Thing is, you're looking at this backwards, from a position of "doing something must be better than doing nothing." Frequently, it's not. The onus here should be on the multivitamin to prove that it is effective, not for study authors to prove that it does nothing. If you actually dig around you won't find studies that show multivitamins are effective at anything for your average westerner -- or anyone who isn't clinically malnourished for that matter. That's not to say that - in certain cases, when you're lacking a specific vitamin - it won't help manage a specific condition, but scattershot "essence of food" has very little data to support it's efficacy.
More vitamin doesn't mean better health. Water soluble vitamins saturate in your body, and the excess is excreted in urine. Fat soluble vitamins, however, build up, and taking more than you need may lead to various kinds of poisoning (particularly A, D, B3 and B6 [3]).
Generally speaking if you're not eating 100% of your meals at McDonalds, you're getting all the vitamins you need from your diet and more vitamins is at best neutral and at absolute worst fatal.
Here's a study from 2011 that shows no decrease in all cause mortality, no decrease in cancer risk [1]. Here's one that shows all cause mortality was 5% higher in multivitamin takers [2].
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3105257/
[2] https://academic.oup.com/aje/article/152/2/149/87699
[3] https://en.wikipedia.org/wiki/Hypervitaminosis
More vitamin doesn't mean better health. Water soluble vitamins saturate in your body, and the excess is excreted in urine. Fat soluble vitamins, however, build up, and taking more than you need may lead to various kinds of poisoning (particularly A, D, B3 and B6 [3]).
Generally speaking if you're not eating 100% of your meals at McDonalds, you're getting all the vitamins you need from your diet and more vitamins is at best neutral and at absolute worst fatal.
Here's a study from 2011 that shows no decrease in all cause mortality, no decrease in cancer risk [1]. Here's one that shows all cause mortality was 5% higher in multivitamin takers [2].
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3105257/
[2] https://academic.oup.com/aje/article/152/2/149/87699
[3] https://en.wikipedia.org/wiki/Hypervitaminosis