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)?
You are breaking calorie sources down into their biological dynamics. You need to compare calorie values with the carb/protein/fiber compositions. However, within the grouping of carbohydrates and fats you are dealing with a thermodynamic CICO situation because their metabolism is exothermic. Proteins undergo gluconeogenesis (turning them into glucose) which is a net energy loss but requires very low blood sugar levels to trigger it.
I agree that nutrition labels are pretty limited in how accurate they can be and how much room for analysis is left. Since I do a lot of my own cooking I’m forced to do a lot of estimating. I’ve found the best rule for myself is to watch portion sizes, stay a little hungry and keep carb heavy foods to a minimum.
For anyone interested in the gruesome details: https://www.ncbi.nlm.nih.gov/books/NBK26882/
Did you get a chance to read the carbohydrate insulin model of obesity paper?
i.e. the physics isn’t important if you turn OFF fat burning, it’s impossible to lose fat