I am glad to have read this thread.....as I was going to ask the same kind of question....except, not about the food.
I have noticed the past week or two that my 6 yr old has been drinking a lot more water and is also begging for more food which he has NEVER done before!! It was really a red flag for me when he was acting starved and totally gulping his food. I called my vet and we are scheduled to get a full blood work done + thyroid next week, I was told it sounds like there is a medical problem and should have the blood work done first and far most at this time to rule it out but they said it could be.....cushings, thyroid, diabetes, or even a simple urinary tract infection and the blood panel will basically tell them if it is anything medical.
I would encourage you to at least get a blood panel done to rule out anything medical.
Good luck and Keep us posted, I too will let you know what I find out next week when we have our results back.