Low levels of vitamin D associated with higher risk of severe COVID-19

Diet and lifestyle 5. jan 2023 2 min Researcher Nete Munk Nielsen Written by Kristian Sjøgren

When the first wave of the COVID-19 pandemic hit Denmark, vitamin D deficiency was associated with a higher risk of severe COVID-19. It is important to point out, that levels of vitamin D should be within recommended ranges, neither too low nor too high.

A new study carried out by researchers from Statens Serum Institut in Copenhagen, the Harvard T.H. Chan School of Public Health in Boston, USA and others concludes that, as the COVID-19 pandemic raged in Denmark in early 2020, having vitamin D deficiency was associated with a higher risk of severe COVID-19.

The study shows that vitamin D deficiency in relation to COVID-19 was associated with a higher risk of hospitalisation, treatment in an intensive care unit, and death.

However, the researchers strongly warn against the use of excessive vitamin D supplementation as a preventive measure of future outbreaks of COVID-19.

“Our results show that vitamin D deficiency was associated with a higher risk of severe COVID-19 and that people do not gain additional protection from having higher levels of vitamin D as long they do not have a deficiency. Thus, levels of vitamin D should be within the levels recommended by the Danish Health Authority, neither too low nor too high,” explains Nete Munk Nielsen, researcher, Department of Epidemiology Research, Statens Serum Institut, Copenhagen.

The association between vitamin D status and COVID-19 severity in this study was assessed among individuals infected in spring 2020: at the beginning of the epidemic. Several variants of SARS-CoV-2 have appeared since, and a large proportion of Denmark’s population has been COVID-19 vaccinated.

The research has been published in Nature Scientific Reports.

Studied vitamin D among individuals infected in the beginning of the epidemic

The researchers examined data from 447 people who developed COVID-19 in spring 2020 and who had a blood sample stored in the Danish National Biobank. The researchers used the blood samples to determine the person’s level of vitamin D.

The blood samples consisted of residual material from blood samples processed in clinical biochemistry departments at selected hospitals in the Capital Region of Denmark. The samples had routinely been sent to and stored by the Danish National Biobank.

Using the Biobank Registry, which includes information from the Danish National Patient Registry, the researchers identified residual blood samples belonging to individuals registered with a hospital contact due to COVID-19 until early May 2020.

The researchers’ first priority was blood samples taken 1–30 days before a COVID-19 contact was registered; second best was a blood sample drawn on the date of registration or 1–2 days after; and if this was not possible, the most recent blood sample collected up to 24 months before the COVID-19 contact was registered.

The researchers divided the participants according to the concentration of vitamin D in their blood:

  • Vitamin D deficiency (less than 25 nmol/L)
  • Insufficient vitamin D (25–49 nmol/L)
  • Sufficient vitamin D (50 nmol/L or more)
They then investigated the illness trajectories at the individual level:
  • 126 were not hospitalised
  • 205 were hospitalised but not admitted to an intensive care unit
  • 34 were admitted to an intensive care unit
  • 82 died within 30 days of a positive test.

Finally, the researchers explored the association between COVID-19 severity and vitamin D status.

“We adjusted the risk estimates for seasonal variation in vitamin D level, age, sex, obesity and country of origin. In addition, we adjusted for possible comorbidities, including cardiovascular disease, diabetes, dementia and liver and kidney diseases,” says Nete Munk Nielsen.

No benefit of higher vitamin D levels

The study shows that vitamin D deficiency was associated with a higher risk of severe COVID-19.

Overall, the study shows that the risk of more severe COVID-19 among individuals with insufficient and sufficient levels was approximately 50 % of that among individuals with deficient levels.

“Of note, having vitamin D levels of 75 nmol/L or more compared with 50 to 74 nmol/L provided no additional protective effect,” explains Nete Munk Nielsen.

She points out, however, that since the study is an observational study, no definitive conclusion can be drawn on the mechanisms behind the association between vitamin D deficiency and the severity of COVID-19, and that this study does not provide any basis for speculating on the how vitamin D level affects the illness trajectories for other viral diseases.

However, she thinks that the study suggests that the association should be investigated more closely in larger well-designed studies.

“It is important to emphasise that our study does not support use of high doses of vitamin D, for example through dietary supplements. This study supports ensuring that we have optimal levels of vitamin D in accordance with the Danish Health Authority’s recommendation, not higher and not lower,” concludes Nete Munk Nielsen.

Vitamin D status and severity of COVID-19” has been published in Nature Scientific Reports. The Novo Nordisk Foundation has supported research on COVID-19 at the Danish National Biobank.

Denmark is at the forefront of epidemiological research. This is to a large extent due to the unique Danish registers which provide detailed informati...

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