There are so many strange magical powers in this world. Mages compress others into /statues/ (wtf?) for months, people are mutated to have cat eyes, a sorceress turns a cart of soldiers into frogs (is this Disney or a gritty medieval era?), dragons shapeshift into human form with their gear somehow appearing on them, vampires regenerate from being /beheaded/, genetically engineered individuals jump between /worlds/ and time (come on, this isn't immersion breaking? the thought that according to the Witcher universe it's possible for Ciri to suddenly pop next to me at the street, and then vanish with a "whoops, wrong place, wrong time"?). But it's the concept of a /mutant/, whose extent of mutations we are unaware of, controlling his metabolism, that raises annoyed cries defending the lore/immersion?
No it is not because that is how the world is shown to be.
Yes, it is POSSIBLE that Witchers can control their metabolism to an extend. But this mechanic came in in the last game, was never mentioned in the books nor in the previous games. You can explain it, but not in a convincing way IMO. It makes sense from a gameplay POV but even there it has it's problems, and in terms of lore it is something unheard of and something that Geralt never used in previous games, something no Witcher used so far.
The potions and signs being mentioned rarely in the books just shows that Geralt is more of a Swordsman/Close Combat guy than a magic or alchemy guy, but the fact that those things ARE mentioned shows us that it IS used by Witchers, how often is up to the Witcher, and sometimes the author might have chosen not to mention the character takes potions because it would make the story boring if it was mentioned too often. All we needed to know was that sometimes, before hard fights, Witchers use them.
In the end it comes down to personal preference. And I personally just find it not fitting and immersion breaking. Casting signs, sure, the Dragons with the Armor (which was btw. a deliberate decision, since they did not seem to have a problem showing Triss nude when she was "de-compressed"), etc, some of them are minor details (the armor of the dragon), some of them are established things which are just not mentioned often (Signs, Potions).
The metabolism thing though is something completely new. It's like the Thermal Clips in Mass Effect. Yes, it was the best decision to do it, but it made no sense whatsoever. (In that case it would have been better that they KEPT The "overheat" and made it an effect in case you would not have any thermal clips anymore).
Such things are often handled in a way like "What are you talking about? This was ALWAYS that way" although there is no mention in previous source material.
It would just change the whole way we look at the potions, their effect, and a basic thing we did never know about Witchers.
I agree that it CAN be explained. I however do personally not like the explanation and it would be immersion breaking for me personally or let's say I would need to get used to the idea because it is really a strange one, especially since we never heard of it.
I can see the benefits but I can also see the problems with it.
I'm of the belief that controlling the metabolism isn't something so implausible considering all the magical powers at work. It honestly makes sense to me. But even if that wasn't the case - this is where I'd choose to draw the line and say I'm not immersed in the world?
If you come up with a reasonable explanation as to why he or any other Witcher didn't do it before then it would be a little bit easier to accept at least.
