My mother passed away a little over a year ago - severe dementia, most likely alzheimer's. She was almost 93, so her death was not unexpected at that age. The decline over the previous 10-15 years was primarily cognitive.
She had gotten to the point that couldn't get up out of a chair on her own, and didn't understand anything that was going on around her when she was in a hospital about a year earlier, but was strong enough to (literally) fight the doctors and nurses at the hospital when she didn't like that they were trying to run some tests on her. (this frail old woman that they thought was too weak to do anything on her own was hitting them - hard!)
Oddly enough, during her last 2 weeks, when she was completely bed-ridden, and basically not eating or drinking hardly anything (No IV, so her blood sugar would have been significantly lower than it had been on her primarily starch and sugar based low fat diet - in other words, her body would have switched over mostly to ketones to keep bodily functions going), she seemed to have more clarity of thought than I'd seen in several years. She couldn't talk at that point (hadn't for several months, though she tried), but if you asked her something, she seemed to understand what you were asking, and would either nod or shake her head (ever so slightly - muscle function was minimal at that point).
During her last couple of days, we were playing a lot of comforting music from her life (mostly old hymns, but also some old standards that she would have been very familiar with from the 40's-50's), and she responded very positively - you could see that she was attempting to sing along with the hymns (slight movement of lips and mouth - she knew all the words), and most remarkably of all, she was moving her hands and fingers as if she was trying to play the hymns on the organ. Again, only slight movements (maybe 1/2" this way and that way), but considering that she had gotten to the point where she wasn't moving at all, that small movement was a significant response. When we played some of the old standards, she had a genuine emotional reaction to those - some reactions were happy, others were wistful.
She was on a couple of different meds for alzheimers over the years. One med was so bad that she declined very quickly - sounds like it must have been destroying the amyloid proteins in their protective stage, just making things worse. Once that med was discontinued, she improved somewhat - at least she wasn't acting quite as out of it as she was while on that med.
There were some other incidents during those last couple of weeks when her blood sugar wasn't constantly being spiked, and it was clear that she was thinking fairly clearly, but so much damage had been done that she wasn't capable of expressing herself with anything other than slight movements - but those slight movements were very clear and decisive responses to questions, clear declarations of what she wanted at the moment. (One instance late at night had to do with her favorite color, and wanting a robe in her favorite color draped over the rail on her bed, so she could look at it as she fell asleep - the pointing, scratching at the fabric, and nodding were clear indications of what she wanted)
There were attempts to improve her diet the last few years. I live a couple hours away in a different state, but my brother did his best to make sure she was fed fewer carbs, and was given real butter, and some food cooked in coconut oil each day, but my mother loved her carbs (left to her own devices, she had gotten to the point that she was eating far more carbs than I'd ever seen her eat before - she'd pile an inch of jelly on a piece of bread, even though I'd never seen her use more than a tiny schmear of jelly when I was a kid), and always hated eating anything "greasy", so she'd reject any food that seemed too oily to her, hence why it was so difficult to increase the amount of coconut oil.
I should mention that she was still at home until the end, with round the clock caregiver/companions - all these little signs that she was thinking more clearly right before the end would have been missed entirely if she'd been in a nursing home. This is also of course an N=1 experience - I don't know how other dementia/Alzheimer's patients have reacted during their last weeks or days, when they're not eating lots of starch and sugar, and not on an IV.