Is a BMI over 25 sinful?

Are overweight Christians sinning against God?

19 Their destiny is destruction, their god is their stomach, and their glory is in their shame. Their mind is set on earthly things.(Philippians 3:19) NIV 

Only the Holy Spirit can tell you if you are sinning not me.

Three are types of diets that I would recommend. Keto, Slow Carb and vegetarian they are really not diets more eating protocols. It is not about calorie restriction. It is about eating natural foods. Foods that comes in a box are not natural.