Many low-carb diets actually do not provide sufficient carbohydrates to your body for daily activities. Therefore your body will start to burn stored carbohydrates (glycogen) for energy. When your body starts burning glycogen, water is released. Therefore the drastic initial drop of weight at the beginning of a low-carb diet is mostly the water that you lose as a result of burning glycogen.
The fact is that low-carb diets are also often calorie-restricted! Mostly only eat an average of 1000 – 1400 calories daily, compared to an average intake of 1800 – 2200 calories for most people. To lose one pound a week, you only need to eat 500 fewer calories per day in your normal diet. Therefore, it doesn’t matter if you eat a high- or low-carb diet, you will lose weight if you reduce your caloric intake to fewer than needed to continue your weight.