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.
The question consists of two parts relating to - cellular respiration and metabolism. The second part of the question - that is, "Why is human metabolism measured in calories?", appears to equate "cellular respiration" with "metabolism". In my post here I provide clarification on the process of metabolism.
Metabolism is a process by which cells in animal bodies produce material and energy necessary for sustaining life. In contract cellular respiration, which is a part of metabolism, is only concerned with producing energy. Also it must be noted that energy generated, used, or stored in body is measured in calories, but human metabolism is not measured in calories.
Metabolism consists of two phases called anabolism and catabolism. During the process of anabolism cells combine molecules of food they receive from outside to assemble new new organic material. These new material produced by anabolism include substances that are used for producing energy as well as for producing tissues for body repair and growth. During the process of catabolism the stored material break down to produce energy and heat.