Could low levels of vitamin D help explain why we’re prone to infections in the winter?

Here in the UK the weather has turned distinctly autumnal, and the drop in temperature will no doubt herald an upsurge in the risk infections including those affecting the ‘respiratory tract’. There is some thought in natural medicine that certain nutrients have a particular part to play in ensuring a healthy immune response, and may therefore have a bearing on whether we succumb to whatever bugs happen to be lurking in our environment.

But why is it that the colder, winter months bring with them an increased risk of infection? One explanation might relate to vitamin D. In the colder, darker months, sunlight exposure is generally reduced, and therefore so are vitamin D levels in the body. While vitamin D has an established role in the preservation of bone health and an increasing reputation as something that can help ward off disease (including cancer and multiple sclerosis), there is also evidence that this nutrient has the ability to help the body repel microbial infection.

In a study published last month in the America Journal of Clinical Nutrition, 800 Finnish men had the level of vitamin D measured in their bloodstream, after which they were followed for a six-month period. The researchers who conducted the study then assessed whether there was any relationship between blood vitamin D levels and the number of days taken off sick due to respiratory tract infection (e.g. chest infection).

The researchers found that individuals with vitamin D levels less than 40 nmol/l, compared to those with levels greater than 40 nmol/l, took twice as many sick days of work due to respiratory tract infection (4 days of average rather than 2). This study suggests that one reason that individuals are more prone to infection in the winter has something to do with lower levels of vitamin D in their bodies.

The study found that greater physical activity was associated with enhanced levels of vitamin D. This association may, of course, be related to the fact that active individuals tend to spend more time outdoors. Smoking, on the other hand, was associated with lower vitamin D levels.

While no studies have ever tested this, it could be that boosting vitamin D levels may enhance our ability to stay infection-free during the winter. Getting as much sun exposure as possible may help here. Another tack might be to supplement with, say, cod liver oil. For more information about this, and the research linking vitamin D supplementation and a reduced risk of death, see my blog post of 12th September 2007.


1. Laaksi I, et al. An association of serum vitamin D concentrations < 40 nmol/L with acute respiratory tract infection in young Finnish men. Am J Clin Nutr 2007 86: 714-717.

4 Responses to Could low levels of vitamin D help explain why we’re prone to infections in the winter?

  1. Tiggy 13 October 2007 at 1:37 am #

    Getting as much sun exposure as possible – hmm, I still have doubts that you can get enough in England during the Winter. I mean you can’t spend all day outside and most people are in offices during daylight hours.

    Can you suggest something other than Cod Liver Oil that provides Vitamin D? I thought Cod Liver Oil was too toxic now?

  2. helen 14 October 2007 at 10:36 pm #

    Also consider that sugar depresses the immune system. The equivalent of 2 cans of soft drink (soda) will compromise the immune system within 20 minutes of consumption & it’s effects can last up to 5 hours it is little wonder that with the dreadful overconsuption of sugar in our diets as well as the lack of essential vitamins such as D & others that boost the immune system that diease becomes rampant during times of increased germ production.
    Cod liver oil is also a natural sourse of vitamin A which is also essential to good immune function.

  3. Neil 16 October 2007 at 6:32 pm #

    Given that all our daylight comes from the sun, does cloud cover filter UVB? Wholly or partially? If partially the perhaps exposing skin to daylight will produce Vit D, albeit at a low level. Ideas anyone?

  4. Mo 18 October 2007 at 6:44 pm #

    This ties in with your previous post on cholesterol…

    Vitamin D is made when UVB hits the skin and reacts with a cholesterol precursor, therefore without adequate sunlight, vitamin D levels are low. And cholesterol is also higher.

    Yet, it needn’t just be colder months that create deficiency. Due to latitude, England can be UVB lacking up to 6-8 months per year, on top of which humans wear clothes and spend a lot of time indoors and in enclosed transport. And yes, fog, cloud cover and pollution inhibits UVB. Even windows only let 6% through.

Leave a Reply