Charlotte was given to me as a 6 pound 6 year old lady, and I've had a wild ride with her health.
At first things were good, but then she started getting diarrhea, vomiting, losing weight, not eating, and peeing outside the litter box. I took her to the vet where I spent $2000 on tests to no avail, so we went with a prescription food.
Even then, she wouldn't touch the prescription food that was to help her sensitive stomach. She kept meowing for food, but wouldn't touch what I gave her. She went down to 4 lbs, which was really concerning.
I tried so many things, until recently I decided to just put some canned chicken breast under her prescription food- my god it's working. She doesn't pee outside the litter box anymore (unless I forget to scoop it), and her weight is back up.
Honestly I just think she hated the food I gave her so much that she would rather die than eat it. I also noted she likes diversity in her food, so I swap between canned tuna and canned chicken mixed with her normal prescription food. She also is much, much happier.
For possible dietary requirements, look into wet food with pumpkin, squash, or sweet potatoes. It helps their digestive track and microbacterial gut health.
It's doing wonders for my Kitty's chronic diarrhea.