Vitamin A is a Vitamin

 There's a theory that vital amine alpha is in fact a toxin and you should never expose yourself to it.

 You can piss out vitamin A. https://pubmed.ncbi.nlm.nih.gov/8074070/ ¯\_(ツ)_/¯

 If your body wants to get rid of it, it can dump as much as it wants. It wouldn't accumulate if it's not supposed to.

 This passes the works-in-my-kitchen test. I overdosed on A once. It's how I know you generally shouldn't eat liver. I pissed it out, exactly the same way I piss out excess salt, iodine, and calcium. Acute but temporary symptoms. Kidney stress gives me a characteristic headache.

 Although yes, retinols are toxic. Insulin is also toxic - the fact that something is poisonous doesn't mean you don't need it to live. Retinols are stored in the liver because airtight seals aren't worth the cost, so the reservoir leaks, and you want it to leak up against something that can regenerate, i.e. liver cells.

 Caveat 2: it is highly probable that there's an alternative, the same way vitamin C, vital amine collagen, has an alternative. You might guess what you can eat instead of vitamin C: it's collagen. This is why meat cures and prevents scurvy. 

 If you have a busted enzyme and can't use vitamin A, you don't die. You use the alternative. You do accumulate vitamin A which you can't use, though. Likely the liver packages retinol for transport, it gets to the cells, then it's released chaotically. There's no way for the liver and kidneys to know your vitamin A metabolism is busted. In this case, yes, you could well need a weirdo strict elimination diet.