This page, and this seem to suggest that multivitamins are generally useless as their value is not seen in a statistically significant manner.
This also mentions that some brands use fillers, and goes a bit further by evaluating specific multivitamin brands in terms of the quality of their ingredients (e.g. some shown to lead to prostate cancer, some have as low of a bio-availability as 3%).
My question: what's the cause of multivitamins' inefficiency, or effectiveness, at supplying the body with the nutrients that it requires?
The problem here is that all claims around this topic centre at individuals who are already well nutritioned so that they do not require any added nutrients, yet somehow they consume extra nutrients from such multivitamins. Then a conclusion is drawn that such individuals are not going to benefit from the multivitamins.
This is a problem, as the cause for the ineffectiveness of multivitamins could possibly be due to a deeper cause, which makes their ineffectiveness extend to even individuals that lack nutrients. A hint of possible reasons is shown here where some manufacturers use fillers and poor ingredients that would make multivitamins useless and harmful even to individuals that require the nutrients that are claimed on the labels of such multivitamins.
The following are some thought-stimulating hypothesis to let you better see the angle I'm looking at that question:
Is the reason that multivitamins are usually consumed by people that are already eating normal food, which allows them to obtain enough vitamins and minerals, so that the addition of a multivitamin is too redundant to show an effect?
Is the reason that multivitamins are packaging multiple ingredients together, which makes it a lot harder to test the fitness of the ingredients; effectively allowing some manufacturers the opportunity to cheat (e.g. putting fillers in pills)?
Is it the packaging and the form-factor, which combines multiple ingredients together, which effectively causes some ingredients to nullify, or mask, others?
What if we, instead, buy 13 separate bottles each containing a single essential vitamin, and buy 15 separate bottles each containing a single essential mineral? E.g. a bottle for only VitA, another only for VitC, etc. Would this approach address the problem that multivitamins face?
E.g. would this make it noticeably easier to verify that the manufacturer is actually honestly giving us what they claim, and not mere fillers?
Is the problem fundamentally concerned the ability of extracting essential vitamins and minerals even if packaged in isolation?
E.g. is engineering (or even science) is not advanced enough to get the essential vitamins and minerals in a form that is as bio-available as natural food?