Take for instance a low carb diet--high in protein but very low in carbs, many bodybuilders will use this to get very lean and then slightly carb up to look fuller before a contest. People on Atkins types of diets usually gain weight back very quickly once they go back to carbohydrates. That is why so many bodybuilders put on weight so quickly after a show, they were depleted and the carbs were a trigger to their system to store glycogen, water and other necessary nutrients in their muscles. Sorry not too scientific but that is generally what happens.