What causes insulin? Glucose. What foods contain glucose? Carbs.
I've lost 40 lbs this year on a high-fat low-carb (ketogenic) diet this year. My A1C has gone from 5.8 to 4.7 (>6.0 is pre-diabetes, >7.0 is diabetes). And I have tons more energy.
https://www.reddit.com/r/keto/wiki/keto_in_a_nutshell