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hehhh??? your body can use carbs, fats or proteins as fuel. carbs are most energy dense, and protein is hardest to convert to energy. so you have to eat a much higher volume of beef to get the same caloric intake as bread or something, and even then your body isn't used to processing that. the transition between your previous diet and your new diet, whatever it may be, has a possibility to induce serious fatigue as your body adjusts to digesting and metabolizing different energy sources. (I personally think some of this has to do with gut bacteria levels readjusting to suit the new internal environment.) keto would have the same effect for the same reasons but not as pronounced and starting sooner since bodies burn carbs first then fats then proteins last. calories in calories out is true, and works in a sterile lab setting, but practically, these diets work for a reason. it's just harder to get more calories in on these diets.