Vitamin D May Prevent Colds And Flu

in health •  3 years ago  (edited)


A meta-analysis study released today in the BMJ (British Medical Journal) suggests that Vitamin D supplementation may help to prevent respiratory tract infections (colds, flu, pneumonia etc.).

This interesting because although it is known that Vitamin D is important for the normal functioning of the immune system such studies have had mixed results in the past.

It is open access and you can read/download it here.

I have tried to make this post a bit more succinct than usual and for the really time starved there is a TLDR summary in the "Conclusion" section - feel free to scroll down

Why Is this Important? - Previous Results Were Mixed

There have been a number of previous meta-analyses, some (2 studies) have found beneficial effects in response to taking Vitamin D to prevent respiratory tract infections some have not (3 studies).

The variable results (heterogeneity) have been hypothesised to be due to various factors which might "muddy" the results.

For example if Vitamin D only benefits people who are deficient and not the general population, any effects you see will be diluted if you have a lot of people who have normal Vitamin D levels.

It may also be that certain administration methods such as giving a massive monthly dose (bolus) are less effective than smaller daily doses and viewing both types of data may mask the actual effects.

Similarly if Vitamin D is more useful in people with asthma or COPD (Chronic Obstructive Pulmonary Disease) then you might again get a similar effect where results become inconsistent as a result of the makeup of the potential populations involved.

This meta-analysis is different because it specifically looks at the general effects on all groups and then does further analysis to look at what if any effect these different variables have on the effect of vitamin D supplementation.

The Findings

The main finding is that Vitamin D supplementation reduced the risk of acute respiratory tract infection (ARTI) among ALL participants with an adjusted odds ratio of 0.88.

This means that people taking Vitamin D in this study had a 12% lower risk of ARTI than those taking placebo.

Also of interest is the finding that the way Vitamin D is administered makes a huge difference.

Giving those big "bolus" doses every 1-3 months has a much smaller effect (odds ratio 0.97) vs small doses daily or weekly (odds ratio 0.81).

So to get the most benefit one would presumably need to take it daily or weekly.

For those who were deficient in Vitamin D (<25 nmol/L) and getting regular doses the effect was even greater with an odds ratio of 0.30 - so a 70% reduced risk of ARTI!

This strongly implies that Vitamin D plays an important role in preventing against these types of infections.

Further in those taking Vitamin D there was no increase in serious adverse events (of any cause) above the levels in people taking placebo.

This strongly suggests that there is no discernible disadvantage to taking it which is also quite important.

Discussion, Limitations, Issues

From my reading of this study it appears that the authors have gone to great lengths to ensure that this study eliminates sources of bias and attempts to make the results more powerful statistically.

This extends from their eligibility criteria where they only used double blind randomised controlled trials that were prospectively looking at ARTI to their fine checking of the results for data and typographical errors.

Also the authors only wanted to use participants for whom IPD (individual participant data) was available.

This was to allow detailed analysis of subgroups and variables (e.g. age, Vitamin D levels, lung disease etc.) as mentioned earlier to pursue and control for sources of the previously mixed results in earlier studies.

Of the 532 studies found only 25 were eligible. Further using IPD meant only about 96.6% subjects were eligible.

The total sample size at the end was 10993 which is still quite large for this sort of study.

The authors say that their study makes a definite case for supplementation as a means of preventing respiratory tract infections and I would broadly agree with this.

It might be best to initially restrict this to people who are at higher risk from complications to respiratory tract infections and are deficient in Vitamin D.

I think it depends how cautious you are as a clinician and I personally would wait until putting everyone on supplemental Vitamin D.

Towards the end of the paper the authors suggest looking at using it to fortify food which would basically put nearly all the population on supplementation.

Personally I think it is too early for that and we need further studies because I am naturally cautious particularly given the previously mixed results.

I'm sure others will disagree with me though.

Quick Note - My Own Situation

I suffer from quite severe asthma and a simple cold can result in an escalating situation that requires hospital admission and heavy steroid treatments (which have severe side effects themselves).

In my own case I have also consistently been found to have low Vitamin D levels and have noted myself that since I have been on Vitamin D supplements the number of colds and chest infections I have had has gone down quite a lot.

I used to have about 2-3 colds without fail every year.

I have not had one this year and the last few times I have had them they have resolved without the usual escalation.
Note this is not good evidence though. It is purely anecdotal and consists of a single potentially very biased sample!


This is a very promising study that shows that Vitamin D has an important role to play in preventing respiratory tract infections.

It appears to show a reduced risk of acute respiratory tract infections in all participants which if it holds out in further studies could have significant implications.

ARTIs cause approximately 10% of hospital admissions and 2.65 million deaths worldwide (2013).

The potential to save life as well as healthcare costs is huge but I feel we must also be a little cautious.

For those who have low Vitamin D levels and/serious pre-existing diseases I think supplementation (as long as it is given as regular doses) makes sense but consult your physician first.

For those who have good diets and are otherwise fit and well, it is unlikely to do any harm based on these results, but in my opinion it is still worth waiting until we have more studies to confirm this.

