What I don't understand is this: Out of the thousands and thousands of participants in the study, some of them are sure to have unknown deficits. You'd think then that in studies like these, even assuming multivitamins are useless to the vast, vast majority of people, you'd see some slight reduced risk for things like heart disease and mental decline. Yet, the studies claim there is no benefit.

Unless people get sick from taking the vitamins and almost exactly offset the number of people that get better from them, I feel like something is missing. They clearly also state that multivitamins ARE in fact beneficial for some groups. If those groups were excluded from the studies (which I don't think should be that hard for them to do), shouldn't we then see an increased risk associated with taking multivitamins for people not in those groups?

I'm probably missing something?