels of vitamin D in the population in those countries before the pandemic.

The study reported a correlation between vitamin D deficiency and a complication known as a cytokine storm, which occurs when the immune system goes into overdrive, as well as a correlation between vitamin D deficiency and mortality rates.

Vitamin D modulates the response of white blood cells and can prevent them from releasing too many inflammatory cytokines. According to the study, COVID-19 can cause an excess of these cytokines.

"Cytokine storm can severely damage lungs and lead to acute respiratory distress syndrome and death in patients," Ali Daneshkhah, a postdoctoral research associate at Northwestern and the lead author of the study, said in a press release. "This is what seems to kill a majority of COVID-19 patients, not the destruction of the lungs by the virus itself. It is the complications from the misdirected fire from the immune system."

However, the researchers stress that this does not mean that everyone -- especially those without a known deficiency -- need to start increasing their intake of vitamin D.

"While I think it is important for people to know that vitamin D deficiency might play a role in mortality, we don't need to push vitamin D on everybody," Northwestern's lead researcher Vadim Backman said in the release. "This needs further study, and I hope our work will stimulate interest in this area."

Michael Holick, an expert on Vitamin D research from Boston University, warns that taking too much vitamin D can result in vitamin D toxicity and have negative side effects including kidney damage, bone pain and calcium stones.

But Holick told that someone would have to take "tens of thousands of units of vitamin D for half a year" for those side effects to occur.

"Too much vitamin D can increase your blood calcium and it can start depositing calcium into your blood vessels, which ultimately can cause a lot of harm," Holick said in a telephone interview on Wednesday. "But vitamin D toxicity is one of the rarest medical conditions in the world."

Dr. David Seres, director of medical nutrition at Columbia University Irving Medical Center, told on Wednesday that people should consult a physician before taking vitamin D supplements.

"Vitamin D is a fat, and is only present in the bloodstream bound to a specific carrier protein. The level of this protein drops when there is severe inflammation… But despite years of papers claiming that this represents a deficiency, this is just a phenomenon related to physiological shifts and is not a deficiency," Seres said in an email.

"It should be no surprise that lower D levels predict poor outcomes, since sicker patients have lower carrier protein levels," he added.

Seres said the conclusions drawn in this study can be misleading. He cautioned that the researchers' claims should not be used to change one's behaviour.

"The authors use the correlation between D levels and a marker of inflammation, as well as a few average measures of D levels from the general populations in different countries, to extrapolate a conclusion that vitamin D has a role in COVID-19 severity. There was no direct measurement, and the conclusions are a stretch and ignore what we know about how vitamin D is carried in the bloodstream," Seres said.

Another study in the U.K. also found a link between higher levels of vitamin D and fewer COVID-19 cases and deaths among 20 European countries.

The study, published May 7 in the journal Aging Clinical and Experimental Research, found that patients with higher average levels of vitamin D had fewer coronavirus cases and lower mortality rates.

The study found that Italy and Spain, where populations have experienced high COVID-19 mortality rates, have lower average vitamin D levels than most northern European countries. Patients studied in northern European countries including Finland, Norway, Denmark and Sweden, had higher than average levels of vitamin D and lower rates of coronavirus cases and deaths.

The study did not look at whether increased vitamin D levels could result in better outcomes for COVID-19 patients, but lead researcher Dr. Lee Smith said in a press release that it is a prospect researchers are looking to explore further.

"Vitamin D has been shown to protect against acute respiratory infections, and older adults, the group most deficient in vitamin D, are also the ones most seriously affected by COVID-19," Smith said. "A previous study found that 75 per cent of people in institutions, such as hospitals and care homes, were severely deficient in vitamin D. We suggest it would be advisable to perform dedicated studies looking at vitamin D levels in COVID-19 patients with different degrees of disease severity."

Researchers noted that this study was limited in that it did not account for the level of testing and different public health measures taken by each country to prevent the spread of infection.

Seres said recent hype around vitamin D as a disease and virus cure-all is unfounded.

"Vitamin D has been promoted as a cure for everything from cancer to Alzheimer's. [But] let me be very clear -- there is no data that the effect of sun exposure on vitamin D levels, or supplementation of vitamin D, has any impact on whether you are susceptible to coronavirus infection, or on the severity of the illness if you are infected," Seres said.

There are currently no drugs, vaccines, or other therapeutics approved by Health Canada to prevent or treat the novel coronavirus.

Osteoporosis Canada advises healthy adults aged 19-50 consume 400-1,000 IU of vitamin D per day, and those over 50, or younger adults at high risk, take 800-2,000 IU daily. The organization advises year-round vitamin D supplementation for all Canadian adults.