Food is a necessity. The body uses it for energy, for repair, and for building new structures. Food can be as powerful as a drug when it comes to building lean muscle mass. Lack of food can lead to a catabolic state in the body. Starvation leads to the body eating itself and the first thing to go is the lean, protein rich tissue. So, if food is this important and a necessity, why does it turn into body fat?
First, it is important to understand that body fat actually has a purpose. It began back in prehistoric times. The body needed a way to store energy in between feedings, that might not come for a while. The body fat stores afford that opportunity. A lot of energy can be stored in the fat stores. This is insurance for those times of starvation.
Our bodies are still programmed for this survival mechanism and even though food is plentiful the body will still protect against possible starvation. This is the mechanism that leads to fat storage. This is why calorie restriction diets are often unsuccessful in the long run. The body senses starvation and slows the metabolism to survive, then shuffles the calories consumed to fat stores for future needs.
Every day the body burns a certain number of calories just to maintain normal functions within the body. This is the basal metabolic rate. Then, any additional activity increases the amount of calories needed to support and maintain the body. The body expects this amount of energy to be consumed throughout the day. Any deficit in the caloric intake results in the body using its stores of energy. If there are no energy stores, or if the deficit is significant enough to trigger the survival response, the body will attack the lean tissue for energy. This results in a reduction of muscle and bone mass.
Any calories in excess of the daily needs get stored in fat stores for those future lean times. After an extended period of low calories the body slows the normal processes in the body to reduce the caloric needs. This means the body can survive on fewer calories without turning on itself. Then, when the food becomes plentiful again, the priority becomes making sure the body has adequate energy stores for future periods of starvation.
Different nutrients are stored at different rates due to chemical and hormonal differences. Fat taken in through the diet is chemically closest to the make-up of body fat, however, it is also a great source of energy, especially in the long term. Energy is released each time it is broken down to a smaller unit. Then the final products of fat metabolism can be converted to glucose for energy use. The body prefers this energy source, due to its large quantity, but excess will easily be converted to body fat. Carbohydrates, especially simple carbohydrates, trigger the release of insulin which opens up receptors on fat and muscle stores to receive the calories. This makes the storage of excess carbohydrates (and any other nutrients eaten with them) in fat stores easy. Protein is the farthest from fat, chemically. It is also used for repair of many structures in the body and to produce hormones and other messengers. It is the last nutrient used for energy and the last to be stored, but in excess any nutrient can be stored for future energy use.
Individuals trying to lose body fat are fighting millions of years of genetic coding. The survival mechanism keeps the body from shedding body fat too quickly. The energy from food is stored there for future needs. Any excess calories, beyond regular daily needs, are shuffled to the fat stores for this reason. The body is an amazing machine with survival as the main program.