As rates of obesity and diet-related diseases continue rising worldwide, nutrition labels on processed and packaged foods provide valuable data to help guide consumer food selections. Among the most useful pieces of information displayed is the calorie content of products.
This allows shoppers to factor energy intake into daily meal plans and assess products accordingly.
This ScienceShot explores the main practices applied to measure calories in foods that ultimately get printed on labels.
Defining the Calorie
First, it helps to precisely define what constitutes a calorie.
In chemistry and physics, a calorie refers to the amount of energy needed to increase the temperature of 1 gram of water by 1° Celsius. On food labels, however, the term Calorie denotes a kilocalorie, which equals 1,000 calories.
A kilocalorie (kcal) contains the amount of energy required to raise the temperature of 1 kilogram of water 1° Celsius. Sometimes nutrition information expresses Caloric values in kilojoules (kj), a metric unit, where 1 kcal = 4.184 kj.
Thus, the Calorie on food packaging
denotes a unit 1,000 times larger than the
calorie employed in the physical sciences.
The first method used by scientists to determine the energy content of foods was bomb calorimetry. This method directly determines caloric values by burning items and monitoring temperature changes. A bomb calorimeter is a water-filled sealed, insulated container. A food sample would be placed within the chamber and completely combusted by the researchers. Scientists could directly quantify the energy content of edibles by tracking the resulting temperature increase in the surrounding water.
The results would then be averaged across food types. However, this method necessitates the destruction of samples during testing. As a result, despite providing the most direct calorie calculation, bomb calorimetry is rarely used today. Direct calorimetric testing on individual items, while once necessary for calculating energy density averages across nutrients, is now an unnecessary waste of resources. As a result, alternative approaches based on chemical composition now predominate in calorie quantification.
The Atwater System
The Nutrition Labelling and Education Act (NLEA) now governs how calories are calculated for food packages. It was enacted in 1990 and requires nutrition information disclosure, including calorie counts, for all products using a consistent system. The Atwater system is the accepted method for calculating energy content indirectly through chemical components. Instead of literally burning items, this method calculates and applies averages for energy obtained with the bomb calorimetry method.
Fibre is subtracted from total carbs since it does not contribute metabolites for energy generation. It then employs predetermined energy densities of 4 kcal/g for useable carbohydrates and protein and 9 kcal/g for lipids. Instead of directly tracking heat output through combustion, the totals are added together to generate an overall Calorie estimate based on chemical composition.
An energy bar containing 10 g protein, 20 g total carbs with 5 g fibre, and 9 g fat, for example, would be calculated as (10 g protein x 4 kcal/g) + (15 g useable carbohydrates x 4 kcal/g) + (9 g fat x 9 kcal/g) = 160 kcals.
The calorie densities for macronutrients used in the Atwater method, as previously stated, are derived from bomb calorimetry tests. However this method takes into account as well the variables that influence energy extraction across food types. Cooking and chewing, for example, aid in the breakdown of compounds, allowing for more complete digestion and, as a result, increased calorie delivery versus raw foods with comparable macronutrient profiles.
So the method provides approximations
based on realistic human consumption rather
than pure chemistry.
Alternative Calorie Testing Methods
While bomb calorimetry and the Atwater system dominate calorie calculation for packaged goods, some alternative methods exist that provide more direct measurements. For example, digestibility corrected metabolizable energy testing quantifies heat output from excreta in animals fed specific foods. This captures energy lost through digestion instead of pure chemistry.
Isoperibol calorimeters serve as scaled-down bomb calorimeters, burning small samples instead of complete combustion. They provide more precise direct readings but remain impractical for large-scale testing requirements.
In summary, bomb calorimetry provides the most direct way to measure energy content in foods by literally burning items and tracking temperature rises. This initially helped establish averages across nutrients for energy release during digestion. For packaged products, the legally required Atwater system serves as the approved modern technique, calculating calories based on chemical makeup rather than direct testing. This saves resources while still promoting informed consumer choices through macronutrient-based energy approximation.
Thus, bomb calorimetry supports Atwater averages and the Atwater system suffices for informative food labeling. More involved testing often costs excessive time and resources without significantly improving accuracy.
- National Data Lab web site at http://www.nal.usda.gov/fnic/foodcomp/
- Nutrition Analysis Tool at http://www.nat.uiuc.edu