I haven't had a chance to read the study, so I don't know what sort of doses were studied - but what I have heard was that it was a wide range of doses from modest supplements, to 'mega doses'. Hopefully, I'll have a chance to read the full text tomorrow.
Originally posted by: Philippine Mango
Tell us more! Why are they toxic? How come they're not water soluble while Vitamin C is?
Vitamins A, D, E and K are fat-soluble because they are. Their chemical structure means that they are oils - which are, in general, insoluble in water, but soluble in other oils/fats. Vitamin C, by contrast, is an acid, and, like acids in general, is highly water soluble.
Because fat soluble chemicals are more difficult for the body to excrete, they tend to be concentrated in the liver. This is why fish liver oil is often used as a food supplement, it is a highly concentrated source of these oily vitamins. They are also, to some extent, more prone to being overdosed on - because even a modest excess taken for a long time, can build up. In contrast, even taking 10x the recommended dose of Vitamin C, will have only a short duration of any effects, as the kidneys will filter it out within a few hours.
As to why vitamins are toxic - the simple answer is that they serve to control, or facilitate, certain functions in the body. If you have too much of a vitamin, it can subvert the normal control mechanisms.
For example, Vitamin A is required for cells to reproduce, and mature. It acts like a 'growth factor' - a signal that tell cells that they are required to reproduce, and develop new tissues. Deficiency, therefore causes problems with tissues that need to be renewed regularly - the skin, hair, blood, the immune system, etc. Excess, causes these tissues to grow in an uncontrolled fashion, yet not develop correctly- so you get peeling, dry skin; thin abnormal hair that falls out easily; blood disorders, etc. The other problem is that the vitamin A can build up in the liver, and exceed the liver's ability to store it. The liver than tries to 'detoxify' it, by processing it into a more soluble for that can be excreted - the catch in this case, is that due to bad luck more than anything else, the technique the liver uses to detoxify things causes the Vitamin A to be converted into a nasty poison, which can cause serious injury to the liver.
Vitamin D is used to regulate calcium in the body - it allows calcium to be absorbed from food, processed in the body and stored in the bones. Too much vitamin D, causes excessive amounts of calcium to build up in the blood. This causes kidney problems, dehydration, salt imbalances, etc.
