Vitamin A isn't toxic, perhaps you mean other toxics in polar bear livers? What may be possible is for example what happens with apples. It's seeds are poisonous, but the antidote is found in the flesh surrounding it. In other words, if you get just the toxins it's bad (not entirely true for apple seeds but let's skip that) but if you eat the whole liver you get the chemicals that compensate with it? Which makes some sense because it didn't kill the polar bear either. Just guessing here, I'd have to read up on it. Do you have an article perhaps?
Vitamin A isnt toxic correct. BUT the large doses of it found in Polar Bears liver is. Toxicity from eating liver The liver of certain animals — including the polar bear, seal, and husky — is unsafe to eat because it is extraordinarily high in vitamin A. This danger has long been known to the Inuit and has been recognized by Europeans since at least 1597 when Gerrit de Veer wrote in his diary that, while taking refuge in the winter in Nova Zembla, he and his men became gravely ill after eating polar bear liver.[5] In 1913, Antarctic explorers Douglas Mawson and Xavier Mertz were both poisoned, the latter fatally, from eating the liver of their sled dogs.[6] Vitamin A itself was not discovered until 1917. http://en.wikipedia.org/wiki/Hypervitaminosis_A