Vitamin D Supplements Aren’t Curing Asthma

Reports of the miraculous power of vitamin d are misleading at best

Gideon M-K; Health Nerd
5 min readOct 6, 2017
Pictured: Not a miracle

Anyone who has seen my picture (if you haven’t, it’s just up there in a little circle^) knows at least two things about me. One: I love cats. Especially fat ones who sit on my lap and purr.

On the left is Aurora. She is fat and cuddly. On the right is Luna. She is less fat, also cuddly, and a pain in the arse at 2am

And two: I’m very white.

What this means to my life, other than being woken up at 5 when the cats decide it’s time for breakfast, is that I burn incredibly easily. The sun in Australia is crazy, with the UV index getting up to 15 regularly on a summer’s day. For reference, the UV index was originally only intended to go up to 10, but in the theme of Australia being a place where everything is out to kill you it turns out that the sun is worse here too.

It is a cloudy, cold, spring day today. And the majority of Australia is covered in what I like to call “hellish purple”

In conclusion, I don’t get a lot of sun. The knock-on effect of this is that I have low vitamin d levels.

Why am I telling you all this? If you’ve been looking at the news recently, you might have noticed the headlines screaming that vitamin d supplementation was going to stop all asthma from being a problem ever.

Sadly, that’s not quite the case.

The Bloody Awesome Study

I’m often quite complementary of other scientists’ work, partly because I try to be a nice guy and partly because I live in the world’s thinnest glass house. But when I say that this is probably my favorite study this year, you should understand that it is no exaggeration.

It’s an awesome piece of work.

Building on previous studies that showed a decrease in the risk of certain outcomes of asthma from taking a vitamin d supplement, the researchers did what is called a systematic review and meta-analysis.

This type of research is generally considered to be the peak of evidence-based science. It means that the researchers dug through the literature, looking at thousands of studies, to see what the effects of vitamin d supplementation on asthma were. Not only this, they did it in a rigorous, controlled way which means that anyone who wants to can replicate their results. It means that you can confidently say — as long as they followed thorough methodology, which in this case they did — that this study represents the best scientific evidence on the subject that currently exists.

The exception to the rule, of course, being memes

They found that taking a vitamin d supplement, either through an injection, a daily supplement, or both, reduced the risk of having an asthma attack severe enough to make you go to the hospital. It also reduced the likelihood that asthma sufferers would need extra medicine to control the asthma attack.

Sounds great, right?

Don’t put away your puffers yet.

Minor Effects

The first thing to note is that most news stories wildly exaggerated the benefit that this study found. Despite the study itself using the absolute risk to describe the improvement when taking vitamin d — a decrease of 3% when compared to the people who didn’t take vitamin d — most news outlets reported it…differently.

Pictured: …differently

Yes, most media stories used the more misleading relative risk, which showed a 50% decrease in asthma attacks. I’ve written many times about why this is bad, but basically it boils down to the fact that relative risk is often a large overstatement of what the risk actually means.

There was also some evidence that the vitamin d supplementation was only helpful for those who had low levels of vitamin d. This makes sense — supplementation is usually pointless if you don’t have a deficiency — and potentially means that these results are only important to people who are not getting enough vitamin d, although it’s important to note that the researchers were not able to confirm if this was true.

The results of the study also may not have much clinical significance. This is actually acknowledged by the researchers, who note that whilst this treatment has the potential to help, it is not conclusively so in part because the effect sizes were small. A reduction in severe asthma attacks sounds great, but not everyone gets severe asthma attacks, and the numbers of attacks requiring additional medications in both groups were very small (about .3% vs .43% in the control). So the treatment might help, but not be worth the risk.

Which brings us to a very important point, that not a single news story mentioned: the risk.

The Risk

We generally consider vitamin supplements to be safe. They are things that our body requires to function, after all.

But, it turns out that too much of a good thing can be bad for you. Vitamin d is a great example of this: there is a fairly well-established increase in the risk of fractures and falls for older people.

And that was a fairly major issue with this study: whilst it looked amazingly well at the potential benefits of vitamin d supplementation, there was no attention paid to the potential harms. We assume — perhaps correctly — that there aren’t any, but previous experience has shown this to be a dangerous assumption.

Ultimately, these results shouldn’t impact your life all that much. If you are deficient in vitamin d, it’s probably a good idea to talk to your doctor regardless. If you aren’t, there’s not much evidence that taking a supplement is going to help, and there is some evidence that it might hurt.

If you’ve got asthma, talk to your GP.

Just don’t believe all the hype.

If you like cats, or dislike scary stories about supplements — or both — click the clap button below and show some love! You can also follow me here or on twitter. I also have a whole bunch of other stories about everything from why supplements don’t work to the perils of raw milk just waiting to be read!