In 1970, two time Nobel prize winner Linus Pauling published a book called “Vitamin C and the common cold”. In it he argued that large supplemental doses of vitamin C could be used to decrease the length and severity of colds. This was the beginning of decades of controversy surrounding vitamin C (a.k.a. ascorbic acid) and its role in preventing respiratory infections, and resulted in Linus Pauling spending the last few decades of his life being derided as a quack by the medical establishment. But was he wrong?
In 2013, the Cochrane collaboration produced a systematic review and meta-analysis looking to answer this very question. The review included 63 trials, of which all were placebo controlled, and almost all were randomized and double-blind.
If you haven’t already done so, I would recommend reading my guide to scientific method in health science before reading the rest of this article, so that you are comfortable with all the terms used.
The lengths of the interventions varied from two weeks to five years. In order to be included in the analysis, participants had to be getting at least 200 milligrams (0,2 grams) of vitamin C per day (for reference, the recommended daily intake (RDI) according to the US Food and Drug Administration (FDA) is 90 milligrams for men and 75 milligrams for women). The review looked at whether vitamin C could decrease the frequency of colds, whether it could decrease their duration, and whether it could decrease their severity.
29 trials with a total of 11,306 participants were included in the meta-analysis of frequency, while 31 trials with a total of 9,745 episodes of common cold were included in the meta-analysis of duration, and 9 trials with a total of 2,143 participants were included in the meta-analysis of severity.
Back in the 1970’s, when Linus Pauling was writing books arguing for vitamin C supplementation as a way to protect against the common cold, he was recommending an intake of at least 1 gram per day, so although this review included all studies with at least 0,2 grams of vitamin C, we also want to know whether the higher 1 gram dose has an effect. Otherwise, if there is no effect, one could argue that it was simply due to the low dose used in the studies. Luckily, the review did sub-group analyses of just the high dose studies, so we have all that data too.
What were the results?
Overall, among the people taking vitamin C, there was a 5% reduction in the relative risk of having at least one cold during the intervention period. This result was highly statistically significant (p = 0,001).
However, most of this benefit came from a sub-group of studies (involving 598 individuals) that looked at people who were engaging in strenous exercise (marathon runners, skiers, and soldiers). In this group, vitamin C supplementation decreased the relative risk of one or more colds by more than 50%! As before, this result was highly statistically significant (p = 0,00001).
When the people doing strenous exercise were removed from the analysis, there was a 3% reduction in relative risk, which was no longer statistically significant (p = 0,06).
What about if we only look at the people who were taking very high doses, 1 gram per day or above?
17 of the studies, involving 6,661 individuals, were giving participants 1 gram per day or more. In these studies, there was a 2% reduction in relative risk, but it wasn’t statistically significant.
So, virtually no effect on frequency of colds, unless you’re engaging in high intensity exercise, in which case there seems to be a very big effect. What about the duration of colds?
The meta-analysis was separated based on whether the participants were adults or children. Among adults, there was an 8% reduction in the duration of colds, and among children, there was a 14% reduction. Both results were highly statistically significant (p = 0,0002 for adults and p < 0,0001 for children).
When only studies giving at least 1 gram of vitamin C were included, the duration reduction was still 8% for adults, but increased to 18% for children. The average common cold lasts 7-10 days, so on average you could expect to recover roughly one day faster if you are an adult, and two days faster if you are a child. That’s not a huge effect, but it’s noticeable.
Finally, the reviewers looked at symptom severity. This was determined by letting participants fill in how severe they felt their symptoms were on a scale. What they found was that vitamin C decreased symptom severity by on average 14% . Again, that isn’t huge, but the effect was highly statistically significant (p = 0,0018), so it’s probably real.
One thing this review didn’t look at, which would have been very interesting, was the levels of vitamin C in the bloodstream at the start of the studies. As I described in a previous article, vitamin D lowers the frequency of respiratory infections hugely for people who are deficient, but only has small effect for people who have normal levels. It is quite possible that vitamin C would show a similar pattern.
In light of the covid-19 pandemic, it is also clear that the common cold can kill you if you’re old and frail to begin with, since covid-19 has a similar mortality rate to the so called “common cold” coronaviruses (very low in the general population, but quite high among the old and frail). It would be very interesting to see some studies looking at whether vitamin C supplementation improves the odds of surviving a cold in this group – it actually makes sense that it would, since vitamin C clearly shortens duration and decreases symptoms.
Another thing that I think is interesting is that the shortening of the duration of colds was twice as big in children as in adults. This could be because both the children and adults were both getting 1 gram per day, so the relative dose the children were getting was higher (because children are smaller than adults). It would be interesting to see a study in which adults were getting even higher doses, to see if the duration could be shortened even further.
In spite of the fact that we’ve been talking about vitamin C as a potential treatment for the common cold for fifty years, there are many things we still don’t know.
One important thing to note is that the studies looking at the effect on duration and severity of symptoms were giving vitamin C daily regardless of whether people had any symptoms or not. In other words, vitamin C was being given prophylactically, not therapeutically.
When the reviewers looked specifically at studies giving vitamin C only after the beginning of symptoms, they were not able to find any effect on symptom duration or severity. So, if you want to shorten the duration and severity of your colds, it’s not enough to take vitamin C just when you have symptoms, you have to take it all year round.
So, what conclusions can we draw?
Vitamin C supplements don’t seem to noticeably decrease the frequency of colds, unless you engage in high intensity physical exercise, in which case there seems to be a big reduction in frequency. However, when you do get a cold, vitamin C supplementation will likely shorten the duration by a day or so, and will likely also decrease the severity of the symptoms you experience a bit. The effect isn’t huge, but since vitamin C is cheap, safe, and widely available, neither are the downsides to taking it.
So, Linus Pauling was right, at least when it comes to the ability of vitamin C to shorten the duration and decrease the severity of the common cold. Who’d have thought it? Not me, that’s for sure. I was taught in medical school that there was zero evidence that vitamin C had any effect on the common cold, and that the issue had been firmly settled. I guess that just shows you should never trust your teachers – always double check the data yourself.
Should you take vitamin C?
That’s a harder question, and I guess depends on two things. Firstly, do you engage in high intensity exercise? If that is the case, then supplementing probably makes sense, since you are halving the number of colds you are likely to experience.
Secondly, do you think it’s worth taking a pill every day of the year to have a few extra symptom-free days per year and slightly milder symptoms when you do get a cold? If yes, then it’s probably worth supplementing. If no, maybe not. If you’re old and frail, then it makes more sense to take it than if you are young and fit, since the risk of severe disease is higher and the benefit is therefore likely to be bigger.
If you do decide to take a regular vitamin C supplement, what dose should you take?
Most of the studies involved in this meta-analysis were giving 1 gram of vitamin C per day, and in terms of symptom duration the results actually became a little bit stronger when studies giving less than 1 gram per day were excluded. Based on that, 1 gram per day seems a reasonable dose to supplement with. According to the FDA, the tolerable upper limit for vitamin C is 2 grams per day, so 1 gram is perfectly safe to take.
You might also be interested in my article about whether vitamin D supplements protect against respiratory infections, or my article about whether vitamin D can cure covid-19.