My personal opinion is that the human body may be too complex to say we fully understand a seemingly straightforward solution like just taking a multivitamin. We evolved over an awfully long time before industrialized agriculture and supplements. I’m not trying to demonize them because they have solved the number 1 concern humans had for the last 10,000 years of not getting enough calories, but I think it’s wise to temper the hubris of thinking we understand the human body well enough to expect a simple fix from a pill.
Biodynamic farmed food seems like the best bet if you have access to it. At least you're avoiding the worst of the mega-farming shortcuts.
It's simpler to add iodine to salt -- and maybe magnesium to the soil
I call it The Precog. It'll be a hit in SV.