Vitamin C holds an almost legendary status among drugstore supplements. It’s common to take a few tablets when we’re worried about getting sick or mix up a glass of Emergen-C at the first sign of a cold. It’s nice to imagine it as a cure-all, but reality doesn’t quite match up with our hopes.
Vitamin C probably wouldn’t be where it is today without Nobel laureate Linus Pauling getting really excited about it. (His enthusiasm was, ironically, infectious.) But even today, the Linus Pauling Institute at Oregon State University writes: “It is widely thought by the general public that vitamin C boosts immune function, yet human studies published to date are conflicting.”
When vitamin C helps
As we noted in a previous post on immune boosting supplements, vitamin C’s proven benefits are small and probably don’t apply to most of us. People undergoing severe stress due to extreme levels of exercise (like marathon runners and soldiers training in subarctic conditions) may be less likely to catch a cold if they take vitamin C.
A few studies have found that people who regularly take large amounts of vitamin C may suffer from slightly shorter colds (clearing up a day or half a day early) but the vitamin didn’t prevent them from getting sick in the first place. That said, a 2013 Cochrane review concluded that the evidence is mixed, and even the shortened colds aren’t reliably seen in research.
The National Institutes of Health summarizes the research like so:
Overall, the evidence to date suggests that regular intakes of vitamin C at doses of at least 200 mg/day do not reduce the incidence of the common cold in the general population, but such intakes might be helpful in people exposed to extreme physical exercise or cold environments and those with marginal vitamin C status, such as the elderly and chronic smokers.
When it doesn’t
“Taking supplements once cold symptoms have already begun has no proven benefits,” the Linus Pauling Institute says. If you’re already coming down with a cold, the opportunity to maybe-sorta-possibly reduce the duration of your cold has already passed.
The institute’s information page on vitamin C sifts through the research on other conditions, and I’d recommend reading it if you want to know more. People who routinely get enough vitamin C have lower risks of some health conditions, including heart disease.
Fortunately, vitamin C pretty common in fruits and vegetables, and if you’re eating an overall healthy diet you probably won’t have any difficulty meeting your needs. If you want to hedge your bets by taking it in pill or powder form, there aren’t many downsides. High doses can sometimes cause diarrhea, but studies haven’t reliably identified any long-term or serious consequences of mega-dosing the vitamin. To be cautious, though, a maximum of 2,000 mg per day is considered the safe upper limit.