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Issue: Ir Med J; Vol 113; No. 4; P58
Optimisation of Vitamin D Status for Enhanced
Immuno-protection Against Covid-19
D.M. McCartney1, D.G. Byrne2,3
1. School of Biological and Health Sciences, College of Sciences & Health, Technological
University Dublin - City Campus, Kevin Street, Dublin D08 NF82, Ireland
2. Department of Internal Medicine, St. James’s Hospital, James’s Street, Dublin 8, Ireland
3. Dept. of Clinical Medicine, School of Medicine, Trinity College Dublin, Dublin 2, Ireland
Vitamin D deficiency (serum 25(OH)D<50nmol/l) is common in Ireland, particularly amongst older adults, hospital
inpatients and nursing home residents. Vitamin D deficiency is associated with increased risk of acute viral
respiratory infection and community acquired pneumonia, with several molecular mechanisms proposed to explain
this association. Vitamin D supplementation has also been shown to reduce the risk of respiratory infection.
Vitamin D and Covid-19
Correction of vitamin D deficiency is thought to suppress CD26, a putative adhesion molecule for Covid-19 host cell
invasion. Vitamin D may also attenuate interferon gamma (IFNγ) and interleukin-6 (IL-6) inflammatory responses,
both potent predictors of poorer outcome in critically-ill ventilated patients including those with Covid-19.
Vitamin D Requirements
Irish adults require 25-30µg/d of vitamin D3, an intake not achievable by diet alone, to reliably maintain serum
25(OH)D levels >50nmol/l. Supplementation with doses up to 100µg/d has been shown to be safe for adults, and
many agencies and expert groups now advocate supplementation in older adults, albeit at lower levels than this.
Conclusions and Recommendations
Vitamin D deficiency is common and may contribute to increased risk of respiratory infection including Covid-19. We
recommend that all older adults, hospital inpatients, nursing home residents and other vulnerable groups (e.g. those
with diabetes mellitus or compromised immune function, those with darker skin, vegetarians and vegans, those who
are overweight or obese, smokers and healthcare workers) be urgently supplemented with 20-50µg/d of vitamin D to
enhance their resistance to Covid-19, and that this advice be quickly extended to the general adult population.
Vitamin D is a steroid hormone which may be synthesised endogenously from the effect of UVB irradiation on skin,
or consumed from exogenous dietary sources or supplements. Recent studies have shown an inverse relationship
between serum vitamin D levels and risk of acute respiratory tract infection1. Notably, a September 2019 meta-
analysis by Zhou and colleagues incorporating data from 21,000 subjects across eight observational studies showed
that those with a serum vitamin D level <20ng/ml (i.e. <50nmol/l) had a 64% increased risk of community-acquired
While the latter data are associative, and do not in and of themselves indicate a causal role for low vitamin D levels
in community-acquired pneumonia, there is existing experimental evidence which suggests several mechanisms by
which optimisation of vitamin D status contributes to enhanced resistance to viral respiratory tract infection3,4,5.
Moreover, notwithstanding the heterogeneity of infection types included and population groups captured, a recent
systematic review which evaluated the findings of 7 meta-analyses incorporating data from 30 randomised
controlled trials concluded that vitamin D supplementation, particularly in those with low serum levels at baseline, is
likely to reduce the risk of respiratory tract infection6, a finding corroborated by two further systematic reviews the
Relevance of Vitamin D to Covid-19
With regard to Covid-19, it is salient that while the virulence mechanisms of this virus have not been fully
characterised, a number of molecular virulence mechanisms including dipeptidyl peptidase-4 receptor (DPP-4/CD26)
binding, Papain-like protease (PLpro)-mediated replication, MDA5 and RIG-I host-recognition evasion, and disruption
of M-protein mediated type-1 IFN induction have been identified in the closely-related Covid-MERS virus8. Of these,
human DPP-4/CD26 has recently been shown to interact with the S1 domain of the COVID-19 spike glycoprotein,
indicating that it may also be an important virulence factor in Covid-19 infection9. Critically in this regard, DPP-
4/CD26 receptor expression has been shown to be significantly reduced in vivo upon correction of vitamin D
deficiency10. There is also evidence that optimisation of vitamin D may attenuate some of the critical downstream
immunological sequelae thought to elicit poorer clinical outcome in Covid-19 infection, such as prolonged
interferon-gamma response4, and persistent interleukin 6 elevation, a negative prognostic indicator in acutely-ill
pneumonia patients11, including those with Covid-19.
