A healthy body will definitely affect positively on the functionality of your brain chemistry though... doesnt mean you have to look like arnold shwarzenegger or wear a backwards snapback with huge headphones, but to be physically fit is vital for things like confidence, assertiveness and proper hormonal regulation. Diet and exercise can assist and in some cases completely cure certain mental illnesses in some children and young adults (adhd for example). Imagine the benefits to the "normal" person, personally I always feel I think more clearly, have a more stable mood pattern and interact with greater fluency when I'm on top of my training and diet.