Vitamin D appears to help reduce serious complications among coronavirus patients, according to a study.
Researchers from Trinity College Dublin, the University of Liverpool and the Irish Longitudinal Study on Ageing (TILDA) examined the association between vitamin D levels and Covid-19 mortality rates.
Dr Eamon Laird and Professor Rose Anne Kenny, who co-authored the paper, found vitamin D can help support the immune system through a number of pathways involved in fighting SARS-CoV-2, the virus that causes Covid-19.
After analysing more than 20 years' worth of European data on vitamin D, and comparing it with current statistics on Covid-19, the researchers showed that the highest infection and death rates had been recorded among those populations with low vitamin D concentrations.
These includes the likes of Spain and Italy which, despite their lower latitude positions and heightened exposure to sunlight, both suffer from high rates of vitamin D deficiency.
The northern latitude countries of Norway, Finland and Denmark, which have recorded comparatively lower Covid-19 infection and death rates, have higher vitamin D levels despite less sunlight exposure, because supplementation and fortification of foods is more common.
The correlation between low vitamin D levels and death from Covid-19 is "statistically significant", said the authors of the report, who also urged the Irish government to change recommendations for supplements.
"Whereas there are currently no results from randomised controlled trials to conclusively prove that vitamin D beneficially affects Covid-19 outcomes, there is strong circumstantial evidence of associations between vitamin D and the severity of Covid-19 responses, including death," said Professor Kenny of Trinity College.
"We call on the Irish government to update guidelines as a matter of urgency and encourage all adults to take supplements during the Covid-19 crisis.
"Deficiency is frequent in Ireland. Deficiency is most prevalent with age, obesity, in men, in ethnic minorities, in people with diabetes, hypertension and in nursing homes."
Co-author Dr Laird added: "Here we see observational evidence of a link of vitamin D with mortality.
"Optimising vitamin D intake to public health guidelines will certainly have benefits for overall health and support immune function.
"Research like this is still exploratory and we need further trials to have concrete evidence on the level of vitamin D that is needed for optimal immune function."
The Irish study reinforces evidence which suggests Covid-19 patients with high vitamin D levels are more likely to survive the disease.
Last week, a study from Northwestern University in the US found that patients with severe deficiency are twice as likely to experience major complications.
The researchers also found a strong correlation between low vitamin D levels and the cytokine storm response, a hyper-inflammatory condition caused by an overactive immune system, seen among some coronavirus patients.
"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's McCormick School of Engineering, said last week.
"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."
Vitamin D is produced in the skin from UVB sunlight exposure and is transported to the liver and then the kidney where it is changed into an active hormone that increases calcium transport from food in the gut and ensures calcium is adequate to keep the skeleton strong.