Many people lack a clear understanding of calories. A calorie isn’t a tangible substance but a unit measuring energy. It represents the energy needed to raise the temperature of one milliliter (or one gram) of water by one degree Celsius (specifically, from 14.5 to 15.5 degrees C). The term “calorie” was introduced by French chemist Antoine Lavoisier to describe the body’s internal heat.
A food calorie is actually a kilocalorie, signifying the energy required to raise the temperature of one liter of water by one degree Celsius. Let’s explore how the energy in food is measured.
Measuring Calories: From Calorimeters to the Atwater System
Originally, a calorimeter measured the calorie content of food. A dehydrated food sample was placed in a container submerged in a known quantity of water. The container was sealed, oxygen was introduced, and the food was ignited. By measuring the water’s temperature increase, scientists calculated the food’s calorie content.
This method had drawbacks. Foods contain components like fiber, which combust in a calorimeter but aren’t absorbed into the bloodstream, thus not contributing calories. Today, food producers use the “Atwater indirect system” to calculate calories.
The Atwater System: Calculating Calories from Macronutrients
The Atwater system determines calories by summing the calories provided by energy-containing nutrients: protein, carbohydrate, fat, and alcohol. Because some carbohydrates contain indigestible fiber, this fiber is subtracted from the total carbohydrate content before calorie calculation.
Alt text: Atwater factors for calculating food energy content from protein, fat, carbohydrates, and alcohol.
The Atwater system utilizes average values: 4 Kcal/g for protein, 4 Kcal/g for carbohydrate, 9 Kcal/g for fat, and 7 Kcal/g for alcohol. These values were determined by burning these substances in a calorimeter. (Note that simple sugars provide slightly less, and polysaccharides slightly more, than 4 Kcal/g, leading to some rounding).
Example: Calculating Calories in a KitKat
Consider a 45-gram KitKat containing 3g of protein, 29g of carbohydrate (22g of which are simple sugars), and 12g of fat. Using the Atwater system:
- Protein: 3g x 4 Kcal/g = 12 Kcal
- Carbohydrate: 29g x 4 Kcal/g = 116 Kcal
- Fat: 12g x 9 Kcal/g = 108 Kcal
Total Calories: 12 + 116 + 108 = 236 Kcal. The label would read approximately 230 Calories (rounding).
Energy Density: Donuts vs. Dynamite
Calorie calculations reveal interesting data. A doughnut (approximately 450 Kcal) has roughly the same calorie content as a stick of dynamite. The key difference is that dynamite releases energy instantly upon ignition, while a doughnut releases energy in the body more gradually. So, a doughnut won’t make you explode, at least not literally. This example underscores the concept of energy density and how the rate of energy release differs significantly between various substances.
Conclusion: Understanding Food Energy Measurement
Understanding how energy in food is measured is crucial for making informed dietary choices. While the original method involved direct calorimetry, the Atwater system offers a more practical approach by calculating calories based on the macronutrient composition of foods. This method allows for more accurate estimations of the energy the body can actually derive from food, excluding indigestible components like fiber. It’s important to consider not only the calorie count but also the source of those calories and how quickly the body can utilize them.
