Newbie question: Fat in food slows effect of insulin?

Hi all, my vet told me that a food high in fat will interfere with the absorption of insulin, and because of that, the fat content in food is equally as important as carbs.

Anyone else heard this? What is a good canned food to buy that is low fat, low carb, and low phosphorus. My vet says the Fancy Feast is too high in phosphorus and prefers that my Tessie Lou stays on Purina DM for this reason. I find it all so confusing. I have scoured the food charts, and called some companies, etc., but I still haven't found a good canned food with the proper balance. Tessie Lou is not crazy about DM, but she loves her Fancy Feast Classics Chicken and Chopped Grill. But, Fancy Feast gives her gas! What to do?