I read somewhere (citation needed) that most people are deficient in magnesium because we clean our food better than our ancestors did and thus eat less mineral-rich dirt, whereas the body, though it needs magnesium for many functions, does not store magnesium because it expects to be getting a fairly steady supply from our dirt consumption. So I guess we should be yanking carrots and taters out of the ground and eating them right there.
I think I've read the same thing about vegans and B12. People who eat animal products get the B12 because the animals are eating the dirt, and it gets into them and thus into the animal products, but vegans generally need to supplement. (Or eat dirt, I suppose, but I think I prefer my lemon-flavored B12 supplement to that.)