There is currently insufficient evidence to support taking vitamin D solely to prevent or treat COVID-19, according to new guidance.
However, experts confirmed current advice for everyone to take a vitamin D supplement throughout the autumn and winter.
The National Institute for Health and Care Excellence (NICE) conducted a rapid review of evidence in response to a hypothesis that vitamin D might help in the prevention or treatment of COVID-19 because of its role in the body's immune response to respiratory viruses.
The review was undertaken in collaboration with Public Health England, and the Scientific Advisory Committee on Nutrition (SACN).
Data reviewed by the panel included the best available scientific evidence published so far, including both observational studies and randomised controlled trials.
Evidence Is 'Inconsistent'
The scientists agreed that low vitamin D status was associated with more severe outcomes from COVID-19. However, they said it was not possible to confirm causality because many of the risk factors for severe COVID-19 outcomes are the same as the risk factors for low vitamin D status.
Other problems with the available evidence were cited as inconsistencies between studies, and differences between supplementation doses, settings, populations, durations of trials, and definitions of outcomes.
Prof Ian Young, chair of SACN, said: "This evidence review confirms that currently there is not enough available evidence to determine that there is a causal relationship between vitamin D and COVID-19."
The guidance recommended that more research be conducted on the subject, stressing the use of high-quality randomised controlled trials in future studies.
It said there should be a particular focus on subgroup analyses, including among older people and people from Black, Asian and Minority Ethnic groups where poorer outcomes are seen in people with COVID-19.
Dr Paul Chrisp, director of the Centre for Guidelines at NICE, said: “As research continues on the impact of vitamin D on COVID-19, we are continuing to monitor evidence as it is published and will review and update the guidance if necessary."
No 'Magic Bullet'
Commenting on the guidance, Naveed Sattar, professor of metabolic medicine at the University of Glasgow, said it was important for the public to know that vitamin D could not be considered a "magic bullet" against the SARS-CoV-2 virus and that "people who wish to supplement with vitamin D tablets over the winter period should not think they will be immune to COVID-19 infection".
Adrian Martineau, clinical professor of respiratory infection and immunity at Queen Mary University of London, welcomed NICE's call for more research in the area. "Pending results of these trials, it makes sense for the UK population to follow existing government guidance to take a daily supplement," he told the Science Media Centre. "There is a chance that it might reduce risk and/or severity of COVID-19 into the bargain – clinical trials currently in progress will hopefully shed light on this question."
One recent study, published in the journal Nutrients, suggested that a high-dose booster therapy of vitamin D in its 'parent' form, cholecalciferol, regardless of baseline serum 25(OH)D levels, appeared to be associated with a reduced risk of mortality in acute inpatients admitted with COVID-19.
The observational study involved 444 participants with a median age of 74 from Tameside General Hospital in Ashton-under-Lyne.
The authors call for further work to determine what an adequate serum level of 25(OH)D might be from large-scale population studies, as well as clinical trials of cholecalciferol therapy, at multiple doses, in order to assess maximum efficacy.
They point out that if beneficial, high dose therapy with vitamin D would be inexpensive and widely available.
Vitamin D Supplement Recommendations
The rapid guidance also included advice on the importance of taking a vitamin D supplement.
The panel was concerned that not everyone was aware, or were following, UK Government recommendations to take a supplement between October and early March when people in the UK do not make enough vitamin D from sunlight.
They stressed that everyone should consider taking a supplement containing 10 micrograms (400 international units) of vitamin D daily.
The panel also stressed that this was particularly important during the COVID-19 pandemic, when people may be indoors more than usual.
'Alarming' Vitamin D Deficiency in UK Ethnic Groups
New research found that 57.2% of people with Asian backgrounds living in the UK are deficient in vitamin D (levels below 25 nmol/L) during the winter and spring.
The study in the journal Clinical Nutrition also reported vitamin D deficiency affecting 38.5% of people with Black African ancestry during winter and spring.
That compared with 36.5% deficiency among those of mixed backgrounds, 33.1% for ethnic Chinese, and 17.5% for those with White European ancestry during the winter and spring seasons.
Vitamin D deficiency remained widespread at other times of the year, with 50.8% of people from Asian backgrounds showing deficiency in the summer and autumn. That compared with just 5.9% deficiency among White Europeans during this time of the year.
The study used data from 440,581 UK Biobank participants.
"The rate of severe deficiency was much higher in most population groups than we would have expected, so these results are really very alarming," commented Prof Elina Hypponen from the University of South Australia, who led the study.
The authors said the results strengthened calls for a mandatory vitamin D fortification programme in the UK.
Prof Adrian Martineau is chief investigator of the CORONAVIT trial – the UK’s national clinical trial of vitamin D supplementation to reduce risk and/or severity of COVID-19.