Of course they know. It's not a question of knowledge.
The problem is evolution designed us to eat more than we need so we don't starve in lean times - for all of human history up to, what, 150 year ago or so the biggest threat to your life was starvation. Of course if you have the willpower you can avoid eating too much. But most people don't.
The problem is evolution designed us to eat more than we need so we don't starve in lean times - for all of human history up to, what, 150 year ago or so the biggest threat to your life was starvation. Of course if you have the willpower you can avoid eating too much. But most people don't.