Cwillie is right--it has to do with her frontal lobe degeneration affecting her impulse control.
My mom has this kind of dementia (I'm pretty sure--my dad never pushed her GP doctor to give any diagnosis other than non-Alzheimer's dementia). 5-7 years ago, she went go through a period of several years where she gained a lot of weight because she was constantly eating candy. Back then she was still able to drive and would go out to the store and buy herself several bags of mini chocolate bars. She hid them under her bed and ate them in her room every chance she got. Eventually that compulsion died out when she could no longer drive or walk to a store and she got back down to a reasonable weight.
Now her compulsion is to eat the same thing for every lunch at exactly 12:00 and she begins looking for her dinner at exactly 6:00 and gets extremely agitated if she can't see the food (it's in the oven or we picked up takeout).
This will be a phase and your m-i-l's family members will do what they can to deal with it until some different behavior takes its place. I found these articles helpful: https://www.brightfocus.org/alzheimers-disease/article/what-are-stages-frontotemporal-dementia
My Luz was just the opposite. In the last year she dropped nearly 30 pounds. The last few weeks she developed problems swallowing. I did leave snacks and liquids out for her and she would munch and sip all day. I also had to help her brush her teeth since she only brushed the lower teeth. But we had fun doing this.
The frontal lobes are where our impulse control lives so it makes sense that overeating (and overdoing lots of other things too) are common in this kind of dementia.
Overeating is a symptom of FTD, especially sweets and carbs. You could restrict her access to these but that might upset her. Dental care is most important with the increased sweets. Make sure she brushes twice a day. Oral bacteria can cause infections.
My mom has this kind of dementia (I'm pretty sure--my dad never pushed her GP doctor to give any diagnosis other than non-Alzheimer's dementia). 5-7 years ago, she went go through a period of several years where she gained a lot of weight because she was constantly eating candy. Back then she was still able to drive and would go out to the store and buy herself several bags of mini chocolate bars. She hid them under her bed and ate them in her room every chance she got. Eventually that compulsion died out when she could no longer drive or walk to a store and she got back down to a reasonable weight.
Now her compulsion is to eat the same thing for every lunch at exactly 12:00 and she begins looking for her dinner at exactly 6:00 and gets extremely agitated if she can't see the food (it's in the oven or we picked up takeout).
This will be a phase and your m-i-l's family members will do what they can to deal with it until some different behavior takes its place. I found these articles helpful:
https://www.brightfocus.org/alzheimers-disease/article/what-are-stages-frontotemporal-dementia
https://www.caregiver.org/frontotemporal-dementia
The last few weeks she developed problems swallowing.
I did leave snacks and liquids out for her and she would munch and sip all day.
I also had to help her brush her teeth since she only brushed the lower teeth.
But we had fun doing this.