Vitamin D is classified as a fat-soluble vitamin but functions physiologically as a steroid hormone. Its receptors, vitamin D response elements, are present on virtually every cell in the body, including every type of immune cell: macrophages, dendritic cells, T cells, B cells, and natural killer cells. Vitamin D deficiency does not simply reduce one immune function. It impairs the calibration of the entire immune system. The UK government advises supplementation for all adults from October to March, acknowledging that sun exposure in northern latitudes is insufficient to maintain adequate levels through winter.
Innate Immunity: Antimicrobial Peptides
One of the most important immune functions of vitamin D is its role in stimulating the production of antimicrobial peptides, particularly cathelicidin and defensin, by macrophages and epithelial cells. These peptides punch holes in bacterial and viral membranes, providing a fast front-line defence. Macrophages cannot produce adequate cathelicidin in the absence of sufficient vitamin D. This mechanism explains the well-documented association between vitamin D deficiency and increased susceptibility to respiratory infections, including tuberculosis, influenza, and COVID-19.
Meta-Analysis Evidence for Respiratory Infections
A 2017 meta-analysis by Martineau et al. published in the BMJ, the largest and most methodologically rigorous to date, pooled individual patient data from 25 randomised controlled trials involving 11,321 participants. The analysis found that vitamin D supplementation reduced the risk of acute respiratory infection by 12 percent overall, and by 42 percent in those who were deficient at baseline. Daily or weekly dosing was more effective than large bolus doses, suggesting that maintaining consistent blood levels is more important than periodic high-dose correction.
Autoimmune Disease and Vitamin D
A large randomised trial, VITAL, published in the New England Journal of Medicine in 2022 by Hahn et al., found that supplementation with 2000 IU of vitamin D3 per day for approximately 5 years reduced the incidence of autoimmune disease by 22 percent compared to placebo. The effect was stronger for inflammatory arthritis, polymyalgia rheumatica, and psoriasis. This is the first large randomised trial evidence for vitamin D in primary autoimmune disease prevention, moving the question from association to causation.
- Supplement with 1000 to 2000 IU of vitamin D3 daily from October to March at minimum
- Check serum 25-hydroxyvitamin D: the optimal range is 75 to 150 nmol per litre
- Take vitamin D3 with vitamin K2 to support calcium metabolism if supplementing above 2000 IU
- Vitamin D is fat-soluble: take with a meal containing fat for optimal absorption
- Adults with darker skin or those who cover their skin for cultural reasons require year-round supplementation
Sources
- Martineau, A.R., et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 356, i6583.
- Hahn, J., et al. (2022). Vitamin D and marine omega-3 fatty acid supplementation and incident autoimmune disease. BMJ, 376, e066452.