How does a lion get away with eating liver (high in vit.A) while humans can't , according to your theory?
Carnivores have a higher ratio of hepatic stellate cells to hepatocytes. The stellate cells store retinol. the hepatocytes are the normal liver cells that are responsible for the usual processes in the liver. That doesn't mean they thrive on retinol, it just means they can get away with eating other animals.
No one really knows what the stellate cells do beyond storing vitamin A, so these cells are likely just a defense mechanism to protect the body.
Carnivores are also able to excrete retinyl esters through the urine, something humans can't do. This means that carnivores detoxify the dangerous unbound form of retinol conveniently via urine, while in humans elevated free retinyl esters are very dangerous and can build up easily.
These differences between carnivores and humans is the primary reason lions can get away with eating liver. This also shows that humans were never designed to eat high A foods like liver.
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