I am often asked about what individuals can do to protect themselves from infections, particularly the common cold.
If one goes to any convenience store it's easy to find several "immunity boosting" concoctions consisting of several vitamins (B-complex, C, etc) which are non-trivial in cost.
Does vitamin supplementation have any effect on the rate of respiratory infections or the like? (One major caveat, I am assuming the consumption of a regular diet in the developed world by an immune competent individual.)
The answer is no.
Studies have repeatedly shown that vitamins, especially D and C, have no effect on the rate of upper respiratory infections. Of course, these facts don't stop the hordes of people that swear by them from spending money on what could be duplicated for about $1--with better taste, in my opinion--by consuming a can of V8.
Moreover, vitamin supplementation--as such--has not proven to be efficacious for any reason (mortality, etc) prompting a great editorial in Annals of Internal Medicine entitled "Enough is Enough: Stop Wasting Money on Vitamin and Mineral Supplements" with which I totally agree.