It is true that in the short term, most people lose weight very quickly on low carb diets. The catch is that the majority of the weight lost is loss of water and muscle tissue – not fat. When carbs are in short supply, your body doesn’t just turn to fats for energy but also protein (muscle), breaking it down and turning it into glucose. When a normal diet is resumed, some of the lost muscle tissue is rebuilt, water is restored and weight quickly returns, but mostly as fat. What you are left with is a decrease in muscle tissue, which will lead to a decrease in the number of calories you need every day to maintain your weight, thus making it significantly harder to keep your weight under control when you stop following the low carb diet.