It's that time of year again--the start of cold and flu season. If you work in the health care industry, or spend a lot of time reading health related information, you know that it's the time of year to be encouraged to wash your hands, get vaccinated for flu prevention, and try to stay home from work if you are sick to avoid infecting your colleagues. Of course you are probably also going to get overloaded with all kinds of information about how to "cure" or "avoid" colds and flu with all type of "natural" or "alternative" methods. One that I still see creeping up this time of year is the use of vitamin C in various forms to help fight colds and reduce symptom time. Is there any evidence? Let's find out.
Fortunately for us Skeptics, there happens to be a large body of research indicating that large doses of vitamin C do not decrease the length of colds or prevent them once symptoms have started. One such large scale review is available for free here: Cochrane review on vitamin C. So, number one, no evidence that this helps.
If that is not quite good enough for our non-skeptical friends, keep in mind that your body is saturated with vitamin C after 500 mg, so anything that you take in beyond that is lost through the urine. Keep in mind that a lot of the vitamin C products being marketed to us, like Emergen-C and Airborne, are bombing us with 1000 mg of vitamin C per serving. You apparently pay about 1 cent per milligram of vitamin C if you buy any of those products though, so that adds up to some pretty expensive urine if you purchase it. (You will pay less money if you buy generic vitamin C capsules, but I would rather not pee out any of my hard earned dollars quite frankly).
Are there any other problems besides expensive urine? Possibly. People who are already at risk for kidney stones might become more so during episodes of vitamin C bombing. There was also a study done back in 1938 that showed that the equivalent of several POUNDS of vitamin C could kill an average person, but that study was also done in rats so I wouldn't really want to test it (Link).
Are there some people who might benefit from vitamin C supplementation? Yes, but not necessarily for colds. There is a disease called scurvy, which is normally associated with a sea-commerce past in which sailors that had to spend months at sea without access to fruits and vegetables. In modern times, people can still get scurvy if they have a condition, like subtance abuse, that causes them to neglect their diet; if they are a frail elderly person or young child whose caregiver has neglected to provide them with adequate fruits and vegetables, or if they are on a prolonged no-carb diet. Since you only need about 20-40 mg per day to prevent scurvy, and even eating one potato a day will give you about 20 mg (other fruits and vegetables will give you more), we haven't seen an epidemic of scurvy lately.
Now, if you read the Cochrane review above (you did, didn't you?) you will note that for marathon runners and people who live in very cold climates vitamin C bombing might reduce their risk of colds. I know that's not me, and it's probably not you either. Your 30 min jog three times per week does not count.
Take home message--don't spend your hard earned money on vitamin C supplementation for cold prevention or to lessen symptoms.