Vitamin D supplementation does not decrease incidence of viral upper respiratory tract infections

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Key study points:

1. Vitamin D supplemented individuals showed no significant difference in number, severity, or duration of swab-positive URTI episodes when compared to participants taking placebo

Primer: Calcitriol, the active metabolite of vitamin D, is formed when inactive vitamin D from the sun is absorbed by the skin and then activated by the kidneys and liver. It can also be provided through a diet rich in fatty fish, eggs, fortified milk products, or supplements.  Active vitamin D has also been linked to innate and adaptive immune function, with recent studies exploring the relationship between vitamin D levels and a variety of autoimmune disorders as well as disease susceptibility. Many researchers have investigated the vitamin’s connection to upper respiratory tract infections (URTIs), proposing that serum vitamin D levels are inversely related to the development of URTIs; however, results have been inconclusive. The current study aims to elucidate the possible relationship between vitamin D supplementation and URTI susceptibility.

Background reading:
1. Overview of general physiologic features and functions of vitamin D1,2,3,4 [The American Journal of Clinical Nutrition]
2. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey [Archives of Internal Medicine]

This [randomized, double-blinded, controlled ] study: 322 individuals from New Zealand, aged 18 years or older, were randomized to a treatment group receiving vitamin D3 for 18 months or a placebo group following the same dosing regimen. Participants underwent initial demographic evaluations, monthly visits for medication administration and reporting of URTIs, and were also told to contact staff at the onset of URTI symptoms such as rhinorrhea, sinus congestion, sore throat, or cough. Individuals experiencing symptoms underwent a standardized evaluation and a nasopharyneal sample was sent for viral identification.
No significant difference was found between the treatment and placebo group regarding the duration, severity, or number of swab-positive URTI episodes. Most participants experienced URTI symptoms at least once during the study, with only 13 participants not having symptoms. Rhinoviruses and coronaviruses were most commonly identified viral strains on positive swabs. Results were controlled based on season and baseline vitamin D levels.

In sum: The current study found no association between vitamin D supplementation and viral URTI regardless of season, implying that supplementation may not prevent URTI episodes. The researchers recognize that these findings are both supported and refuted by previous studies with a variety of strengths and limitations. While this study has a relatively large sample size, it draws from a select New Zealand population and all participants randomized to the treatment group underwent the same dosing regimen. Researchers propose that working with a population more prone to vitamin D deficiency might yield different findings and encourage future research working with a varied study population on different dosing regimens. Although this study’s findings do not highlight any benefit of supplementation as viral prevention, vitamin D supplementation for a variety of other reasons is not discussed or discouraged.

Click to read the study, published today in [JAMA]

By LHC and MS

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