What is a Calorie?

Have you ever wondered - what is the value of counting and burning calories?

For many of us, counting Calories can feel a bit abstract: Low calorie foods are often a health claim on food labels, and eating too many calories results in weight gain, so at face value calories are really bad, right? Isn't that why we need to go to the gym so we can burn them away?

A Calorie is a Unit of Energy.

Also known as a kilocalorie (kcal), a Calorie is defined as the amount of heat energy needed to raise 1 kilogram (1 liter) of water by 1 degree Celsius

(McArdle, Katch, & Katch, 2015). If you take a piece of food and put it in a closed container, then surround that closed container with another container full of water, you can light that food on fire and measure the temperature change of the surrounding water. If 1 liter of water surrounds that food and upon ignition, it raises the temperature of that water by 20 degrees Celsius, then that piece of food contained 20 kcals. This is one method used to actually measure the calorific value of foods, and the device described is called a bomb calorimeter. Thus, a set weight of food can be measured to identify the amount of heat energy, or calories, that it contains.

The average heat of combustion for dietary fat is 9.4 kcals per gram The average heat of combustion for carbohydrates is 4.2 kcals per gram The average heat of combustion for protein is actually 5.65 kcals per gram

(McArdle, Katch, & Katch, 2015)

Now if you're familiar with counting calories you might recognize that protein's caloric value sounds a little off. Proteins get broken down to amino acids which have a nitrogen group bound to them. The body cannot metabolize the nitrogen component of an amino acid, which accounts for 19% of protein's potential energy. The body gets rid of the nitrogen through other means, but its metabolic and usable energy is reduced to approximately 4.6 kcals per gram (McArdle, Katch, & Katch, 2015).

Putting it into Practice

What happens to food after we consume it can feel abstract; it's doing something, but it's easy to feel confused about what's precisely going on. There's a fear that some foods are stored more easily as fat, and that reducing your Caloric intake will resolve that. Many people apply the "more is better" approach to this strategy, and dangerously under-eat the amount of Calories they require. They aspire to be healthy, but miss the mark and actually step into a habit that is dangerously destructive for their health. Your body requires energy to function, and Calories are a unit of a energy! You have baseline Caloric needs to keep your body functioning, and the more physically active you are, the more Calories you require on top of that baseline figure.

There are many formulas out there to help you calculate your target Caloric range. The Harris-Benedict formula is common, and applied frequently for sedentary populations. The Cunningham formulas is less common, and applied more appropriately on physically active populations.

For help with applying these formulas, you can check out our online course Complete Lifestyle with RD Haley Hughes. And for specific nutrition guidance beyond my exercise science scope of practice, check out Haley's website: https://rdrxnutrition.com/


McArdle, B., Katch, F., & Katch, V. (2015). Exercise physiology: Nutrition, energy, and human performance (8th ed.). Baltimore: Lippincott Williams & Wilkins.