vitamin A

Also known as retinoic acid (C1C=CCC(C)(C)C1C=CC(C)=CC=CC(C)=CC=O(O)) it is the organic part of vision (as 11-cis retinol). It is also used in the skin. Alledgedly found in carrots, as an aid to night vision, its precursor (carotinoids) can be found in most vegetables. Eating too many carrots can make you turn orange :-(

More seriously, polar bear liver contains large amounts of vitamin D (as well as vitamin A) which is why you shouldn't eat (any :) liver (I hate liver). Only fat-soluble vitamins, like A and D (and some Bs) can be overdosed on. C is water soluble, and is simply excreted if taken in too large a quantity.

You can die of vitamin A overdose by eating polar bear liver. This has happened to a few hapless arctic explorers.

The Alchemist is right in that eating too many carrots can turn you orange. Sort of. It's not because of vitamin A overdose though. It's a care of carotenaemia, or too much beta-carotene, which is a metabolic precursor of vitamin A.

vitamin A Deficiency
The opposite of the rare situation of a vitamin A overdose, is the vitamin A deficiency which affects up to 230 million people (mostly children) in over 75 countries. The main effect is irreversible damage to the eyes, and it even may result in blindness. People with the deficiency do have little white spots in the iris of the eye; at first blocking only that part of their sight.

Check the WHO website at http://www.who.int/vaccines-diseases/diseases/vitamin_a.htm (or http://www.micronutrient.org/publications/vadkey.shtml) for more information and their plans how to combat the deficiency. Besides providing dietary supplements and "hitch hiking" with large scale immunization programs, there are ongoing projects to stimulate consumption of vitamin A-rich foods. An additional positive effect of the last mentioned approach is, that it stimulates the regional agri-business and is more sustainable than keeping on providing supplements. An example is the use of sweet potato (camote in spanish; another good source of beta-carotene, the precursor of vitamin A) in weaning food, which has been invesitgated at the CIP (www.cipotato.org).