According to a book I am reading, diet science currently agrees that there is one way to loose weight: A calorie deficit.
For example, if I need 2000 kcal a day and eat only 1500 kcal a day, I will loose some weight over the next weeks/months.
To my understanding, calories here are totally interchangeable, if we are only concerned with loosing weight (and ignore nutrients etc).
Calories are basically measured by burning food and measuring how much energy was set free.
My question is: Why and how does it work so good and why are calories interchangeable?
In more detail: Why can we translate the burning of calories with fire to processing the calories in food with our digestion system so perfect? Why is there no difference (concerning weight loss), if I eat 1500 calories as pure sugar or eat them as pure protein (where I would assume the body needs more energy to break down the protein)?
Because thermodynamics exist. Food is compact nature way of storing energy. If you overeat, then your body will store excess energy in fat. Because it hasnt been all used. If you undereat energy is being used from for but rest of it will have to come from fat. Energy, measured in calories, had to come from something. You poop some stuff out too i would guess.
So calorieswise proteins, carbs, or fat - doesn’t matter. This matter when talking about proper diet, person needs protein to grow, fat to syntetise some hormones, and carbs as source if quick energy. For good poop you need fiber too.
You are correct, body uses up energy to process proteins and other matter. It is assumed overall that all of these processes are part of that body upkeep of 1400kcal daily.