It sounds to me like you're a little confused about the advice you're reading. Even I got confused reading your post and had to go over it several times to understand what you were saying.
Quote:
Say I ate 1800 calories/day for a year or so and lost 100 pounds, but to get to my goal and maintain it will take my body 1500 calories a day..
This makes no sense.

If your maintenance calories are 1500, then you're not going to lose eating 1800 calories. If your maintenance calories are 1500 (and by the way, we're assuming here that all these numbers are just theoretical, because as Mel says, no one can tell you what an exact number is or isn't for you), then you'd have to eat at or below 1500 calories to get to that goal.
BUT ... here's where the higher calorie figure comes in.
Let's say there's a woman who is 5'5" and works out 3x a week. And these figures are for maintenance - not to lose, not to gain, just to maintain. (Again, these numbers are hypothetical and I'm rounding off for easier math.)
At 300 lbs, she needs 3000 calories to maintain her weight.
At 200 lbs, she needs 2500 calories to maintain her weight.
At 140 lbs, she needs 1800 calories to maintain her weight.
If you looked at it mathematically, you'd say that it would make sense for her to just start eating 1200 calories, get down to 140, and then add back in 600 calories to stay at 140 lbs.
That makes logical sense.
The problem here is that our bodies aren't machines in that respect. Even though you are dieting and want to lose weight, your body still needs to be nourished. And if you take away TOO MUCH nourishment at once, you freak out your body and cause it to stop losing weight. Our bodies are programmed for survival - over any other need. And if you suddenly drop the amount you're feeding your body *so* drastically below what it needs, your body starts holding on to everything you give it. Hence ... stalling, plateaus, etc.
So the better thing to do is to slowly reduce your calories. As long as you're at a calorie deficit (and assuming there are not other medical issues) you *will* lose weight.
So instead if you were the hypothetical woman above ... instead of freaking out your body and going from 3000 calories to 1200 calories ... go from 3000 to 2500. When your weight loss slows down, you drop down a little more, to 2200 or 2000. When your weight loss slows down even more, you drop down to 1800 or 1500 even. And then as you hit your goal, you start to stabilize.
This allows your body to adjust slowly to eating fewer calories. Instead of freaking out your system and throwing things out of whack, it's a healthy, reasonable way to lose weight. And you'll get more consistent results if you *don't* freak out your body.
That's why eating drastically lower than you have to is really not a good idea. The whole body freak out thing is something to be avoided.
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