Diet. A four letter word.
It means to mentally control one's desire to eat. Which is impossible.
The key is the word "desire". There are different levels of desire, and hunger is one of them.
What we see is that there are those people who can seemingly eat all that they want, and there are those who, if they eat all that they want, will be very fat.
And the key here is also the work "want".
I find personally that I "want" more food than I need. I know that a certain portion will satisfy my hunger. But after eating that portion, I still have a desire to eat more. Not hunger to eat more, just a desire. I want to pack it in, as much as I can. I say "well, I still have room for that", and then I eat that. Until I am fully stuffed, way past being hungry.
So, there is a desire for food that is a desire past hunger and is not hunger.
It must be an addiction.
It may be a throwback to evolution, where those that ate more than they needed, survived the coming famine. Being fat in good times helped one survive the lean times.
Again, it is a matter of desire. My thin friends eat all they want and are still thin because when they eat enough, they dont want any more.
So dieting, that is controlling mentally your eating desires, is doomed to failure.
The trick must be to either fool your desires, or never to develop the desires.
To fool the desire, you eat things that make you feel full, and satisfy.
To never develop the desires, that has to start as a child and continue as an adult.