How is the calorie value of a food sample determined?

It can be calculated from bomb calorimetry data, subtracting 1.25 Cal/g protein for incomplete digestion. More commonly, it is calculated using 4 Cal/g for total carbohydrates, 4 Cal/gram for proteins, and 9 Cal/gram for total fats. It can also be calculated using data for specific food factors for FDA approved ingredients.