You have to be really careful when looking at vitamin studies. A drug study on viagra for example, will only use VIAGRA, brand name, standardized dose, etc. A study on a vitamin is *REALLY* difficult to guage unless you specifically look for certain things. What brand of vitamin is used? What chemical formula is used? Is is from a natural or man made source? What is it taken in conjunction with? ( most vitamins have certain co-factors which readily increase their absorption) Is it taken as part of a complex? Take calcium for example, TUMS has millions of pregnant women thinking that they can get all of their calcuim from popping their antacids. But TUMS contain calcuim carbonate, which is crap calcium (pardon my french) from groung up chalk - the same stuff that is in your toothpaste. Unless you are very young and have very high levels of stomach acid, you barely get anything out of it.
Many OTC vitamins are pure garbage. You don't absorb hardly any of it, and you end up with very expensive urine and stools. Some vit's are packed so hard, that they simply do not break down in the body. Others are filled with artificial sweeteners (poison in itself). The human body does not readily assimilate most forms of manmade vitamins.
I do not recommend artificial sources of vitamins. I do suggest that they be taken with food ( for the most part), right before bed (so that they have time to sit in you). It is even better to split the dose if you can. Also, take complexes if they are available - ie B-complex, c-complex, etc. That is how that vit. is found in nature, & for reasons which are still not completely understood, your body assimilates them way more efficiently.
Not that I love Standard Process, but they have some excellent articles on vitamins and how they work.
Also, the previous poster hit on a very important topic, the modern diet is not what it used to be. Our soil is delpeted and full of toxins. We need to get every advantage that we can get.