To lose weight you must create a negative energy balance: calories taken are less than calories expended. Result: Fat, glycogen and muscle are used for energy to make up the caloric deficit and ideally weight is lost and fat stores are reduced. However, losing weight and losing fat are two different stories. Weight loss does not necessarily equal fat loss.
Dieting: Starvation Protection Mechanism
The secret with any serious and long-lasting attempt at weight loss is to “trick” the starvation protection mechanism of the body. Very low calorie diets more often than not, reduce food/ caloric intake to the point that the body believes that it is starving (<1200 calories for women and <1800 for men). This is because your body uses a brilliant primal survival system that is thought to have evolved over thousands of years as a defense against starvation and means the body becomes super efficient at making the most of the calories it does get from food and drink. When you are crash dieting your body does not know that there are a dozen restaurants and supermarkets around the corner and takes steps to protect it. Your body does not know the difference between starvation and crash dieting and little or no food for extended periods of time equals danger.
The main way your body responds to very low calorie diets or crash diets, is to protect its fat stores and instead use lean tissue or muscle to provide it with some of the calories it needs to keep functioning. Fat is the body’s energy reserve and when you consume fewer calories than your body needs, your body turns to fat for energy. However, when your body feels threatened (crash diet) the body’s leptin levels (a hormone that liberates stored body fat) decrease and only allows fat cells to release energy in order to sustain your most basic bodily functions. This means that although you are dieting, you are not losing fat (or losing only a small proportion of fat) because your body has shut down is its fat-burning ability in order to preserve energy. To make matters worse, when you eat your body starts to store the nutrients/ energy in your fat cells (as a calorie reservoir) for later use, rather than using it to provide energy for your normal day-to-day activities. This means that your fat cells may be increasing in size, instead of getting smaller. The bottom line is that your body fat may actually be INCREASING!