How are calories calculated in food?
Do they sum all the energy in every molecule? By for example, by burning the stuff up and calculating the energy output.
Or is this value supposed to represent the energy that a human body will extract from that food?
Or do they only sum up the elements that we are supposed to digest (Protein, Carbs, Fat) ?
For example, wood contains a lot of energy. We can see that by burning it. But the human body is not able to break done the fibers to extract the enery from wood. We do not eat wood but I suppose there are multiple elements contained in food that we do not digest but that contain energy.
Also, Fat contains a lot of energy compared to carbs or proteins but the human body has a much more difficult time to process fat and extract energy from it (catabolism).
Is it the energy outputed from the catabolism of fat that is calculated or again the theoretical quantity energy in fat (burning it).