Counting Calories on Food Packages

S. Connery, a reader of the Scientific American, wondered how in the world food packagers calculate the number of calories in the foods they sell. The magazine turned to Jim Painter, an assistant professor of food science and human nutrition at the University of Illinois, for this somewhat complicated answer.
Painter tells us that the Nutrition Labeling and Education Act of 1990 (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.
More useful, perhaps, is Painter’s reference to the USDA’s Nutrient Data Laboratory, where serious nutrition wonks can look up the nutritional value of just about any kind of food you can buy.