In answer to a reader submitted question, Simon Whistler, host of the informative video series “Today I Found Out” explained what a calorie is, how calories were counted in the past and how food manufacturers currently calculate a calorie when labeling food.
In 1990, under The Nutrition Labeling and Education Act of 1990, the FDA began requiring food manufacturers to label the amounts of nutrients and calories contained in their products. This act resulted in moving away from the bomb calorimeter method for determining the number of calories in food. One reason for this was to make it easier and less expensive for manufacturers to meet the standard. Another was that the bomb calorimeter method measures all available calories in the product. Since most foods contain indigestible components, like fiber, that pass through our system and get excreted in the form of bum brownies, this would lead to a consistent overestimation of ingested calories using the bomb calorimeter. Instead, they used an easier method known as the Atwater system. This approach adds up the calories available through the energy containing nutrients in the food item, like protein, carbohydrates, fats, and alcohols. They use standard reference tables for common ingredients based on their energy densities.