Does vitamin D really help combat Covid-19?

Until there is firm scientific evidence we should avoid overblown claims about near-magical substances.

(Photo By BSIP/Universal Images Group via Getty Images)

If there’s one thing you can rely on in this pandemic, it’s this: at least once a month there’ll be a new article in the media arguing that vitamin D helps prevent Covid-19.

The case is simple: we know vitamin D – the “sunshine vitamin” – is involved in immune function, so could it help us fight off the virus?

Some, such as the Labour MP Rupa Huq, are pretty certain of the answer. On Tuesday (12 January) she wrote an article in the Evening Standard where she described vitamin D as “wonderstuff” that “offers us all hope to stamp out this dreaded disease”. She wrote that its benefits are “hushed up” – perhaps, she suggested, because cheap vitamin pills can’t be monetised by Big Pharma, and are less glamorous than a vaccine.

Huq’s article appeared a few days after the publication of an Observer piece that described how she and David Davis, the Conservative MP, had become unlikely allies on this topic, campaigning to reduce what they see as the UK’s vitamin D deficiency. Huq and Davis are frustrated that bodies such as Public Health England say there is insufficient evidence to promote the vitamin, which Davis argues “could potentially save tens of thousands of lives” during the pandemic.

Why do the experts disagree with Huq and Davis? What does the evidence say on vitamin D and Covid-19? You might think that after nearly a year’s worth of studies, we’d have a pretty good idea of the answer. Far from it.

Most of the Covid-19/vitamin D research is in the form of observational studies – where researchers look at the correlation of blood vitamin D levels with the risk of getting Covid-19, or the severity of the disease. By now, there have been quite a few such studies, and researchers have done meta-analyses to pool all the data and try to draw broad conclusions.

If you just skim those meta-analyses, the case might look optimistic. One meta-analysis found that, although vitamin D wasn’t associated with risk of getting Covid-19, severe cases were more likely to involve a deficiency. Another found that, depending on how you measure it, a higher vitamin D level is sometimes associated with a lower risk of infection and hospitalisation. A third found lower levels of vitamin D in Covid-19 patients compared to those without the disease.

Exciting, right? Not really. Much of the research is low-quality, with small samples and questionable analyses. More importantly, observational research is bedevilled by what epidemiologists call “confounding”. For example, if older people, or those with darker skin, are more at risk of vitamin D deficiency, and they’re more at risk of severe Covid-19 for other reasons, this kind of study might mistakenly link the deficiency and the disease – even if one doesn’t cause the other.

What we need are randomised trials, where researchers give Covid-19 patients either vitamin D or a placebo and test whether the vitamin causes a better outcome. Surprisingly, only two exist to date. The first was a Spanish study that found that Covid-19 patients given a type of vitamin D supplement tended to do better. The aforementioned Observer article said this study “came close to incontrovertibly proving low vitamin D levels have a pivotal role in causing increased death rates”. That’s dramatically far from the reality: this was a pilot study with only 76 participants, and some clear flaws in its design.

The second is a somewhat larger study from Brazil, which fairly convincingly found no benefits of vitamin D supplementation for severe Covid patients. It’s not yet published in a peer-reviewed journal, but it’s oddly not mentioned by the vitamin D proponents.

You might be thinking: what’s the downside? Vitamin D is cheap. We’re all staying indoors during lockdown and therefore getting less sunlight. Even if vitamin D doesn’t help combat Covid-19, surely it’s a good idea to take supplements anyway? But this is a motte-and-bailey argument – where someone makes a very specific, controversial claim, that claim is attacked and they retreat to a far more general – but more defensible – position. We weren’t talking about the general benefits of vitamin D – we were talking about benefits for Covid-19. And the honest answer at this point is that science has failed to give us any clarity: we simply don’t know whether it has an effect.

And there are downsides. There is a new, more transmissible variant of Covid-19, and we need people to take the utmost caution not to catch it. Promoting the idea that there’s a simple, dramatically effective solution – “wonderstuff”! – that boosts our immune system and prevents the disease could easily lull people into a false sense of security: “I’ve taken my vitamin D capsule, so maybe I can do without wearing a mask today.”

Many more trials of vitamin D and Covid-19 are on the way, so we’ll get a more definitive answer soon. Until then, let’s lay off the overblown claims about near-magical substances and sinister conspiracies. It might feel unsatisfying but, as with so many parts of the coronavirus debate, the only sensible scientific view on vitamin D is one of uncertainty.