The human brain uses 25% of the calories that you consume. Food is not just calories; it is information. When you consume bad food, you are essentially giving your brain bad information. Moreover, there are foods out there that increases inflammation, which is associated with depression and dementia.
As your weight goes up, the actual physical size and function of your brain goes down. Your brain is the main central unit that controls what you think and how you act. So even if you don't really care about how you look, you should pay attention to how your brain thinks.
Simply changing with your will power usually only last for a short period of time. For a long lasting change, we need to transform our minds, refocus on what is good. We all have to challenge our thoughts and focus on the Truth - the scriptures and God. The Truth will set us free.