I want to implement a way to combine food together with some penalty/diminishing returns, but I want it to be dynamic. I want the item itself to always be the same, but just with a modified nutrition. Out of curiosity I managed to make it work by making a new Item which I know is wrong, but it does "work", aside from giving me edible air.

So what I'm asking is just: How can I change the nutrition of an item that I made after it's been registered?

That's a shame, so my best bet at the moment would be to just make a few separate items I suppose. Well thanks for pointing me in the right direction and apologies for making the air edible!