My DM died a year ago just before her 90th birthday, she was in a nursing home due to dementia. She also had diabetes which was well controlled.
We had the opposite problem, the home was very reluctant to give her much if anything in the way of sweet treats or puddings, which she loved, despite us saying that we were happy for her to have desserts or afternoon cake.
She had a healthy appetite to the end but was not at all overweight, she was very petite and weighed about six stone dripping wet.
Sadly she had very few pleasures left in life in her final years EXCEPT eating (she always loved her food, goodness knows how she stayed slim), so we had many spats with the home; we felt she ate a healthy and varied diet and should have a piece of cake or a couple of decent biscuits (NOT bloody Rich Tea) in the afternoon, especially as she could see cake being given to other residents, and did not understand why she couldn't have any. I thought that was really unnecessarily cruel.
So we would take her a small cake or couple of biscuits in when we visited, and leave a packet in her room, (some of the carers agree to give them with her bedtime drink). But generally It was bloody difficult to get past the Home's mindset that "diabetics can't have anything sweet' despite it being insulin controlled.
And the stupidest thing of all was that they were absolutely fine with giving her a (very sugary) commercial fruit yoghurt (yoghurt - it must be healthy!) for her pudding while other residents got steamed sponge and custard.
She died of pneumonia in the end, after a bout of shingles. I still feel anger and guilt that she was denied little treats in her final years when it would have meant so much to her.