My mom was just diagnosed with Dementia about a month ago after almost a year of going back and forth to a neurologist. She doesn't think that there is anything wrong with her most of the time. She always tells me "I'm not crazy" (especially if I'm trying to add another medicine to her regimen). I am constantly being accused of stealing and hiding things from her. I can't keep enough food in the house. My mom eats constantly. She will eat a half gallon of ice cream in 1 1/2 day. When I say something about it, she doesn't remember that she's just eaten 3 bowls. Now I have tried limiting the amount of food but she has someone take her to the store to buy more.
When there is an eating issue in the house the rest of the family needs to change their way of eating until this phase of dementia passes, sounds like Mom is getting comfort out of eating. No more half gallons of ice cream for awhile, or buy flavors she doesn't like. What other food items that Mom gets into? If she tends to eat easy to eat items, then no more chips, cupcakes, etc. Go for the boring such as apples, pears, grapes, raisins, nuts.
The being accused of stealing and hiding items is yet another phase of dementia, eventually that will pass and something else will take it place. Hang in there, strap on your seat-belt, this is going to be quite some journey.
My concern would be for her health in the present. If she gains 5 pounds a month for the rest of her life, she might wind up pretty uncomfortable in, say, three years. And it might be much more difficult for in-home care to take care of her if she is 180 pounds heavier than she is now!
So while I wouldn't necessarily insist she eat broccoli instead of ice cream, I guess I would try to prevent the worst consequences of her current eating patterns. I like vstefans medical approach a lot, especially if something medical is contributing to the problem in the first place. Talk to her doctors.
Also keep in mind that this particular pattern may not last, and the weight gain may start tapering off. That is no reason to totally ignore it now, but the "180 pounds in 3 years" is a worst-case scenario and may not come to pass in any event.
If at all possible, don't make food a contest of wills issue. Distraction, redirection, and other activities are better (if they work at least some of the time). Adding exercise is a great option, if your can get it to work. While she is out walking with you or a companion she is not only not eating but she is burning some calories.
Most people with dementia cannot live alone. It sounds like this is the case for your mother. She needs more supervision and interaction if she is getting her neighbors to take her food shopping. My first suggestion would be an adult day health program (adult day care). Generally a bus or van picks participants up, they spend the day with other adults with scheduled activities, are given a hot lunch, and are returned home. She would certainly not have as many opportunities for over-eating in that environment. And she might really enjoy it. Usually it is available for 1 to 5 days a week.
Or if she had a companion with her for several hours a day at home that might give her something else to do besides eat.
As to other aspects of the newly-diagnosed dementia, she is right. She is not "crazy." She is a mature, intelligent, self-directed independent adult who happens to have a disease that makes her brain malfunction sometimes (or all the time, as the case may be.)
I don't need any more pills! How many times to I have to tell you that I am not crazy!
Of course you are not crazy, Mom. You are an intelligent, well-educated woman! You now have a disease that interferes with that intelligence sometimes, and this pill is to help support your intelligence by keeping your brain healthy. (Agree with her as much as you possibly can.)
The accusations of hiding or stealing are very common. It becomes a vicious circle. They can't find something. Their "logical" conclusion is that someone must have taken it. So they hide something else to ensure it isn't stolen too. And when they don't remember hiding it that it reinforces the paranoia. You must absolutely not take that personally. Don't argue about it. Be reassuring and acknowledge her distress.
"Oh mom, those nice sunglasses with the little stars are missing? That is really too bad. I know how much you like those, and I do too. I'm pretty sure your neighbor would not take them or anything else of yours, but it is very sad you can't find them. Let me help you look." This particular phase will pass and you will not be dealing with this forever. The next phase will be a new challenge!