IIRC the amount of vitamin A in even a fairly small piece of polar bear liver is over two orders of magnitude higher than needed to give a human hypervitaminosis A symptoms.
on the other hand, getting it in “pro-vitamin” form like beta carotene lets our bodies convert what it needs to vitamin A, and we just dump the excess.
There are two ways vegans get sufficient B12:
Modern way is to take supplements containing B12
Old-style is low-level contamination of various food sources. Pre-industrial low-level contamination of food with things like insect bits, rodent feces, and the like provided sufficient B12.
It should also be pointed out that the body stores B12 pretty well, so that it can take years for a deficiency to show up when a person goes from an omnivore diet to a vegan one. An occasional “cheat” like having an egg or something with dairy in it at long intervals can prevent deficiency symptoms for a long time.
From what I’ve learned about it, fungi do not produce B12, only bacteria do.
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Bacteria and archaea. Everything else in the food chain gets it from those two.
The bioavailability thing is a big one - there are apparently many variants on B12 and not all are usable by humans, or usable to the same degree.
This is basically how herbivorous animals get B12. Cattle, deer, and others ingest small insects and other animals along with the vegetation they eat. An occasional wormy apple is an essential part of the diet. They may also deliberately consume birds eggs or other animal matter.
An argument for unfiltered honey!
Known, when I was at university, as ‘Dead Bee Honey’. Some Vegans differ on the acceptability of honey in the diet, but none of the ones I knew objected.
Polar bears are obligate carnivores in approximately the same sense that cats are. Cats must have animal protein in their diet (or else, perhaps, Taurine supplementation).
UNLIKE brown bears, the polar bear’s metabolism is specialized to require large amounts of fat from marine mammals, and it cannot derive sufficient caloric intake from terrestrial food.
“Although seal predation is the primary and an indispensable way of life for most polar bears, when alternatives are present they are quite flexible. Polar bears consume a wide variety of other wild foods, including muskox, reindeer, birds, eggs, rodents, crabs, other crustaceans, fish, and other polar bears. They may also eat plants, including berries, roots, and kelp however, none of these have been a significant part of their diet, except for beachcast marine mammal carcasses.”
" Although ungulates are not typical prey, the killing of one during the summer months can greatly increase the odds of survival during that lean period."
Polar bears may attempt to consume almost anything that they can find.
It’s not clear whether zoos give blubber to their polar bears, however they DO specifically include lard in the polar bear diet.