Higher vitamin D level linked with reduced risk of infection

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.

9 Responses to Higher vitamin D level linked with reduced risk of infection

  1. Vivian 28 June 2010 at 10:30 pm #

    It’s only anecdotal, but my incidence of colds and flu has decreased dramatically since I started taking VitD3. Last cold was Feb ’09. Since that time my husband has had 4 colds, and nearly everyone in the office was sick over the winter. Didn’t catch any of it – in the past I would have been sick at least a couple of times over the winter. I’ve since got my 76 year old mother to agree to taking VitD and she’s noticed the same.

  2. Dave in Ohio 29 June 2010 at 1:11 am #

    I highly recommend Michael Holick’s “The Vitamin D Solution”, published in April 2010. A comprehensive treatment of what we know today about Vitamin D. (Ignore his saturated fat advice)

  3. Donald G 29 June 2010 at 1:56 am #

    Dr. Briffa,

    I would like to know your current view of the most desirable blood level of 25 OH vitamin D, especially in old people.

    The latest issue of Sidney Wolfe’s Worst Pills, Best Pills (I think a solid publication) warns against excessive use of all vitamins, including D, with the latter sometimes causing, among other problems, the deposition of calcium in arteries.

  4. Reijo Laatikainen 29 June 2010 at 8:29 am #

    There is also evidence from randomized controlled trials that vitamin D supplementation potentially decrease viral infections, at least influenza A (Urashima M et al. .Am J Clin Nutr 91: 1255-1260, 2010)

  5. Liz Smith 2 July 2010 at 10:00 pm #

    My sister in Australia had her blood tested and it came out at 19. They said they wanted it between 90-160 and it took her two years to get it up to 92.

    She says its a serious problem down under as no one will go out in the sun and when tested most blood levels are seriously low.

    If I cannot pronounce the ingredients of the sun screen ‘stuff’ I do not use it, and keep out of the midday sun. I’m waiting for my latest results to come back.

  6. Margaret Wilde 3 July 2010 at 2:09 am #

    Years ago I went as a volunteer – human guinea pig – to the Common Cold Research Centre (it closed down some years ago) at Harvard Hospital, near Salisbury. This was unpaid – just expenses. The virologists there taught us quite a bit about the common cold and their discoveries about it. Apparently more colds are caught on New Year’s Day than any other day because of people going to New Year’s Eve parties, which tend to be inside in the warmth, with lots of people crowded together, and some will have colds and be coughing, etc. Germs may be transferred to, say, a door handle and then transferred to your hand when you touch the handle. Then if you rub your eye, say, you can by doing that introduce germs into your body through the eye. Alcohol at the party will have opened the pores of your skin and warmed you up a lot. – When you leave to go home, even if you have wrapped up well, your opened pores encounter the shock of the cold air and you get chilled – and when you are chilled, your body’s defences are lowered and so the germs that have entered your system can now get a foothold. – That was their explanation at that time.

  7. Jill H 3 July 2010 at 10:09 pm #

    I think about this a lot. At the end of last year, my 89 year old dad suffered a serious stroke. He had already had several ‘mini strokes’ and vascular dementia was heightened to a point where it was no longer safe to be in his own home even with 24 hour care – I was strongly advised (told) that for his own safety he needed to be looked after in a home. I then set about trying to find one that would fit the needs of a much loved person. I settled on a home that had a beautiful garden with a bird table. My mother, when she was alive, kept a beautiful garden with a bird table that many birds and squirrels visited and I had hopes that perhaps dad would sit in the garden and remember those times in some way. But each time I visited no one was in the garden. I would put dad’s flat cap on his head and walk with him in the garden and talked to staff about perhaps starting a ‘wheelchair club’ where several residents of the home could be taken out to have a cappuccino or tea or something. Dad died several months later – an infection – kidney failure – dementia. We never got the wheel chair club going. Making sure that elderly people always have access to drinks so that they do not become dehydrated – a walk in the sun to help ensure a little vitamin D to perhaps improve chances of fighting infections. Little steps or big strides in our care of the elderly in homes.

  8. stefano 6 January 2012 at 1:35 am #

    i have the same experience about vit d definceny and used to have flu/colds almost every month of the year despite vaccines.
    2009 since i got vit d oover 35ng/ml flu has become a one day illness with just some low fever and some bone pains, no vaccines
    2010 vit d over 50ng/ml, no flu
    2011 vit d over 70ng/ml no flu, anybody with bad flu and antibiotics for 15days in this period, still no flu for me


  1. More evidence links higher vitamin D with lower risk of infection | Dr Briffa's Blog - A Good Look at Good Health - 7 June 2011

    […] have previously reported on another study which found higher levels of vitamin D were associated with reduced risk of flu and reduced […]

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