Calorie question!

It's not a stupid question.

http://www.scientificamerican.com/article/how-do-food-manufacturers/

"The Nutrition Labeling and Education Act of 1990 (NLEA) currently dictates what information is presented on food labels. The NLEA requires that the Calorie level placed on a packaged food be calculated from food components. According to the National Data Lab (NDL), most of the calorie values in the USDA and industry food tables are based on an indirect calorie estimation made using the so-called Atwater system. In this system, calories are not determined directly by burning the foods. Instead, the total caloric value is calculated by adding up the calories provided by the energy-containing nutrients: protein, carbohydrate, fat and alcohol. Because carbohydrates contain some fiber that is not digested and utilized by the body, the fiber component is usually subtracted from the total carbohydrate before calculating the calories."

Calories are derived using a combustion method called calorimetry. A substance (in this case, the food) is combusted in a vacuum chamber surrounded by water. The combustion raises the water temperature and the calorie (unit of heat) is determined.

At least that's what I remember from chem...