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)?