Food often has information on the packaging on how many calories/joules it contains. Calorie is the physical unit to express the chemical energy of a thing and is measured by fully burning a sample of said thing and then measuring the rise in temperature of the container this experiment was conducted in. Using this method, the calories of the substance per weight unit are calculated.
However, how much of that chemical energy is absorbed through digestion is another story. I imagine there could be done some measurements by monitoring the blood sugar after food intake.
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