These kind of people are at much lower risk of complications from these respiratory infections.

Whatever your situation I would still strongly advise that you discuss the situation with your physician BEFORE starting any new kind of supplement.


  1. Martineau, Adrian R., David A. Jolliffe, Richard L. Hooper, Lauren Greenberg, John F. Aloia, Peter Bergman, Gal Dubnov-Raz, et al. 2017. “Vitamin D Supplementation to Prevent Acute Respiratory Tract Infections: Systematic Review and Meta-Analysis of Individual Participant Data.” BMJ 356 (February). British Medical Journal Publishing Group: i6583.

Thank you for reading

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  ·  3 years ago (edited)

Thank you for your article, I found another publication from 2010 confirming a similar hypothesis. Would you mind discussing other important functions of Vitamin D from your professional perspective? I have been interested in the topic for several years and have been supplementing Vitamin D in relatively high doses (1000 IU 2-3 times a week, sometimes daily) and subjectively I have had a LOT less colds, so my personal experience also speaks for what the researchers found. However, there seem to be many other benefits of having adequate levels of Vitamin D, more in the range >50 nmol/L I have read several times, but the <25 nmol/L are commonly referred too, why is there such a discrepancy? Other sources I have read claim for example that:

  • Vitamin D reduces the risk of multiple sclerosis1
  • Vitamin D decreases chances of developing heart disease2
  • Vitamin D might help with depression3 4
  • Vitamin D seems to have a positive effect for those who try to lose weight5

From my point of view it seems that Vitamin D is way too undervalued/underrated when considering possible risks. Vitamin D is less toxic than water...assuming 2.5 liters/day are recommended, four times that, e.g. 10 liters/day could kill you, except of course you're sweating a lot. For a very high "safe" dose of daily Vitamin D intake of 4000IU6 it would need 25 times that ammount to even start showing symptoms of an overdose. But I'm getting sidetracked here =). My main question for you is: What is the reason Vitamin D gets relatively little attention compared to the possible benefits of supplementing it?

Even increasing the commonly recommended daily dose of 400IU to let's say a 1000UI per day seems to be impossible, regardless of all the studies that show evidence of higher doses being beneficial for the mostly "indoorsy" lifestyle people in the western world have. Why would that be? Do interests of the pharmaceutical industry possibly play a role here? (Vitamin D is relatively cheap).

Looking forward to reading your comment or another article about the subject!

My main question for you is: What is the reason Vitamin D gets relatively little attention compared to the possible benefits of supplementing it?

The main reason I think is that the previous meta-analyses have had mixed results.

Do interests of the pharmaceutical industry possibly play a role here? (Vitamin D is relatively cheap).

It's possible but I think if Vitamin D did become "big" they would come out with a branded version. I think it is more likely that being a vitamin it doesn't get as much attention as other new agents because it is not "glamorous".

Hi! We hope to discuss your post today on our podcast!
Feel free to join us at 3pm CST / 9pm GMT at!

Thanks will be around for sure as have some papers to write and review:) I love to have it on while I work.

This is great news and thanks for the nice explanation. Balanced diets and moderation are always advisable. People want to find a magic pill for everything; there isn't one. It is important to have a balanced diet, including plenty of foods that contain Vitamin D. If supplementation is needed, then lots of multivitamins have added bigger doses of D in recent years (especially kids' vitamins). But people should not suddenly start taking huge doses of Vitamin D - all things in balance and moderation.

Exactly. Thanks for your sensible words:)

  ·  3 years ago (edited)

I must be the counterexample then... I was tested as deficient in D-vitamins a couple of years ago. I am however very rarely sick (this happens once every two years, but then I get the big one :p ).

PS: my deficiency is definitely connected to my life style, spending most of my time inside in front of a computer...

I think mine too. Personal examples are not great though :)

statistically speaking, they don't count, on top of being super-biased :)

Yes exactly.

Interesting post. I think anything that strengthens the immune system would help to prevent, or at least lessen the effects of, colds. I am not a Doctor or medically trained person, I just am interested in cybernetics and like to know how the body (like any other system) regulates itself and how the immune system fights off infection (or tries to).

There are many herbs that are suitable for this purpose and could be added to meals, although it's a good idea to check online about side affects (I am taking a supplement that is supposed to be good for memory etc, but the side affects include bloating which isn't as bad if it's taken using butter or anything else fat soluble).

Yes that is the reasoning behind this but quite often things that should help don't so that is why we need to have proper research to make sure.

Understandable. As I said, I'm not a Doctor so I am only basing my comments on what I have read and what I do understand about the immune system (not a lot admittedly). Let's hope we can find a number of great natural supplements that working together and don't have anything that can cancel another out :-)

Yes for sure:)

@thecryptofiend #til Vitamin D may help prevent colds and flu thanks to you
I should go out often in the sun
I used to take vitamin D complements just because my skin was getting white patches .. I didn't know it could also help against cold... we lack that here during seasons when the sun is hiding in the clouds much

It's tricky though because sun exposure can increase your risk of skin cancers!

@thecryptofiend - true so do everything in moderation :D
applies here, too