Undereating is a major issue for the majority of the world's population, for anorexics, and for you, if you become overly obsessed with your weight.
Not eating enough calories can be a health calamity.
Around the World in 80 Days is one of my favorite books. Towards the end of the trip, the hero, Phileas Fogg, knows his 80 days is running out. He's on a steam ship in the Atlantic, and he's burned up all the fuel rushing back to England.
Feeling the time pressure and unable to arrive in England on time without fuel, he buys the ship and begins to disassemble it. Everything not essential to staying afloat is torn down and thrown into the furnace to provide energy for the engine.
When your body runs out of energy, it does the same thing. It must keep your body temperature correct, or all the chemical processes necessary to life will not function properly. It has to power your brain, or you will not be able to think. It must keep your heart beating, your lungs pumping, and almost all of your organs doing their jobs so that you do not die.
If you do not provide it with food, and thus with calories, it will begin to burn the fat it has stored for just such an emergency.
The problem is, your body can only burn fat so fast. Fat has a lot of energy, but that energy is released slowly. Undereating somewhat is no problem if you're providing your body with some calories.
But if you provide your body with no calories—or too few—it will have to turn to something besides your fat to burn.
Nutritionists' solution is to eat at least 1200 calories a day, even when you are on a diet. If you do that, your body will have no problem providing all your additional caloric needs by burning stored fat. You'll lose the weight you want to lose.
Eat less, and fat may not be enough. You'll wind up starting the process described below.
The next best source of energy is your muscles. Large muscles are not essential for life, and protein provides energy much more rapidly than fat does.
Your body will burn muscle on any day that you don't eat. That's not that big a deal. Your body is well-equipped to handle the occasional day without food, and the minor amount of muscle mass burned on a day you fast is easily restored when you eat again.
If undereating goes on, however, and you burn all or most of your stored fat plus a large amount of your muscles, your will begin to burn the protein in your organs.
On top of that, your body begins shutting down any "inessential" systems. This includes even your immune system, making you highly susceptible to disease.
At that point, your body begins to break down, you get sick, and you die. That's what happens to anorexics, 1 in 5 of which die within 20 years of developing the mental disorder.
Don't join them!!!