In a YouTube video by Epipheo called “How To Lose Weight: The Real Math Behind Weight”, they talk with Dr. Carson Chow an MIT physicist and mathematician. He explains that for our body to function we must burn energy. When you do not consume enough calories, your body burns fat.
Energy does not come free and if you do not eat enough calories to provide the body with energy, then it will burn stored fat. One pound of fat burned is about 3-500 calories of fat.
This is a large amount of energy.
When you eat 500 calories less a day in one week, you can lose 1 pound per week. The problem with rapid weight loss is that it is hard to sustain and often you gain the weight back. The rule of burning calories and losing weight rapidly often level off. The new rule is that for every ten calories less you eat, you lose one pound but may not see the results for 3 years. This is because weight loss is slow and takes more time to achieve than we realize. Slow weight loss is better because you may be able to keep the weight off once you lose it.