If you've got a big gut and you start doing sit-ups, you are going to get bigger because you build up the muscle. You've got to get rid of that fat! How do you get rid of fat? By changing your diet.
I'm a big, big blues fan and the last several years I've really invested in the blues a lot, and I think my playing is getting better because of it - not necessarily better on a technical level, but certainly on a level of appropriateness.