Asklemmy

Calories are interchangeable like this percisely because a calorie is a unit of energy.

This "energy" we speak of is in stored as chemical potential energy of molecules.

When the human body digests foods, it breaks down molecules to build new ones through chemical reactions. Some such reactions release energy, while others require outside energy to happen. Some molecules are, likewise good stores of energy for the body because they take part in reactions that release energy.

But, at the end of the day, energy is energy. Another type of chemical reactions that release energy is burning. It just so hapoens to be much faster and easier to create and control than the work an ingestive tract does.

The only difference is that burning converts things into a slightly different set of molecules than digestion would (with burning releasing all energy and digestion leavinf some untapped), so energy released by burning isn't 100% on par to the energy extractable to a human digesting it.

That being said, the difference between the "theoretical" energy (burning) and usable energy (ingestion) isn't too important. You may put in the 1500 calories on the label, but you won't utilize all of them. However, taking into account the fact that whenever energy is measured, it's measured by burning we stay consistent. We may not be 100% percise, but we're at least consistently wrong. And the amount of unavailiable energy is incredibly small - humans are actually more efficient than machines from an "energy efficiency" standpoint. Given the fact that each person has a different metabolism (and metabolism changes regularily throughout the day, year and with age), neither does trying to be 100% percise make sense, since your values for today will be different from your values for tomorrow.

About losing weight: Weight is lost when energy is taken in, and gained when it used.

Since a human uses about 2000 calories a day, 1500 was discovered as the best middle ground between starving and not gaining weight altogether.

It really doesn't matter where the calories come from because the only important thing for tracking weight is net energy, gained or lost. 100 calories "trapped" in sugar is the same as 100 calories "trapped in fat". With the human body being as efficient at sucking out energy out of stuff, the only real difference is in how long the process takes - energy in sugars is practically instantly availiable, while energy in protein takes some time to be extracted.

A net gain or loss of 200 calories is the same, wether it's through sugars or proteins. But, for the body, it's all the same. If it has a sufficit of energy it'll store it (and you'll have a net weight gain). If it has a deficit, it'll seem you've lost weight, as that energy went into something other than your body's reserves.