Lack of context: The claim did not mention that this study and others failed to find an association between vitamin D levels in the body and the risk of COVID-19 infection. This raises the possibility that the results observed in the study are due to selection bias or confounding factors, rather than the effects of vitamin D.
FULL CLAIM: “Habitual use of vitamin D supplements was associated with a 34% lower risk of COVID-19 infection”; “This is an observational study so causation cannot be established”; “Only large randomized-controlled trials will be able to test whether vitamin D plays a role in COVID-19”
An Instagram post claimed that a study had shown that vitamin D supplements reduced the risk of COVID-19 infections by 34 percent.
The study, published in The American Journal of Clinical Nutrition, used data from the U.K. Biobank, a large collection of data from more than 500,000 people in the United Kingdom. The U.K. Biobank is commonly used by researchers studying potential associations between lifestyle, health, and genetics.
The study only included 8,297 people who had records of a COVID-19 test between 16 March 2020 and 29 June 2020, as well as complete records on vitamin D supplement use and blood levels. Of these, 363 people regularly took vitamin D supplements.
The researchers found significant differences between vitamin D supplement users and non-users, including age, gender, ethnicity, social deprivation, diet, and the prevalence of several diseases such as cancer and chronic obstructive pulmonary disease.
There was no statistically significant difference in the number of COVID-19 cases between the two groups. But when researchers adjusted the figures to account for differences in the groups’ characteristics, such as those listed above, there was a weakly significant difference in the number of COVID-19 cases.
The Instagram post correctly stated that the vitamin D users had a 34 percent lower risk of COVID-19, after adjustment. However, there is a wide margin of error on this number, from 3 percent to 55 percent. This means we cannot have much confidence in the 34 percent figure and that any benefit from vitamin D supplements may be too small (as low as 3 percent fewer cases) to make a significant clinical difference.
The authors of the paper also analyzed levels of vitamin D as well as genetic factors that affect vitamin D levels for both supplement users and non-users. These would be expected to have similar effects on COVID-19 infections as vitamin D supplements. However, neither of these factors affected the risk of COVID-19.
As the authors of the study stated:
“In conclusion, an inverse association between habitual use of vitamin D supplements and the risk of COVID-19 infection was observed in the current study. However, we cannot rule out the possibility that the observed inverse association was due to residual confounding or selection bias, since we did not find a consistent inverse association between baseline circulating vitamin D levels or genetically predicted vitamin D levels and the risk of COVID-19 infection. Further clinical trials are needed to verify such an inverse association between the habitual use of vitamin D supplements and the risk of COVID-19 infection.”
Furthermore, during most of the time period that the study analyzed, COVID-19 tests were not widespread, which the authors acknowledged as a limitation: “COVID-19 tests were largely restricted to participants with symptoms in hospitals during the spring period, who might not represent the whole population in the United Kingdom”.
As the Instagram post explained, a large randomized controlled clinical trial would be needed to provide high-quality evidence that vitamin D levels cause a reduction in the risk of COVID-19 infection and to rule out other factors that could have influenced the results, such as selection bias in the study population and confounding factors.
The primary source of vitamin D in our bodies is through skin exposure to sunlight. Having darker skin, spending more time indoors, and living at higher latitudes during winter can increase the risk of vitamin D deficiency.
Researchers have studied vitamin D many times since the start of the COVID-19 pandemic. Vitamin D is considered a possible contributing factor to the increased disease burden among nursing home residents and ethnic minorities. However, other studies that also used the U.K. Biobank data arrived at the opposite conclusion, reporting that vitamin D levels in the body aren’t associated with decreased risk of COVID-19 infection[2-4].
Previous research into vitamin D identified plausible biological mechanisms for how it could benefit immune defenses against infection. An analysis in 2017 of 25 randomized controlled trials showed that vitamin D supplementation reduces the risk of acute respiratory tract infection, especially for those who are very deficient. Earlier reviews by Health Feedback also discussed the subject of vitamin D and its effects on preventing and treating acute respiratory infections like the flu and COVID-19 (see here and here).
Although there is not enough evidence to say that vitamin D supplementation reduces the risk of COVID-19 infection, some health bodies, such as the U.K. National Health Service and Northern Ireland’s Public Health Agency, recommended its wider use to counter the increased risk of vitamin D deficiency that people face from spending more time indoors due to COVID-19 restrictions. Because of its low toxicity, many in the scientific community argued that the potential benefits of using vitamin D supplements in preventing COVID-19 infection would be worth the risks of vitamin D overdose. That being said, those who decide to take vitamin D supplements should still take care to avoid vitamin D overdose, which can lead to bone pain and kidney stones.
- 1 – Ma et al. (2021) Habitual use of vitamin D supplements and risk of coronavirus disease 2019 (COVID-19) infection: a prospective study in UK Biobank. The American Journal of Clinical Nutrition.
- 2 – Hastie et al. (2020) Vitamin D concentrations and COVID-19 infection in UK Biobank. Diabetes & Metabolic Syndrome.
- 3 – Raisi-Estabragh et al. (2020) Greater risk of severe COVID-19 in Black, Asian and Minority Ethnic populations is not explained by cardiometabolic, socioeconomic or behavioural factors, or by 25(OH)-vitamin D status: study of 1326 cases from the UK Biobank. Journal of Public Health.
- 4 – Hastie et al. (2021) Vitamin D and COVID-19 infection and mortality in UK Biobank. European Journal of Nutrition.
- 5 – Lagishetty et al. (2011) Vitamin D metabolism and innate immunity. Molecular and Cellular Endocrinology.
- 6 – Martineau et al (2017) Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. British Medical Journal.