Starvation diets (e.g. fasting, Cabbage Soup diet, Bikini diet)
Starvation diets or very low calorie diets rely on cutting the total calories we need to approximately 1000kcal. This might seem like the best way to lose weight but starvation or very low calorie dieting slows your body’s metabolism (the rate at which we burn calories) by as much as 40% which can make it progressively more difficult to lose weight and keep it off. The body goes into starvation mode and adapts to the reduced calorie intake by becoming very efficient at making the most of the calories it does get from food.
Technically the body will lose body fat, but it will also lose lean muscle mass.
Because of the very low calorie intake the body will use some body fat as energy (ketosis) and lean tissue or muscle (gluconeogensis) to provide it with the calories it needs to function. This process known as catabolism causes muscle mass to drop. Given that muscle burns calories, the less muscle a person has the fewer calories they require.
In addition to the loss of water and muscle, low calorie diets deprive the body of essential vitamins and nutrients needed to keep it healthy. The initial weight that is lost is primarily muscle and water and is quickly regained when the old eating habits return.