>>76506223
your bmr - basal metabolic rate is how many calories your body burns to keep itself alive unchanged
(if you eat at your bmr then your weight would remain about the same if you never moved [tdee is bmr plus daily movement which are your maintenance calories])

if you eat in a deficit the body will reduce the bmr by a bit (actual amount is unknown and varies)
with bigger deficits the reduction is bigger
but the reduction in bmr can never cancel a caloric deficit

once the body gets more calories it brings the bmr back up
you can't break your metabolism permanently, it only undergoes temporary changes


deciding on the amount of calories for cutting can be done through a simple subtratcion of 500 or less calories from your tdee of by setting up a diet based on required micronutrients as in at least 0.8g protein per pound of lean body mass, at least 3grams of omega3s and some saturated fats for hormones (just about the lowest you can go without sacrificing health or muscles)