As a result, vitamin D has attracted a lot of interest because of its ability to prevent and treat COVID-19. But most studies are observational and have produced conflicting findings.
The first trial, which enrolled 6,200 adults (16 years and older) who were not taking vitamin D supplements at the time of registration, was conducted in the UK between December 2020 and June 2021.
Half of the participants in the vitamin D test had a blood test (3,100 people). Those with low levels (2,674; 86%) were given 3200 IU/day or 800 IU/day vitamin D supplements for six months. The other half (control) had no trial or pill.
During the six-month follow-up period, neither dose of vitamin D affected the number of acute respiratory infections or COVID-19 that were identified. There were no significant side effects associated with the supplements studied, and the number of side effects was comparable between the groups.
The second trial, which used cod liver oil and was low in vitamins D, A and omega-3 fatty acids, was conducted in Norway from November 2020 to June 2021.
Approximately 34,741 adults (18-75 years) not taking vitamin D supplements were given 5 mL of cod liver oil or 5 mL of corn oil as a placebo every day for six months. Most of the subjects (86%) who underwent the trial started the trial with adequate vitamin D.
Again, when compared with placebo, the researchers found no difference between cod liver oil and PCR-confirmed acute respiratory infection or COVID-19. The cod liver oil group had only milder side effects compared with the placebo group.
Results should be evaluated further as both studies used a highly effective vaccine.
Does vitamin D protect against COVID-19?
Despite this, both studies have several advantages, such as excellent participant compliance and the use of RT-PCR swab tests to confirm infection. The findings support other studies that have demonstrated no protective effect of vitamin D on the risk of COVID-19.
Vitamin D supplementation does not appear to reduce the incidence of COVID-19 or other acute respiratory infections.
In a related editorial, Professor Peter Bergman of the Karolinska Institutet in Sweden said that vaccination remains the most reliable method to prevent COVID-19 infection. Vitamin D supplements and cod liver oil should not be given to healthy people with normal vitamin D levels.
Instead, he recommends that clinicians focus on high-risk populations, such as those with darker or less sun-exposed skin, pregnant women and older adults with chronic diseases.