Vitamin D prevents colds and flus

A meta-analysis published in the British Medical Journal has found vitamin D supplementation cuts the rate of upper respiratory infections people experienced.


A meta-analysis looks at numerous studies which investigate the same topic and combines the data to look for trends.  This one looked at 25 studies with 10933 participants.  All the studies gave vitamin D to an intervention group and looked at the proportion who experienced one acute respiratory tract infection compared to those taking a placebo.

The studies took place in 14 countries over four continents and included both sexes and those aged from birth to 95 years of age.

The studies gave the dose of vitamin D in different ways

  • Bolus doses given at 1-3 months happened in 7 studies
  • Weekly doses were given in 3 studies
  • Daily doses were administered in 12 studies

The studies followed the participants from 7 weeks to 1.5 years. and all were randomised and double blinded.  This meant the participants and the investigators didn’t know who were in each category.

19 of the 25 trials took blood from participants at the start to check their baseline vitamin D concentration, they ranged from 18.9 – 88.9 nmol/L.

The meta-analysis obtained the raw data from the studies and reassessed it.

Less people taking a vitamin D supplement had a respiratory tract infection

Overall the study found that significantly fewer participants experienced one acute respiratory tract infection if they were taking vitamin D supplementation compared to not taking it.

The authors found that the bolus doses (those given in bigger doses every 1 – 3 months) really weren’t that helpful and if the weekly and daily dosed individuals were assessed alone then it became even more significant.

If you take those who had low vitamin D levels to begin with <25 nmol/L you only need to give 4 people a vitamin D supplement to prevent one person from having a respiratory tract infection.  If vitamin D level was above 25 nmol/L to begin with then this rises to 15 people needing to have the supplement to prevent an infection, but is still a significant result.

It found that side effects from vitamin D are rare.

So why is it that the bolus doses didn’t offer much protection, the study has suggested that if you give somebody a bolus of vitamin D then you are going to see a spike in the vitamin D blood concentration, which could have a knock on effect to the usual body processes that lead to activated vitamin D being made active or degraded.

What are the negatives of the meta-analysis?

The disadvantages of the study were that some data may be missing due to perhaps unpublished data, also some subgroups that were assessed had limited data.

Also, The studies didn’t tend to look at who adhered to taking the vitamin D, this could be a plus as the meta-analysis was an intention to treat study (this means it includes everybody, including those who were given the supplement and chose not to take it).  This means that anybody who replicates this study by taking the vitamin D religiously should see at least the benefit that the study found.

The last issue was that very few of the studies actually confirmed an infection with laboratory confirmation.  However, since these type of infections are normally diagnosed from history and examination, it is unlikely to be a big issue.