Dementia definitely messes with appetites! Some people eat more, some change their tastes, often wanting more sweet things but I think the majority just go off eating altogether.
It was certainly an issue with my mum and I thought it was due to the CH not offering appetising choices. But I discussed it at length with mum and she said she was just no longer interested in food and didn't seem to get hungry. I spent many hours trying to come up with tempting ideas, things she used to love, new things that might take her fancy. She was even happy to talk about food - always wanted to know what I was having and liked to look at recipe pictures in magazines, though her reaction was always to pull her face and say whatever it was looked ghastly. Her standard response when offered anything was, 'I'm just not hungry.'
Obviously people who are moving around less (and using less calories) don't need as much to eat but it is very worrying when you see the weight dropping off and nothing seems to tempt them.
But please don't think this is a deliberate act by your wife or that she is refusing food on purpose. Her dementia means she is just not capable of that sort of planning. I think the part of the brain that controls appetite often goes quite early in dementia.
Take care - thinking of you...