Prevalence of Deficiency
In Ireland, as a consequence of poor dietary intake, low supplementation rates and sub-optimal sun exposure, the
prevalence of vitamin D deficiency is high, particularly amongst older adults, the most vulnerable constituency to
Covid-19 mortality. In the last nationally representative sample, 35.7% of adults aged 50-64 years, and 44.0% of
adults aged 65-84 years had serum vitamin D levels less than 50nmol/l on a year-round basis, while these figures
rose to 55.4% and 48.1% respectively in winter12. These data are critical, as they suggest that one half of our older
adults currently have serum vitamin D levels below the threshold at which viral respiratory infection risk is known to
increase. It is also noteworthy that vitamin D levels are even poorer amongst nursing home and hospital inpatients in
Ireland, with 37-42% of these individuals having serum levels less than 25nmol/l13.
Intake Requirements and Supplementation Guidelines
Existing guidance from the Food Safety Authority of Ireland (FSAI) recommends that older adults should supplement
with 10 micrograms of vitamin D per day14. However, most countries in Europe now recommend intakes of 15-20
micrograms per day for these older age groups, with the Institute of Medicine (IoM) and the Endocrine Society in the
US recommending intakes of 20 micrograms per day and 37.5-50 micrograms per day respectively for older adults
since 201115. Two well-designed modelling studies have been conducted to estimate the oral dose of vitamin D
required to achieve and maintain adequate serum levels in Irish adults on a year round basis. The first of these
proposed a daily dose of 28.0 micrograms to maintain serum vitamin D levels above the critical 50nmol/l threshold
in 97.5% of healthy Irish adults throughout the year16, while the second suggested a daily requirement of 24.7
micrograms for Irish adults aged 64 years and over to achieve and maintain these serum levels17.
Safety of Vitamin D Supplementation
While documented cases of vitamin D toxicity do appear in the literature, these are rare, and invariably relate to
extremely high doses taken over an extended period of time18. There is no evidence however, that vitamin D
supplementation at 20-50 micrograms per day has any adverse effects on health. Indeed several studies have
explicitly cited the safety of vitamin D3 supplementation at doses of up to 100 micrograms per day19,20, with a
further review proposing a tolerable upper limit (TUL) of 250 micrograms per day21. These findings are perhaps
unsurprising, given that cutaneous synthesis yields a typical ‘dermal dose’ of ~70 micrograms per day from regular
sunlight exposure during the Summer months, and that one single whole-body minimum erythemal dose can
produce a rise in serum vitamin D levels which is equivalent to an oral dose of ~250-625 micrograms22. For context, a
minimum erythemal dose can be produced by as little as 10–15min of whole-body sun exposure at mid-day in mid-
summer in a pale-skinned individual, and is therefore not an uncommon occurrence. Further research and clinical
data demonstrating the safety of vitamin D supplementation at doses of 20-50 micrograms per day abound in the
literature23,24, highlighting its viability as a means of addressing this common but important nutritional deficit.
Conclusions and Recommendations
Vitamin D intakes and status are low in Ireland, particularly amongst older adults, hospital inpatients and nursing
home residents. Low serum vitamin D has been associated with increased risk and severity of viral respiratory
infections including community acquired pneumonia, whilst there is also evidence that vitamin D supplementation
which raises serum vitamin D levels above 50nmol/l may ameliorate this risk. Among the proposed protective effects
of vitamin D are several which may reduce the risk of Covid-19 infection, or which may attenuate the immunological
sequelae responsible for its fulminant respiratory effects. There is existing guidance from health authorities in
Ireland and globally that older adults should supplement with vitamin D, and there now exists a wealth of evidence
which demonstrates the safety of vitamin D3 supplementation at doses of 20-50 micrograms per day.
In the face of the impending Covid-19 epidemic, and in the absence of a vaccine or any effective anti-viral drug
therapy to treat those infected, these findings call for the prioritised supplementation of all hospital inpatients,
nursing home residents and community-dwelling older adults with vitamin D at a minimum daily dose of 20
micrograms per day. It is further recommended that supplementation be targeted at other vulnerable constituencies
(e.g. those with diabetes mellitus or compromised immune function, those with darker skin, vegetarians and vegans,
those who are overweight or obese, smokers and healthcare workers), and ultimately extended to the rest of the
population in order to mitigate the grave public health risks associated with Covid-19 infection.
Declaration of Conflicts of Interest:
The authors declare that they have no conflict of interest.
Dr Daniel McCartney,
School of Biological and Health Sciences,
Technological University of Dublin - City Campus,
Dublin D08 NF82,
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