I can tell you it was damned hard to change the way I ate, and I'm not stupid. At least, I'm not stupid in the "underclass too stupid to do the obvious things that will fix their lives" kind of way that you're talking about. Intelligence wasn't the solution to the problem; long-term focus on accustoming myself to a different way of living was. For about eighteen months, maybe closer to two years, I paid more attention to losing weight than I did to anything else, including my personal life and professional development. And guess what? Even now I have to pay a certain amount of attention to maintain my weight. It just won't go on autopilot. I'm still smart enough and knowledgeable enough, but it's a small constant drain on my energy to execute.
The drain on my energy is much lower now, but I was lucky to be able to devote so much time and energy to get to where I am now. I was even unemployed for a significant amount of that time, which I could afford thanks to working in a pretty high-paying profession, and which was a big help in being able to focus on changing my lifestyle. Plus I happened to have a friend who decided to turn me into a running partner at exactly the right time. How lucky could I get?
All that happened in my mid-twenties, so I spent more than a decade being fat and bright (at least by IQ-type standards: test scores, grades, academic competitions, etc.) before I made any lasting progress on my weight.