The principle of homeostasis is one of the most basic in medicine. My experience with parents of autistic children has been that they don't understand this.
Oh, don't get me wrong -- it's not their fault. It's hardly reasonable to expect every parent of an autistic child to have an understanding of it.
To expect at least most them to have heard about it, yes -- it's covered in most high-school biology courses (or it's supposed to be). Understanding, however... let's just say that American high schools often fail in their goal of promoting understanding of the subjects they're supposed to teach.
In essence, however, the principle states that organisms exist in a state of balance, maintained by a series of feedback mechanisms. Too little of something important can be bad -- but so can too much of the same thing. While the relationship between insulin and blood sugar is perhaps the most common example of this referenced in a purely medical context in popular culture (mostly because of the conditions of hyperinsulinemic hypoglycemia and diabetes, each of which is defined by the a type of breakdown of this balance), there are countless others.
So -- why is this important for understanding the problems with megavitamins?
Well, the principle of homeostasis accounts for why the same thing can have radically different effects depending on how much of it is involved... or, for that matter, why extremely low amounts of it and extremely high levels of it can have the same or similar effects.
It's all about the dosing... which is where the problem comes in.
Vitamins, like all other chemicals (and yes, vitamins are chemicals -- get over it), are subject to this. Simply put, it's quite possible to overdose on them.
I have heard -- anecdotally -- mentions of a child overdosing on Vitamin C from drinking too much orange juice... but most of the stories I hear of involve supplementation.
No, not megavitamin supplementation, either. I'm talking about regular, normal, over-the-counter dietary supplements. There's a very good reason why modern multivitamins are really, really careful about their iron content, for instance (although that's technically a mineral, not a vitamin).
Vitamins are one of the classic cases of "a little medical knowledge is worse than none" -- and, as I write this, I'm visualizing an exchange:
"But it's good for you!"
"Yeah, well, it's good for you to have enough... too much and it becomes a poison. Homeostasis and all."
Anyway, the point of all of this (beyond general medical education) is pretty simple -- as with all drugs, it's quite possible to overdose on vitamins.
And yes, vitamins, when taken as suplements (at least -- their status as such when consumed in food is more debatable), are drugs.
There's even a medical term for this -- it's called "hypervitaminosis".
So, in short -- people always tell you about the health benefits of vitamins. They don't tell you about the hazards thereof. Going overboard is stupid and can risk serious health problems.
Next time, I'll detail exactly what happens with overdoses of certain vitamins. I think my readers will be unpleasantly surprised.