Most people have a range (buffer zone) of calorie intake where increases or decreases of calorie intake within this range result in no meaningful change in bodyweight. The corollary of this is that calorie intake generally needs to be significantly lower than one would expect (not uncommon for females to have to diet on 1,000-1200 kcals/day and males to have to diet on 1,500-2,000 kcals/day) in order to yield consistent fat loss. It is common for someone new to dieting to think to themselves:
“It seems like I’m maintaining weight on “X” amount of calories per day. This must mean that if I decrease my calorie intake by 500 calories/day, I can lose 1 lb. of fat per week! (500 calorie deficit x 7 = 3,500 kcals. 3,500 calories equals about 1 pound (0.45 kilogram) of fat).”
No. That’s not the way this works.
This also explains why after a diet—assuming a gradual increases in calorie intake and NO BINGING—bodyweight will remain stable (sometimes bodyweight will even drop slightly) while the person adds somewhere in the range of ~500-1,000 kcals to their lowest calorie intake that they were previously dieting on. More specifically, after a fat loss phase, when gradually increasing kcal intake while still in the buffer range, the metabolism seems to sometimes adapt and relatively overshoot, which results in a loss of BW/fat (not indefinitely though). To be clear: this isn’t the result of the whoosh effect/loss of water/decrease in cortisol or increased NEAT (constrained TDEE precludes this).
Let me illustrate with my own metabolism:
I can finish a lean mass gaining phase maintaining weight on about ~4,000 kcals/day. However, my metabolism will swiftly adapt—with no real meaningful change in bodyweight/fat loss—to a calorie intake between 2,200-4,000 kcals/day. It is not until my calories are lowered to around 1800-2000 per day that I can consistently lose fat. In fact, I can ride out an entire fat loss diet on 1800-2000 kcals/day.