Black people with high vitamin D levels may be less likely to catch the coronavirus, research suggests.
The benefits of the so-called sunshine supplement have been debated since the outbreak emerged, with many experts optimistic the vitamin's anti-inflammatory properties may ward off the infection's complications.
In December 2020, the UK's National Institute for Health and Care Excellence (Nice) said "there is currently not enough evidence to support taking vitamin D solely to prevent or treat COVID-19 [the disease caused by the coronavirus]".
To better understand its potential benefits, scientists from the University of Chicago analysed the vitamin D levels of more than 4,600 people up to 365 days before they were tested for the coronavirus.
Results reveal the Black participants with "deficient" vitamin D were more than twice as likely to catch the infection than those with the highest levels.
The same was not true for the white individuals, however, with previous studies suggesting Black people are more likely to have insufficient vitamin D.
Vitamin D "has diverse physiological effects", impacting a person's calcium regulation, bone density and immune response, the scientists wrote in the journal JAMA Network Open.
Previous studies have linked vitamin D supplementation with reduced respiratory viral infections.
With its effect on the coronavirus being less clear, the Chicago scientists analysed tens of thousands of people – average age 52 – who were tested for the infection between 3 March and 30 December 2020.
The participants had also had their vitamin D level measured up to 365 days before being swabbed.
Vitamin D levels were grouped as under 20 ng/mL – "deficient", 20 ng/mL to 29 ng/mL – "insufficient", 30 ng/mL to 39 ng/mL, and 40 ng/mL or above.
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The results reveal vitamin D levels were lower among the more than 2,200 Black participants, with over a third (36%) having a deficient amount compared to one in 10 (16%) of the white individuals.
"Socioeconomic factors and structural inequities clearly contribute" to the disparity, wrote the scientists.
"Biological susceptibility" may also play a role, with "lighter skin increasing vitamin D production in response to sunlight".
The Black participants with deficient vitamin D were more than twice as likely to test positive for the coronavirus than their counterparts at 40 ng/mL or above. It is unclear how vitamin D may ward off the infection.
The scientists calculated 9.7% of the Black participants with vitamin D levels below 20 ng/mL had the infection, compared to 3.8% of those at 40 ng/mL or above.
Every 1 ng/mL increase above 30 ng/mL reduces a Black person's risk of catching the coronavirus by 5%, the results suggest.
The findings "support the idea supplementation might decrease COVID-19 risk by increasing vitamin D levels", wrote the scientists.
"While the increasing availability of COVID-19 vaccines is likely to reduce the spread of COVID-19 and hence any potential benefits of vitamin D supplementation, the presence of new strains resistant to existing vaccines may increase the potential benefits of vitamin D supplementation," they added.
"Vitamin D supplementation may be useful in populations not receiving the vaccine.
"Weakened host responses may also enhance the conditions for development of viral mutation, so supplementation might have benefits at the population level in reducing the risk of mutant strains."
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With sunshine in short supply during the UK's cold months, the NHS has always maintained "everyone should consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter". This is equivalent to 10,000 ng.
The supplements were also recommended during the UK's first lockdown, introduced in late March 2020, due to people spending a considerable amount of times indoors.
When it comes to the coronavirus specifically, Nice has argued it has "not [been] possible to determine a direct relationship between vitamin D and COVID-19 based on the available evidence".
Some experts believe the vitamin's anti-inflammatory properties could ward off coronavirus complications. Patients may become critically ill when the immune system over-reacts to the infection, triggering a widespread inflammatory response that damages vital organs.
Inflammation may also be responsible for long COVID, where complications linger after a former coronavirus patient tests negative for the infection.
Those in favour of vitamin D as a coronavirus strategy argue the supplements are inexpensive and safe, even at high doses.