Help cat gain weight!

Hello,

I have a 16 year old cat. She has always been small but over the last couple years she has lost even more weight. She was 5 pounds in April and now she is 4.5 pounds. We got her up to 5.5 last year after using Hills AD food. But she started refusing to eat in a few months ago.

I brought her to the vet for her 6 month and told her the food issue. They recommended trying Royal Canin recovery to see if a flavor change helps. They were empathetic but didn't have any other suggestions.

She's getting so small! Doctor says it could be IBD which they can't do much about. We already have a hunger stimulant for her ears.

Does anyone have any other tricks that helped? People food that wouldn't be bad for her that would help?