If vitamin issues to health were not so critical, they could be a lot of fun. Here is an article "Sure fire tricks to get the most out of your vitamin supplements" ref that actually has some truths, tongue-in-cheek style. But, what about the cases when certain nutrients are found to prevent dementia, or slow down heart disease, etc. without the many side effects of drugs.
While Scientists like to paint in black and whites, and their studies reflect this aberration, real life exists mostly in gray tones. Vitamins are not either all good or all bad, they can be one or the other as well as 50 shades of gray in-between. Again, individual biological differences account for many of these variations. Often these are expressed as gene mutants. Nature gone wrong. There are many wayward mutant genes that exist to change the way vitamin D is processed. If one does not factor in these, study results can widely vary, as they do.
The same can be said for soy reactions since the number of people with the ability to convert Daidzein, an isoflavone phytoestrogen found in soy, to equol shows great variation. ref Equol is proving to be the beneficial element. It needs a certain bacteria in the intestinal tract for this conversion and only 35% have this bacteria in the USA while in Asia as high as 50% have it.