John is loosing weight because his body is burning all of the carbs he has stored. After the initial burning of the stored carbs, Johns body will begin to burn stored fat. If and when he introduces carbs back into his diet, the weight is likely to return. There can be a negative of low carbs in the diet because carbs are the main energy source for the cells mitochondria (energy producers). It is not recommended to go on a very low carb diet. It is recommended that a well balanced diet be consumed for weight loss and maintenance.