Synopsis

Source of validity: Scientific
Rating (out of 5): 3

Vitamin D has been studied for its role in supporting the immune system and potentially helping to prevent or treat infections. The scientific rationale arises from vitamin D’s function in modulating both innate and adaptive immune responses. Immune cells such as macrophages and dendritic cells express vitamin D receptors and are able to convert circulating 25(OH)D to the active form, calcitriol, which then influences the expression of antimicrobial peptides like cathelicidin and defensins. These peptides enhance the body’s ability to destroy invading pathogens, especially in the respiratory tract.

Epidemiological studies have found associations between low vitamin D levels and increased susceptibility to infections such as influenza, pneumonia, and respiratory tract infections. Several randomized controlled trials (RCTs) and meta-analyses have investigated whether vitamin D supplementation reduces infection risk. A 2017 meta-analysis published in BMJ found that vitamin D supplementation modestly reduced the risk of acute respiratory infections, particularly in individuals with marked vitamin D deficiency. However, results across studies are mixed, with some trials showing no significant benefit.

While vitamin D is not a treatment for active infection, maintaining adequate levels may help reduce the risk or severity of infections. The evidence is strongest for respiratory tract infections, while data for other infections are less robust. Thus, scientific evidence supports a preventive role for vitamin D in certain infections, but further research is needed to clarify optimal dosing, target populations, and effects on specific pathogens.

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