Starvation Mode
When you're trying to lose weight, your main goal is to create a calorie deficit (burn more calories than you take in). If you burn 500 calories more than you eat everyday, your body will be forced to make up that 500 calorie shortfall from sources other than food - mainly body fat with the result being weight loss. The bigger your calorie deficit, the faster you'll lose weight. Some may ask, "Why not try to eat as little as possible?"
Your metabolism is like your body's power plant. The food you eat is fuel and your metabolism converts the food to energy (a very simplistic explanation). What would happen if you stopped delivering coal to a local power plant? The plant would have to scale back energy production unless it started getting more fuel.
Just like the power plant, when you drastically cut your calorie intake (fuel supply), your metabolism will slow down and burn less food to convert into energy. So what does this have to do with weight loss? When your calories are severely restricted, the body thinks it's entering a prolonged period of time without any 
food. Just like engineers at a power plant will shutdown energy production, your body will will work to conserve energy if you don't eat - mainly by slowing down your metabolism.
This is when you enter starvation mode. Long ago when our ancestors were hunters, food wasn't as readily available as it is today. Our body developed this starvation mechanism as a way to conserve body fat for the long periods of time humans went without food. The trigger to enter starvation mode back then was the same as it is today: an intake of very few calories.
This trigger fluctuates from person to person, but generally, starvation mode kicks in below 1,200 calories. When you go below 1,200 calories per day, your body will start hoarding energy. The effect will be a slowdown in the total amount of calories your body burns each day. Your weight loss will stop and even reverse.
The only way to prevent going into starvation mode is to eat more than 1,200 calories (or so) per day. If you need to lose weight faster, consider increasing your exercise level rather than further cutting your calories.
To make sure that you are eating the right amount, visit the calorie calculator. Enter your age, height, weight and activity level to get an estimate of how many calories you burn each day. If your calorie deficit puts you under 1,200-1,400 calories, skip the calorie cutting and create a deficit by exercising more and increasing your activity level. If you're already experiencing signs of being in starvation mode, you need to increase your calorie intake.
For more information on losing weight without starving yourself, visit the Advanced Guide to Dieting. You can also check out how others have successfully lost weight or share your own dieting experience at the diet forums.
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