I've had two cats progressively refuse to eat to the point they wouldn't touch their favorites. Both times, it ended up being IBD, and unless it's caused by food allergies, it's treated with steroids. The first cat went from 16lbs to 11.5lbs and we didn't realize there was a problem because she free fed. We switched to wet food, and that's when we noticed her diminishing appetite. 4 days after the start of prednisolone, she was back to eating and 1.5 years later, she's back up to 16lbs. We've tried to wean her off the pred, but she stops eating.
The second cat just recently stopped eating. He was put on budesonide 5 days ago and is eating again(same food he refused). The clear sign for me was that they were still begging to eat, but turned their noses up at anything I put down(and I mean anything. I tried every food from every store close to me). With diabetes though, treatment is a concern. I chose to go with budesonide this time because it's localized in the intestines and is prone to fewer side-effects, but I would assume it would still affect blood glucose.
One other thing that worked for me was pureeing their food. They devour the food when it's pureed, but don't touch it when it's not.
Good luck,
Jason