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.