system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are the amount of calories and nutrition facts in food determined?
gpt
A food calorie is 1000 calories, 1Kcal, or a Calorie, spelled with a capital. 
In an aluminum calorimeter, a weighed sample of the food is burned in pure oxygen and the amount of heat produced is determined, per gram of food. While it is true that not everyone digests and absorbs equally, that is how the table of values is determined. The label amount is an average.