One a days contain much more than RDA amounts. And no such thing has been "proven". But if you don't think one a day is enough then take two. But don't go telling people they need to buy some expensive ass designer bodybuilding vitamin so they can piss their money away.
Here are some percentages of RDA from my wifes basic "one a day" type vitamin (I don't take it because I need to limit vitamin D intake):
Vitamin A = 160%
Vitamin C = 417%
Vitamin E = 417%
Vitamin B-1 = 5,333%
Vitamin B-2 = 4.706%
Niacin = 400%
B-6 = 4,000%
Folic Acid = 100%
B-12 = 1,333%
It goes on like this. Admittedly it's a better product than some basic drug store brands but no one is saying people shouldn't be smart shoppers
. The mineral portion of the basic vitamin/mineral sups could be where they fall short and this may be the "proven" thing you are speaking of but even then it's only certain minerals like the zinc and magnesium I've mentioned. It would still be very effective for someone to get a basic multi plus an extra mineral sup with larger amounts. The main point is that even if you believe you need more you don't have to get ripped off by outrages sups containing all sorts of underdosed crap you don't need in the first place. But honestly I'm beginning to think that many people WANT to get ripped off.
As I've said, several times, only so much can be absorbed in one dose. This doesn't change whether the multi is separated into several pills, or whether it is expensive or not (assuming a quality product). Hell you could probably get enough of the basic vitamins if you took several Flintstones a day. Again, minerals would be where you may need more (of certain ones).
The simplistic idea of taking super mega huge doses of everything can be a very bad idea. You have to realize that vitamins and minerals can compete against one another in the body. Too much on one can cause defeciencies of another. And there are other interplays. You'll notice that the biotin amount in the above list seems like a lot less than the others. This is done for a reason. Lots of biotin can take over some of the roles of b-12 in the body and mask a defeciency. Which could lead to permanent damage because of the things that b-12 does that biotin can't. And since b-12 defeciencies are more common (except for pregnant women) you find more b-12 and less biotin.
In the case of minerals there can be all sorts of problems. Too much magnesium can cause calcium problems and vice versa. In the case of zinc it has been "proven" that more than 30 to 40 mgs. a day is not only a waste but can also cause copper defeciency. And I guarantee you that a lot of the super mega type vitamins may contain way more than enough zinc and likely no copper at all.
I could go on and on but the point is this stuff is much more complicated and there are reasons behind the recommended amounts. When you train you may need more but only so much more. Vitamins and minerals do not work in a vacumn. Nothing in the body works in a vacumn. Everything affects everything else. The biggest benefit of taking 10,000 percent of the RDA of stuff everyday is if you really like your piss to be neon yellow.