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Friend says pro-formed VA like beta carotene and other cartenoids are safe to eat.

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There is a negative feedback loop that should control the amount of carotenoids that are absorbed & converted. ISX is a transcription repressor that is stimulated by retinoic acid. ISX suppresses expression of SCARB1 (main receptor that allows carotenoid absorption) & BCO1 (enzyme that converts carotenoids to retinal). Therefore if there is enough absorption & conversion of carotenoids then the RA produced causes expression of ISX and absorption & conversion is suppressed.

However, it seems to me from anecdotal evidence that if someone becomes vitamin A toxic this nice negative feedback loop seems to stop working (at least in some people) & they become ‘super’ absorbers. BCO1 SNPs probably define whether someone is also a ‘super’ converter or just accumulates carotenoids. I’ve been trying to find experimental data on this but work is done on VA deficiency & not toxicity so at the moment this is guesswork & just a hypothesis.

If anyone has anything to add on this topic I’d love to hear.

Chemical reactions are sometimes treated as stochastic/probabilistic, so it's possible the suppression wont be complete beyond certain tolerances. Set that against a backdrop where some of the formed retinol is stored (in fat or the liver), there's a constant drain that will be working against the suppression.

Thanks SpokilyDoesIt. That is a good point. Also there is another receptor for carotenoid absorption not involved in this feedback loop. The paper where Zambian children with hypervitaminosis A turn orange in the mango season suggest this for the reason why absorption not suppressed but conversion appears to be. I don’t know...

Not sure I fully understand about the constant drain - do you mean that RA being taken out of gut cell to store? In conditions of VA toxicity I’m thinking more RA would be around? I could quite easily be thinking about this in the wrong way. It just seems to me that this often said ‘the body tightly controls carotenoid absorption/conversion’ does just not hold true for many.

Certainly not for me and my orange callouses. 🙂

I would say food with retinol like diary liver eggs was much worse for me.

There is also the issue that when carotenoids are stored in the skin they can and do spontaneously convert to retinyl esthers via a separate metabolic pathway that is not tightly regulated. Garrett has a post on this somewhere on his web forum.

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