[Mercury]

... or hypervitaminosis.

I understand that excessive intake of fat-soluble vitamins may lead to vitamin "poisoning." But here is a question:

This question concerns only on the effect of excessive vitamin A.

I read elsewhere (I will provide you with the link as soon as I found the site) that vitamin A is used for some kind of treatment for the eyes. And when taken in excess there isn't any visible side-effect on the patients.

Then lately I read about hypervitaminosis A.
Excessive intake of vitamin A = Bad for health.

Why is it so?

[Celeste]

Originally Posted by Mercury
Excessive intake of vitamin A = Bad for health.

High levels can be toxic and cause problems in the liver for some individuals. I have taken large amounts at times (had whooping cough)with no side-effects. With eye diseases like Macular Degeneration (AMD) the benefits outweigh the potential risks. If high levels start causing symptoms, usually you just stop taking the vit for a few days and then resume at a lower level. Vitamin D is another toxic vit in high doses, as well as vit K and Niacin (B3).

As with anything, moderation seems to be the key.

[Biochemist]

Nearly all vitamins are toxic at high enough doses. More importantly, there is scant evidence that vitamin supplementation provides any real benefits, with the exception of folic acid during pregnancy.

The fat soluble vitamins (A,D,E,K) are more dangerous because they are stored longer (in body fat) and are more prone to accumulation.

The water soluble vitamins (all the Bs and C) are less toxic, but still have problems when used in the "pharmacologic" doses that are frequently suggested.

There was (interestingly) a good review section in the Wall Street Journal (of all places) last week with a pretty balanced review of the minimal benefits and significant penalties of vitamin usage.

Fundamentally, unless you are legitimately undernourished, the value of vitamin supplements is probably zero. For doses higher that a typical once-daily supplement, the risk is probably greater than the benefit.