You could eat it. If you are lucky, you would live to regret it. That's because the liver could be lethal! You could overdose on retinol!
That isn鈥檛 some bizarre toxin, retinol is just the chemical name for a form of vitamin A. Polar bears are carnivores, feeding mostly on seals. Lots of seals. Their hearty appetite is due to their need to accumulate lots of fat for insulation against the cold. Seals are rich in blubber. And where does their fat come from? They feed on fish, squid, krill and other sea creatures which in turn feed on various types of plants ranging from seaweed to algae. These plants contain various carotenoids that concentrate up the food chain and serve as precursors for vitamin A. Since vitamin A is fat-soluble, it builds up in fatty tissue, particularly the liver. Polar bears have lots of fatty tissue, so they accumulate lots of vitamin A, particularly in their liver.
Like virtually any substance, vitamin A can be toxic in high doses. But polar bears have evolved to be unaffected by the higher doses of vitamin A, because its accumulation is a consequence of their need for a high-fat diet. Humans do not require the same degree of insulation, so we have not had the need to eat as much fat as polar bears, although some people do give it a gallant effort. A blood level of vitamin A that does not bother a polar bear can kill a human. And it has. Arctic explorers have died when due to a scarcity of food they had to resort to hunting polar bears.
Vitamin A is a classic example of a substance that is beneficial in small doses and toxic in higher doses. It is required by the body for a variety of purposes. It plays an important role in vision, bone growth, reproduction, immune activity, cell division and cell differentiation, the process by which a stem cell becomes a brain, muscle, lung blood or other specific cell. But amounts are important. While vitamin A is needed in small doses for all these bodily processes, in larger amounts it interferes with them. The result can be blurred vision, loss of hair, infections, birth defects and death. Outside of consuming polar bear liver or going crazy with supplements, an overdose of vitamin A is not an issue. Underdosing is a greater concern.
There are two possibilities for meeting our vitamin A needs. Meat contains vitamin A, with liver containing the most. So do eggs and milk. Most non-fat milk products are fortified with vitamin A to replace the amount lost when fat is removed. Some cereals are also fortified. But it is not necessary to eat animal or fortified products to meet our vitamin A needs. We can do so by eating plant products in spite of the fact that no plant contains vitamin A. That seems to be a conundrum. While it is true that plants have no vitamin A, they do contain compounds called carotenoids, that the body can convert to vitamin A. Beta-carotene, found in the likes of carrots, spinach, cantaloupe, papaya, mango and oatmeal, is a common precursor for vitamin A. And you do not have to worry about overdosing on vitamin A by eating too many carrots. When enough vitamin A has been stored in our liver, our bodies stop converting beta-carotene to retinol. The worst that can happen with a carrot overdose is that the skin can turn a yellow-orange colour. Fortunately, it鈥檚 reversible.
