If you go to a pharmacy, there is an entire section devoted to supplements and vitamins. I will target the lens of medical science on one frequent offender: the Vitamin C supplement. The most famous form of this is likely Airborne which, as the product is quick to state, was developed by a school teacher. I'll take the medicine that my doctor prescribes instead, thanks. Anyway, let's look at why dosing up on Vitamin C doesn't do much since, SPOILER ALERT, it doesn't.
Vitamin C has been known to prevent ailments such as scurvy since the days of wooden ships. Since fresh produce was limited on long ocean voyages, illnesses like scurvy and anemia were commonplace. Here in first-world countries, where a diet containing all of your necessary nutrients is hard not to get, those types of problems are unheard of.
What is Vitamin C for, and why do we care so much? Surprisingly, humans cannot naturally synthesize Vitamin C. This evolutionary disadvantage is offset by the fact that it is easy to get a sufficient quantity for normal metabolic processes. Vitamin C is used to produce collagen. Without Vitamin C, you'd quickly find that your tissue wouldn't quite stay together anymore, which is horrifying. Vitamin C also has antioxidant properties along with numerous applications in other biological functions. You need it.
That said, there is nothing particularly unique to Vitamin C regarding immune system function beyond any other necessary amino acid. It's not a panacea nor a super drug. It's just one cog in the machine.
Let's get down to business:
No studies demonstrate medically significantly improved functionality or production of white blood cells, antibody functions, lymphocyte function, or cytokine function.
Two studies found that Vitamin C harms lymphocyte function. That particular type of white blood cell helps hunt down and kill an infection.
Numerous animal studies showed no benefit in guinea pigs (another animal that does not produce Vitamin C naturally) from vitamin mega-doses.
What about human trials for something simple like preventing the common cold
Typical human trials start in small, double-blind, placebo-controlled studies.
There are two main ways to conduct tests that we're concerning ourselves here: lab trials and real-world trials.
In our lab trials, rhinovirus is injected via the nose. The exact strain is known, so the exact response can be anticipated. One group will act as the control group and is given a placebo Vitamin C dose. The other group is given somewhere between 1-15 grams per day of Vitamin C. In our real-world trials, rhinovirus is not injected, and the study instead relies on a percentage of the study population getting sick "in the wild," so to speak.
A New England Journal of Medicine study in 1976 used 868 children for one such lab trial. The children were randomized and given either a daily Vitamin C dose or a placebo. The only difference between groups after the trial was over was that the test group's Vitamin C levels were demonstratively higher. The test group, in general, showed longer-lasting cold symptoms. Those taking a Vitamin C mega-dose were sick for longer. This is thought to have been possibly due to free radical generation being impeded by the higher-than-normal Vitamin C levels present in the test group.
How about repeatability? There have been over 30 similar trials utilizing over 11,000 patients costing 84 million dollars. A meta-analysis of these trials in the Cochrane Review in 2006 found little variance between test and control groups. On average, in healthy people with a regular diet, no preventative function was found from taking Vitamin C supplements. This was consistent in both lab tests and real-world tests. There was no change in symptoms or length of illness.
In the interests of full disclosure, the only shown medical efficacy in any study was when Vitamin C was taken by participants in large doses over a long period before an exceptionally biologically stressful event. Generally, extreme sports athletes and military personnel in combat situations showed an average half-day reduction in symptoms for a week-long illness. The side effects of heavy dosing included diarrhea and vomiting. That doesn't seem to be worth it.
Ok, so Airborne has shown no proof of being better than sugar pills at preventing the common cold.
How about other problems?
Another 2013 Cochrane review found no evidence of Vitamin C supplements lowering the risk of lung cancer in both healthy and high-risk individuals.
A meta-analysis conducted recently found no effect on the risk of prostate cancer.
There have been both "weak" and "no" associations in the risk of colorectal cancer.
A 2011 study found no indication of breast cancer reduction.
A 2013 meta-analysis found no reduction in cardiovascular risk or stroke risk.
A meta-analysis showed an effect on particular cardiac problems relating to blood vessel wall lining.
A 2010 study found no effect on rheumatoid arthritis.
Another study found no apparent effect on Alzheimer's disease.
There has been no significant effect observed on treating and preventing cataracts.
There has been no consistent efficacy observed in the treatment of infections.
Whew. I think that covers most of the bases. For the most part, Vitamin mega-dosing has no positive medical impact. At high enough levels, it will make you sick. Stick to a well-balanced diet, and you'll get all you need.
If you know of a study topic that I missed, be sure to leave a comment.