When there isn’t enough readily available glucose from the breakdown of carbohydrates, the body first turns to stored carbohydrate reserves (glycogen). If there is still no new intake of carbs, and the reserves are depleted, it will be forced to turn to alternative sources (fat and protein) for energy.
It is not a good idea to force your body to go into these states by drastically lowering your carbohydrate intake over long periods of time. The idea of forcing your body to burn fat for fuel by consuming high amounts of protein and low amounts of carbs will backfire, and most of the time will produce the opposite results than the dieter actually expects!
In extreme cases, such as when a person is on a zero-carb, or a very-low carb diet, the body will break down your muscle and convert it into glucose to use it as an energy source. It will also do this when you are following a low calorie diet, and the starvation response is triggered.
Since your muscle is a metabolically active tissue, your body needs to expand energy (calories) in order to maintain it, and by burning up your muscle as an energy source and lowering your metabolism, you body is simply conserving its energy to allow you to survive longer.
Whenever you try to manipulate your body to burn fat as the main energy source, by drastically lowering your carbohydrate intake, it is only a matter of time until your organism will trigger its defense mechanisms and you will be forced to go off your diet.
As soon as you start consuming normal amounts of carbohydrates, after going off your diet, you will regain all the weight back, and will keep gaining weight, because your body will not burn carbohydrates as efficiently as it did before you started using the low carb diet approach.
Consuming moderate amounts of carbohydrates, protein and dietary fat, on a regular basis is the only way to create permanent fat loss. Your major objective is to choose the best sources of carbohydrates, and to make sure that this macronutrient is present in every meal during the day.