We are told to "find out how many calories your body needs, and only eat that much", when those telling us to do that have no scientific method for actually doing this. We are told that, if we are eating fewer calories than we burn, we will lose weight.
Let me inject a little bit of logic into this, okay?
First of all, your body is not a closed system. It is not a calorimeter. You do not use the energy in food by combusting it, as a machine would. There are a whole bunch of little processes involved, each one relying on chemicals that are produced by your digestive system, or your brain, or other various parts of you. But more on that later.
I am going to make up some numbers. Please note that these are hypotheticals, okay?
Let's say that Janine weighs 240 pounds, is five feet tall, and 33 years old. She is moderately active. Janine eats about 2500 calories per day, and has stayed 240 pounds for the past three years.
Janine has decided to lose some weight, so she reduces her intake to 2200 calories per day. After fifteen days, this amounts to 4500 calories less than she would have over the course of those fifteen days. However, something has happened: Janine's weight did not change! But, wait, she should have lost more than a pound! What is going on here?!
Most of you FA folks are nodding your heads. You know this story. Janine's body has noted the intake deficit, and has responded by slowing her metabolism slightly to make up for it. If Janine continues to reduce her intake, she will eventually lose weight, of course, but her body will respond to this by giving her maddening hunger pangs, making her obsessed with food, and slowing her metabolism further.
So Janine has been a "Good Girl", starving herself down to a socially acceptable 140 pounds. Starving is the correct term here; she has needed to overcome her body's natural defense system against famine. It is more sensible, biologically speaking, to slow the metabolism instead of losing body mass. The body's worked very hard to build tissues and store energy, and it needs a pretty darn good reason to dip into those resources. It evolved under the pressure of tens of thousands of years of hunting, gathering, famine, floods, and struggle. It doesn't know that Janine isn't actually going to die of famine, or that she has plenty of food available to her; it just responds to the reduced food intake by slowing down her systems and giving her a psychological drive to prioritize finding and consuming food.
Janine's slim body, despite fitting into smaller clothing, is still hungry. She uses a calorie calculator to determine the amount of food a 140-pound woman should be eating to maintain her weight, and begins eating that much. The thing is, Janine is not a 140-pound woman any more than Michael Jackson is white. Janine is a 240-pound woman who has endured famine, and her body is ready to get back to its normal self again. She eats like a normal person, but, thanks to a slowed metabolism, and a body that's been eagerly awaiting the chance to rebuild itself, she begins to gain weight again. In fact, she gains back all of that weight, plus an extra ten percent. She doesn't eat quite as much as she did before her diet and regain, but she doesn't need to. Her metabolism's slowed down.
Now, where did that extra ten percent come from? Here's the thing: If you had a savings account that was for emergencies, and you wound up needing every penny out of it when the time came, once you were able to start saving up again, wouldn't you save up some extra beyond what you used last time? Janine's body is saying, "Hey, we barely made it past that famine; we used up 100 pounds of tissue to make it; let's save a little more than that for the next time." And, every time Janine starves herself down to 140 and regains it all, the body's going to decide that it needs even a little more than the time before. And so on, and so forth.
I have portrayed metabolism in a very simplistic way here. Yet, how much explanation have I given, while others will reduce the subject down to "Eat fewer calories than you burn, and you will lose weight"? Metabolism is extremely complex in all creatures, and even more so in an animal that is designed to rather effectively survive famine, as we have. Our little shrew friends absolutely will die if they face famine--they will resort to cannibalism quite readily.
The human organism, however, is well-equipped to resist death by starvation; we do this with a staggering web of chemical reactions throughout our bodies that respond to abundance and scarcity, reactions that are not completely understood, even with our high level of technology and science. We've made grave mistakes tinkering with these processes, mostly for the purposes of vanity. We would be far happier and healthier learning to accept that this autonomous function of our bodies is not subject to our will, but is instead a necessary and wonderful quality that has enabled our species' continued survival against harsh odds.
Don't allow anyone to shame you for not having conscious control over your metabolism; you can no more control it than you can your heart rate just by thinking about it--and there's no reason for you to even try. It isn't a crime to be famine-resistant.