Vitamin D maligned yet again

“Two new trials find no link between vitamin D supplements and reduced risk of covid-19.” This was just one of over a dozen similar headlines over the past week, all describing the results of new research into vitamin D and COVID.

Both studies were published in the British Medical Journal. The first concluded that

Among people aged 16 years and older with a high baseline prevalence of suboptimal vitamin D status, implementation of a population level test-and-teach approach to vitamin D supplementation was not associated with a reduction in risk of all cause respiratory tract infection or covid-19.

The second study, which examined the efficacy of cod liver oil, concluded that

Supplementation with cod liver oil in the winter did not reduce the incidence of SARS-CoV-2 infection, serious covid-19, or other acute respiratory infections compared with placebo.

Both studies have serious flaws that cast doubt upon their conclusions.

The first study compared dosing people with either 800 IU or 3200 IU of vitamin D per day, with a control group of no testing and no supplementation. “Suboptimal” vitamin D status was defined as blood 25(OH)D concentration of <75 nmol/L, which is curious, as the current UK definition of deficiency is <25 nmol/L, and levels as low as 50 nmol/L are considered adequate.

In fact, an older BMJ study (from 2017) examining the protective effect of adequate vitamin D levels on general respiratory infections used a cutoff point of <25nmol/L and found that

Vitamin D supplementation was safe and it protected against acute respiratory tract infection overall. Patients who were very vitamin D deficient . . . experienced the most benefit.

Furthermore, studies from early in the epidemic showed that as expected, the beneficial effects of vitamin D for respiratory infections in general extended to COVID-19 specifically, such as in this study which used a <50 nmol/L cutoff point and found that

Patients with vitamin D deficiency [<50nmol/L] were 14 times more likely to have severe or critical disease than patients with ... [levels of >100nmol/L].

The second study recently published in the BMJ suffered from similar problems, most notably that almost 90 percent of the trial participants had adequate vitamin D levels to start with (>50nmol/L).

Furthermore, cod liver oil contains a significant amount of vitamin A which would have acted to dilute the effects of the vitamin D.

There has been a plethora of studies examining the impact of vitamin D deficiency on COVID-19 from 2020 onward, given that prior to the COVID era, it was widely known that too little of this micronutrient was a contributory factor in many diseases including diabetes, cardiovascular illness, and autoimmune disorders. Mainstream media in general have been accepting of the fact that people seriously deficient in vitamin D are in far greater danger of serious consequences from SARS-CoV-2 infection. Mainstream websites such as WebMD have not shied away from reporting on studies such as the one cited above which described the 14-times-greater likelihood of severe disease with vitamin D deficiency, even noting that the trial's results showed that

… the mortality rate for those with insufficient vitamin D levels was 25.6%, compared with 2.3% among those with adequate levels.

Given that vitamin D is not usually promoted as a population-wide intervention but rather as a specific prophylaxis to be used as needed, it is unclear why the BMJ chose to highlight two studies which are essentially straw men. Indeed, an editorial in the BMJ itself highlighted these and other serious flaws in the study; this editorial, however, has not been widely cited in mainstream media.