Diets of this type are based around the reduction of carbohydrates. Typically, they advise that you consume carbohydrates equivalent to only 20-30% of daily food value of suggested carbohydrate consumption. The most famous diet of this type is the Atkins diet. During the first few weeks on it you will be allowed to eat a maximum of 20 grams of carbohydrates per day. Over time you will be able to slowly increase that amount up to 70 grams. However, you will have to continue eating this way throughout your life. At first your body weight will decrease dramatically, but this is not fat loss. You lose glycogen in your muscles due to the fact that you are not supplying your body with the normal amounts of water and glucose it is used to.