Dr John Campbell is a retired nurse educator and YouTube star who makes videos explaining the science behind COVID research and recommendations.
In several of his videos, he has discussed the role of vitamin D in immunity and highlighted studies suggesting that vitamin D supplementation could prevent COVID-19 or reduce the severity of symptoms.
It helps to damp down unwanted inflammation and overactive immune responses, and it enhances the function of immune cells that fight off pathogens like viruses and bacteria. And low levels of vitamin D have been associated with an increased risk of infection and respiratory diseases.
Early in the pandemic, John asked questions about whether a lack of vitamin D could play a role in causing the severe symptoms of COVID-19.
“It seemed that a lot of the complications of COVID-19 weren’t occurring in the first week when there is a significant viral phase of the illness, but they were occurring later on when there was an inflammatory stage of the illness,” says John.
“We know that vitamin D is an immunomodulator, so low levels of vitamin D could inhibit immunity and allow the inflammation to become rampant.”
Does having low vitamin D increase your risk of catching COVID-19?
Studies have linked low vitamin D levels with hundreds of diseases, including depression, rheumatoid arthritis, osteoporosis and even an increased risk of catching respiratory viruses, including influenza.
“At least 100 diseases have been associated with low vitamin D levels because if you’re sick and you have a chronic disease, or a severe acute disease, your levels go down,” says Tim.
Although John acknowledges the limitations of observational studies, which can spot correlations but can’t prove that one thing causes another, he points out that many relationships that are now widely accepted, such as smoking causing lung cancer, were first identified in observational studies.
Can vitamin D supplements help treat or prevent COVID-19?
Randomised controlled trials which give people a vitamin D supplement or a placebo and then compare how supplementation affects COVID-19 have so far mostly failed to show any benefit.
A trial in Spain treated patients with COVID-19 with active vitamin D and looked at whether the vitamin reduced deaths or intensive care (ICU) admissions. Their results suggested an 80% reduction in admissions to ICU and a 60% reduction in deaths. But after they published their results, the work was widely criticised for methodological failings and later withdrawn.
Last year, ZOE and King’s College London did research looking at whether people who took vitamin D supplements during the pandemic were less likely to catch the disease. The study included 1.4 million people but failed to provide conclusive results in favour of vitamin D.
We found that the supplement provided a small beneficial effect but only for women, which may be explained by other factors.
“I think we are lacking an evidence base to rely on vitamin D,” says Tim.
John agrees that studies looking at vitamin D supplementation are often flawed. He highlights that the lack of large, methodologically sound vitamin D trials may stem from a lack of funding and interest from pharmaceutical companies.
We don’t know the ‘normal’ level of vitamin D
John began recommending vitamin D supplementation to people watching his YouTube videos because he believes that people in the UK are unlikely to get enough vitamin D from their diet or sunshine exposure.
“Studies from the UK looking at serum or active vitamin D levels, we find that the active vitamin D levels are really quite low compared with people who live in sunnier environments,” explains John.
While Tim agrees that people with very low levels of vitamin D may have suboptimal immune systems, he thinks that this affects less than 1% of the population.
Tim also highlights the fact that clinically low vitamin D levels aren’t well defined and normal levels can vary significantly from person to person, with up to 50% determined by your genes.
“For me, the danger is that if you oversell the supplement, you undersell real food and environmental and lifestyle factors that make you healthy,” says Tim.