@Ruina has already explained, but because you are a man ;), I’ll say it for you again. LOL In a slightly different way.
Carbs trigger blood glucose and insulin spikes. So lets say you need 1800 calories one day, but you’re eating a lot of carbs. Insulin is high and insulin says “we need to take all this extra blood sugar (from those carbs) and pack it into the fat cells.” So 500 of those calories get stored as fat, but your energy needs were really 1800 calories, so there’s a deficit. And because insulin is keeping your fat stores “locked way,” you’re hungry. So you eat 500 more calories, insulin says “more sugar incoming!!! put 100 calories in fat storage!” so you’re STILL at a 100 calorie deficit…but you’ve over eaten by 500 calories.
It’s a crazy roller coaster with carbs because the more you eat, in a very real way, the more hungry you are but the fatter you get. Then when you say “I’m going to lose weight! Down to 1500 calories I go!” but you’re still eating carbs, so even with the fewer calories, your insulin sees the blood sugar and says “Hell no, those sugars are going into FAT storage and you can’t have them back.” Eventually, if you restrict enough, you have to use fat for energy, but the entire time, because insulin is high, your body is screaming for more energy…so you’re hungry. You can’t listen to any satiety signal because there really isn’t one. Your body also isn’t efficiently using fat for energy because the preferred mechanism, due to insulin, is energy from glucose.
Cut out the carbs, insulin spikes go way down. So now you don’t have insulin trying to force sugar into fat cells because you’re not eating sugar (whether as sugar, fructose, carbs, etc). You don’t have insulin triggering the “EAT MORE” monster. You’re off the roller coaster and you can start “hearing” your satiety and hunger signals much better. Your body starts burning fat for fuel, it accesses your stored fat more easily because insulin isn’t acting as a “gate keeper,” the ups and downs of blood sugar for fuel go away, and you become a more efficient fat burning machine. You now become “fat-adapted.”
At that point, if you start restricting energy sources (fat), your fat burning machinery will just use the fat on your body for energy. It doesn’t lower BMR because you’re still giving it plenty of fuel from your stored fat cells. When you’re a sugar burner and you reduce calories, it’s simply a deficit of fuel because you have relatively little sugar in storage…so that’s where the lowered BMR comes from.
So you have to be a bit careful, in the beginning, to take away almost ALL sugar sources of energy, but then give your body the fat…because initially, your body isn’t good at accessing stored fat due to the still elevated insulin levels. Once your mitochondria become little fat burning machines, you can then reduce the fat on your plate so that you use the fat on your body more.
Hope that makes sense! It’s late and I’m sure I’ve probably messed something up…