If you eat nutritious food there is a better chance that you'll be healthier than if you don't. What about thinking healthy thoughts?

It is a fact that what we put into our minds is what comes out of them.  Studies have shown that people who dwell on negative things such as, hate, violence, and anger, are likelier to suffer from depression, have lower self esteem, have higher levels of stress, and are  even more likely to commit suicide, than those who dwell on positive things.
We know that consuming lots of sugar and fatty foods has negative effects on our health (obesity, diabetes, lack of energy (fatigue), clogged arteries, heart attacks/strokes, etc.).  In fact, doctors preach to their patients, "if you want to be healthy, eat a well balanced diet and exercise".  What they don't preach is that if you want to have a healthy mind, avoid feeding it negativity.  
Where one's mind is their actions will soon  follow.  When one dwells on negativity, their speech becomes negative (can't, won't, don't, shouldn't, never, always, etc.), their attitude becomes negative (irritable, easily angered, depressed, etc.), and their actions become negative (road rage, angry speech, violence, etc.). 
While unhealthy foods make ones body fat, negative thoughts can make ones head fat.  Negative thinking focuses on self, whereas positive thinking often focused on others (as well as self).