I found Nutrisca dry cat food (I get the chicken) which is made specifically for diabetic cats and I have been using that for about a year for my 13 yr old. My very picky 16 year old cat loves it too. It lasts as long time and is a great price about 34.00for 13 pds. I get it online with free shipping or a nearby feed store. Mainly it has a very low glycemic index for diabetic pets, much much less then all others. Rated at 9-10 where most are rated at 55 on the glycemic index! Grain free, no byproducts, a great food so far.
Important also is he gets 3 bowls a day of Evo wet and water and the dry food is for if I'm not home and his body tells him to eat. My vet recommended always leaving dry out for that reason. He is lucky that he can eat the wet as I have another cat age 9 who throws up wet food as well, and yes it's a struggle to find what works with her.
Anyway my diabetic kitties numbers have been lower being on this combination then anything else. Also leave fresh water wherever they hang out at. He has Phemphagas and has been on steroids for over 4 years so ended up with diabetes due to meds. (Prednisolone, important side note..it's much better and safer choice for pets vs Prednisone, although almost the same, it is processed much better in their systems. Prednisone does not work at all and he degrades,where prednisolone works wonders. If he doesn't take steroids he will die a slow miserable death form this rare autoimmune cat disease, so no choice. Took years to even get the correct diagnoses from a specialist finally.)
He is a large ragdoll cat in the first place - at about 24 pds between the steroids and being a ragdoll. He is almost 14 and has had diabetes over 3 years, past history also of urinary crystals but no reoccurance for 8 years since having so much wet food with extra water mixed in. Evo wet (no cat food with fish is ever given) has been great (the Evo dry makes him constipated) Wellness too though he tends to keep weight down a bit better with Evo. His insulin is Vetsulin. This has been what works for him anyway, vet says he is doing great! You would never guess he has either disease by his looks and actions. I know all cats are different and its not easy to figure it out, takes time and research but hang in there! You'll find what works: )
Below is some basic info.
http://www.dogswell.com/healthy-cat-food-treat/cat-food/ talks alot about dogs but they do make this cat food.
From what I first read along time ago, the person that started the company did so because of a family members cats health issues specifically diabetes so they started their own company focused on better health and food for diabetic cats. I can't find the info now but that's what made me so interested. They wanted to make a healthy food,U.S. based and I liked that too.
Their literature said "Nutrisca now offers a low glycemic, high protein food for cats. Each recipe is natural, great for digestion, and uses animal proteins exclusively from poultry or fish to help cats with food sensitivities". Rated 9-10 on the Glycemic Index (<55 is considered low glycemic) NO grains, potatoes, gluten, corn, wheat, or soy/Antioxidant-rich ingredients/Cranberries for urinary health/Cage Free Chickens and Wild Caught Salmon used for their main sources of protein.
Hope this helps! Best of wishes! (Sorry for the long post, got a bit carried away.)