Low carb diet and keto diets are being followed by some to lose weight rapidly in obese people. At least 45 to 55% of daily calories should come from carbohydrates. Normal individuals should not follow low or very low carb diets on long term basis.
Sugar-free is not synonymous with calorie-free diet. You cannot eat whatever you want just because it is sugar free. You should eat balanced food with needed amount of calories and other macro and micro nutrients.