I think it has something to do with our internal systems , the PH, and maybe genetics or even where our ancestors came from.

Maybe I only think that because of the book I read "Eat right 4 your blood type" - it explains all that or their theory about it anyway.

After reading it I realized why I feel cruddy after I eat certain foods - they were on my bad list!

But in general I think that so many foods are processed and out diets are never perfect anyway - we need the extra boost that Vitamins & supplements can give us.

The RDA {recommended daily requirement } is pretty much the minimum that we need to function.
I used to read a lot about this stuff when my kids were little.
One thing is that there are fat soluble vitamins and water soluble.
You could overdoes on fat soluble but it is way beyond the RD limits.