I overhear or am included in conversations dealing with nutrition (outside of school) basically every day. I love it. Seriously. However, there is one topic that has come up multiple times recently, and although I refrain from bursting bubbles face to face, I need to get it out now.
A long time ago, someone (my theory is that it was the Florida orange people...) decided that they were going to tout oranges and citrus as being immune-boosting due to the vitamin C content. And boy did they score with that one. With zero scientific evidence to back it up, the masses bought it hook, line, and sinker. Then, as if that wasn't enough, some school teacher in California came up with Airborne which has taken off as a popular "immune-supportive" supplement, including but not limited to megadoses of vitamin C and zinc. Awesome. Since then, multiple other products have emerged (Emergen-C, etc) that continue to promote this association between resisting/fighting a cold and vitamin C.
Okay. Here's the real deal with vitamin C. Vitamin C (aka: ascorbic acid) is an antioxidant. Simply, it serves as an electron donor to reduce free radical content in the body. The theory behind this is that we inherently have endogenous free radicals in our bodies all the time, but we also have endogenous means of dealing with these free radicals, keeping our bodies in a reactive oxygen species homeostasis of sorts. The problem is that just by doing anything, including breathing, exercising, smoking, or being in the sun, we exogenously increase our free radicals. Bummer. However, we have also exogenous means of combating that as well. Enter: Antioxidants! So, vitamin C is important for reducing free radicals in the body. In addition, we know that vitamin C plays a role in several reactions that occur in the body, namely iron absorption, collagen synthesis (which keeps our teeth in our mouths- a discovery which contributed to the connection of vitamin C to scurvy) synthesis of neurotransmitters, fatty acid utilization, etc.
There is no evidence that supports the idea that taking lots and lots of vitamin C will help you fight a cold. None. Sorry.
I should also mention that taking large doses (We're talking about anything over 500 mg/day here... ) of vitamin C is a huge waste of money and potentially bad for several reasons. 1) The more you take, the less is absorbed. Anything you don't absorb will simply be excreted as waste. 2) Many of the vitamin C supplements come with zinc too. Zinc inhibits absorption of vitamin C. 3) There is also something called renal threshold, which means that when you exceed a certain point in the body, excess vitamin C will be filtered out in the urine, and flushed. 4) A byproduct of vitamin C metabolism has been linked (again, this lacks sufficient scientific backing) to development of kidney stones. 5) Megadoses of vitamin C are thought to be PRO-oxidative, actually contributing to free radical content in the body. This is still being researched, but is generally supported at this point.
Adult men need 90mg/day and women need 75 mg/day of vitamin C. There is an upper tolerable limit (which is the amount that shouldn't be exceeded without experiencing side effects, in this case diarrhea) of 2000 mg/day. A good goal to shoot for is somewhere around 200 mg/day. This is easily achieved through the diet as long as it includes a variety of fruits and vegetables.
The bottom line here is that supplementing with vitamin C probably isn't going to hurt you as long as you're not exceeding by too much the 500 mg mark. If you think it helps you out, by all means, go for it. But there are a lot of misconceptions/myths about vitamin C, and after refraining from telling people to their face that they're flushing money down the toilet (literally), I needed to get it out.