Your body has four compartments of energy from which to draw to meet its metabolic needs: blood glucose, glycogen (stored glucose), muscle, and fat. It draws on these energy sources in a very specific order - first burning the glucose in the blood and next the glycogen reserve.
Once your glycogen is exhausted, then and only then will your body turn to the muscle and fat compartments.This means that every time you replenish your glycogen stores, the fat-burning stops until it the glycogen is once again depleted.
This is one of the big reasons we have to cut calories in order to lose weight.
Everyone agrees that it matters how many calories you’re taking in – but there are a few crucial secrets that can make all the difference between success and failure.
If we take the standard USDA recommendations of approximately 60% of calories derived from "good carbohydrates", 25% from protein and 15% from "healthy fats" and simpy go on a diet and cut those amounts in half, we will have a "balanced diet" with one-half the calories. Yes, you will lose weight. This is why most popular weight loss programs base their programs on a "balanced diet".
Here's the reason. If we continue to replenish some of the glycogen stores every day by consuming carbs and fats, our fat-burning will stop until those have been depleted. This leads to an erratic weight loss.
Secondly, and more importantly, decreasing the minimum daily requirements of protein will lead to muscle loss. As blood glucose drops (from low calorie intake) the body will burn fat...but will also break down muscle. And, as we lose muscle our metabolism slows.
Now when these folks have achieved their goal weight, what is the predictable result? They go back to eating "normal size" meals but their metabolism is slower and they regain the weight, often times ending up heavier than when they started the diet.