Higher vitamin D level linked with reduced risk of infection

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In previous posts (here and here) I have highlighted the benefits vitamin D has with regard to improving the immune response and helping keep infections such as flu at bay. It has been mooted that the upsurge in viral infections during the winter is connected with the generally lower vitamin D levels at this time. The traditional view is that winter infections are due to ‘indoor crowding’. However, it turns out that flu epidemics do not occur in the summer in crowded workplaces, for instance, despite the presence of the flu virus around people who should, technically speaking, be susceptible to infection [1,2].

These facts were plucked from a recently published study which looked at the relationship between vitamin D levels and risk of viral respiratory tract infection such as cold and flu [3]. This study assessed blood levels of vitamin D and viral infection in almost 200 American men and women throughout last autumn (fall) and winter.

Some interesting findings from this study were:

Compared to individuals with vitamin D levels <38 ng/ml (95 mmol/l), individuals with levels of 38 ng/ml or above were about half as likely to suffer from a viral respiratory infection during the study period.

Of those with higher vitamin D levels (as defined above), 83 per cent had no infections at all during the study period, compared to 55 per cent of those with lower levels.

Those with higher levels of vitamin D who succumbed to flu were ill for an average of 2 days per infection.

Those with lower levels of vitamin D who succumbed to flu were ill for an average of 9 days per infection.

This study showed that as vitamin D levels rose, so did resistance to infection. However, the benefit appeared to plateau at about 38 ng/ml (which is why this figure was chosen as a cat-off in the analysis).

Of course, ‘epidemiological’ studies of this nature a cannot be used to prove ‘causality’ (i.e. that higher vitamin D levels protect against viral infection), only that the higher vitamin D levels are associated with improved resistance to infection. However, the idea that vitamin D might actually help protect against infection is at least plausible. As the authors of this study point out:

Vitamin D has known effects on the immune system. The production of the antimicrobial peptides cathelicidin by macrophages and β-defensin by endothelial cells is upregulated by vitamin D [4,5]. These peptides may be involved in the direct inactivation of viruses [6,7].

If it is true that optimising vitamin D can help protect against infection, then this might have particular significance for the elderly, particularly those who are institutionalised. A combination of low vitamin D (at least partly due to reduced sunlight exposure), somewhat compromised immune function and crowding, could indeed be a lethal mix for some. Enhanced sunlight exposure and/or vitamin D supplementation could be a safe and inexpensive way of protecting against illness and preserving life.


1. Dowell SF. Seasonal variation in host susceptibility and cycles of certain infectious Diseases. Emerg Infect Dis. 2001;7:369-374

2. Thacker SB. The persistence of influenza A in human populations. Epidemiol Rev. 1986;8:129-142

3. Sabetta JR, et al. Serum 25-hydroxyvitamin d and the incidence of acute viral respiratory tract infections in healthy adults. PLoS One. 2010 Jun 14;5(6):e11088.

4. Liu PT, et al. Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science. 2006;311:1770-1773

5. Wang TT, et al. Cutting edge: 1,25-dihydroxyvitamin D3 is a direct inducer of antimicrobial peptide gene expression. J Immunol. 2004;173:2909-2912

6. Gropp R, et al. Epithelial defensins impair adenoviral infection: implication for adenovirus-mediated gene therapy. Hum Gene Ther. 1999;10:957-964

7. Daher KA, et al. Direct inactivation of viruses by human granulocyte defensins. J Virol. 1986;60:1068-1074.

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