I find it funny how everyone is calling an endocrinologist wrong and saying he isn't a nutritionist and yet they aren't medical experts themselves... pretty hypocritical. The knowledge most people bring to the table on this sub is info they have gathered from the Internet as well as other redditors. CICO works, but what you have to realize is that the calories in part is easily measurable. The calories out is what has variations. 2 people that are the same weight age and height doing the same exercises will more than likely not lose the exact same amount of weight. It all depends on each persons metabolism. That's how your calories out is measured. You can Google "how many calories will I burn if I run a mile" but that is highly inaccurate. After a while all of your hormone levels will drop with a large caloric deficit and slow down your metabolism (which is 100% proven btw). This is why the Dr was referring to you going back to your old ways because your metabolism WILL slow down and stop burning calories like it used to and once people plateau they tend to give up if they haven't reached their goal and what they are doing isn't working. What he means by forget CICO because you can eat 4000 cal and lose weight... I don't think he means literally 4000 cals. He wants to check your metabolic rate and set you on a diet with the correct proportions of fats carbs and proteins that will keep your metabolic rate high so the calories will be spent easier. It is kind of like CICO, just allowing you to eat more calories (not 4000 but enough where you are full). So let's say now you're on a diet of 1200 cals a day and your maintenance is 1800. CICO works fine. But you could be at a maintenance of 2200 if your metabolism is working properly and eating 1600 calories per day and losing the exact same amount of weight without having that plateau. Wonder why athletes and bodybuilders monitor fats carbs and proteins and don't go by CICO....