Jim Katsoulis explains why your brain fights weight loss: it evolved when food was scarce, so it pushes you to overeat. Sugar hits your reward circuits like cocaine does. You have to deliberately create motivation to override these older drives.
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You have no natural motivation to want to lose weight. If you think about your brain, it evolved over millions of years in a food scarce environment, so that when you see food, your brain is motivating you to eat as much of it as you can, to the point that even eating sugar lights up the same parts of your brain that would light up if you did cocaine. So once you understand this, what has to happen is you need to create your motivation, because in a certain way, when you want to lose weight, you're kind of fighting against the older parts of your brain. The are just driving you to overeat. And understanding this is the first step in being able to master your weight once and for all.