What is the connection between Calories and cellular respiration? Why is human metabolism measured in Calories?

We can imagine the human body as a complex system that needs energy and consume energy . This energy is measured in calories. Many body functions require this energy. Such functions can be, for example, tissue regeneration, maintaining a constant temperature, exercise, fight infections and viruses, etc.. For all this, the food is consumed and  existing reserves in the body are consumed also (fat and glucogen). When energy needs are adequately satisfied, we are in a relatively flat stage: the body weight remained stable. If the consumption of calories is lower than caloric need at a moment, it triggers a negative energy crisis and use body reserves to recover balance (generally, first, decompose glucogen and fat). If this lack of fuel remain a long time, the body will start "digesting" its own organs and muscles.There is an evidence which demonstrates that diets based on starvation, on long term and diets that avoid fat and carbohydrates, can cause serious health problems ,leading to degradation of proteins from muscles and organs and not a breakdown of fat! (This explains why sometimes, after a  drastic diet without fat and carbohydrates, we can not eliminate cellulite and / or fat). All these are reasons to avoid drastic diets that limit the daily intake of calories to 1000 or less 1000.