Fast Food Head vs Good Food Mind...
You Are What You Eat
A diet of fast food and prepared chow is basically junk and nutritionally vacant, full of clogging fats, salts, starches, phosphates, dyes, colorings, hydrologized compounds and unpronounceable chemicals, to mention a few. Not only does it lack much nutritive value but it also has a lot of harmful constituents that are bad for your body and overall health. It stands to reason that an intake of this as food is not only bad for the body but cannot feed the brain what it needs to reckon even ordinary stuff out.
My conclusion, completely unscientific and from mere observation, is most people who survive on fast food and junk live their daily lives the same way they consume food, in an inert head that lacks control. Fast and full of junk time, wasting energy on trivial pursuits and not really thinking about what they are doing, how they live their lives and thinking ahead. This is a generalization but it seems to me, that if you don't think about what you are putting in your body, not considering how you eat, it's an indication of a general lack of concern about personal well-being. It's difficult to tell which comes first, a lack of ability to have a strategy and a purpose or the lack of good nutrition that creates a toxic body and dull head that can't think well because it's undernourished.
On the other hand, eating healthy food, basic unadulterated stuff that provides protein, carbohydrates, fats, vitamins, minerals and other nutrients in proportional balance, is good for you. It doesn't have to be organic, vegetarian or vegan, sparse or dull and can be obtained in a regular grocery store for reasonable cost. It requires thoughtful shopping and most prepared freezer, prepackaged box and canned foods should be counted out. A lot of basic good food doesn't require much preparation and can be fixed relatively simply for a wholesome meal or "grazing" during the day. It provides the building blocks for a healthy body and simultaneously nourishes the mind.
Thinking about how you treat your body by considering what you eat with a modicum of exercise, is a sign of having control over your life and how you live it. It's an indication that you plan ahead, consider the consequences of bad choices, deliberate on important decisions and overall think relatively clearly. There's also a high probability that your moods are better or if you have mood problems you are better able to deal with them. My completely unscientific conclusion is that because you eat better, you also have a better mind.