Your body needs vitamins in order to form complex aminos to operate. But your body only needs to make so many of them - especially if you are an adult, not pregnant, or not suffering from a disease of some sort. The very premise that loading up your body with "excess" vitamins beyond what you need is already pretty fraught. Building a house without enough lumber can lead to long term deficiency - but loading up a con…
There would still be a ton of goiters if not for iodized salt, basically an obligatory vitamin intake. People had no good iodine source living inland where most anything they catch or grow is not going to have sufficient iodine no matter what it is they were eating. I'm not sure what the ancestral iodine source might have been. Fishing villages perhaps along the coast? Hard to say how much coast was relatively popula…
The iodine deficiency issues that haunted the Swiss (and Appalachia) arose from people settling down from nomadic lifestyles, in mountainous regions that easily were leeched of iodine by rainfall, and then farming that already leeched soil until there wasn't any iodine left at all.