Calories & Calorimetry

People discuss calories all the time – it is a common thing to discuss when talking about diets, nutrition, waist line, performance, health, among many other areas of life; however, do we truly understand what calories are? How are calories measured in food? How are they measured in our metabolism? These questions, and more, will be discussed here. In this article, we will cover what calories are, how they are measured, and how the scientific definition applies to the “real world”.

What is a calorie?

A calorie, according to most dictionaries, is a unit of energy defined by its ability to heat a gram of water by 1 degree Celsius [1]. That is a relatively useless definition when trying to apply that to physiology, however. For us to understand its significance we need to understand how heat relates to physiology – if we do not understand that connection, the definition offered is a nebulous definition we simply accept. Heat is used to determine energy in physiology, because the greater amount of heat, the greater the energy.

A perfect example of this is seen in human metabolism. Our metabolism allows us to maintain our body temperature and it does so by increasing or decreasing its energy use (for example, think of a fever – our body heat increases and our body’s use of substrate to create energy also increases, because they are cause and effect)[4]. So, a calorie is not a physical substance, but simply a unit of energy measurement that can be quantified using heat measurement.

Knowing that every part of the body needs energy, and that energy, in its simplest form, is adenosine tri-phosphate (ATP); we know that the metabolic pathways that create ATP release heat with every reaction necessary to fulfill each pathway [4]. So, foods contain the substrates that the body will eventually break down to release energy (ATP) and heat. In a manner of speaking, foods contain untapped energy that will be released and visualized (and felt) via heat production (calories).


What is Calorimetry?

Now, if we think of our definition for calorie as a unit of energy that is expressed by the amount of heat it generates, we can better understand calorimetry. Calorimetry is the measure of heat, expressed as calories [3]. Calorimetry is the way we are able to figure how many calories are in a particular food item, but also tells us how many calories our body (metabolism) uses on a day to day basis [2][5].

Types of Calorimtery?

As we know, calorimetry can be applied to food, as well as to our metabolism do determine calories. All of these methods are similar, and some are even identical with scale being the only difference between them. We will break these down so we fully understand how we are able to establish calories in both scenarios, and to do this, we need to learn the further distinction between direct and indirect calorimetry.

Direct Calorimetry

Direct calorimetry was the first way to determine the calorie amount of a particular food and/or metabolic caloric expenditure of a mammal [6]. It is still used to determine the calories in food items, and is understood as the “gold standard” of calorimetry, because it allows for little error [2][5]. This method of calorimetry was discovered by a French chemist by the name of Antoine Lavoisier in the 18th century. He noticed something that we have previously discussed – we emit heat. Using this knowledge, he created some of the first direct calorimeters, and these calorimeters are still used today.

The direct calorimeters he built were built so as to allow an animal to be placed in the middle of an insulated “room” or “box”. The room was surrounded by ice, and he was able to measure the amount of ice that melted over a period of time and relate that back to the amount of heat given off by the animal [6]. Water equaled heat used, and heat used equaled calories. This method is considered direct, because the subject has a direct impact, via heat production, on the exact measurement of energy being produced.

Later calorimeters use this idea to quantify human metabolism by sticking a person inside of an insulated room, having them stay still, and then measuring the heat displacement from the person, inside the insulated room, to the water surrounding the room (other versions exist, as well) [6]. If all other variables are controlled for, like consumption of food, activity, and even oxygen levels, this reading shown in the increase in water temperature is accurately and reliably related to heat given off by the person within the chamber [6].

Above is an illustration of direct calorimetry with a person inside a highly controlled room. However, keep in mind that this illustration is wrong in depicting a person on a treadmill as the treadmill would skew results.

The same concept is applied to food, using a bomb calorimeter, as food is placed in a metal, insulated box that is submerged in water [2]. The food has a controlled amount of heat applied to it to make it burn, and as the food is burned, chemical bonds are broken (since all food is made of chemicals), releasing energy and heat, and that is then absorbed in the walls of the box, transferred to the water directly outside of the box, and then measured by thermometer to assess the amount of heat (and via heat, energy) released by said food [2]. Then, adjusting calculations for heat added to the food to make it burn, we receive information about how much heat that food produced.

So, keeping this in mind, we can reapply the scientific definition of a calorie; a calorie is the amount of heat necessary to increase the temperature of one gram of water by 1 degree Celsius – this definition is directly applicable to direct calorimetry as we measure the heat of water outside of each chamber to determine heat produced by the subject therein.


Indirect Calorimetry

That said, there is another method of calorimetry, in indirect calorimetry, and it is the preferred version of calorimetry, because it is easier to assess (no need for a chamber), less costly, and allows for the assessment of physical activity [6]. Direct calorimetry is held back in that it is a nuisance to set up, expensive, and above all, it cannot be used to assess physical activity caloric expenditure, because sweating traps heat, exercise equipment creates heat via movement, an uneven distribution of heat release dependent on the exercise, among other reasons [6]. So, how does indirect calorimetry work?

Indirect calorimetry is possible by measuring the exchange of oxygen and carbon dioxide from the mouth and nose, also known as VO2 exchange [6]. This is informative, because the amount of oxygen and carbon dioxide exchange, using the respiratory exchange ratio, allows us to determine the calories used at a particular intensity. Respiratory exchange ratio (RER) is simply a ratio of oxygen intake to carbon dioxide use, which increases as exercise intensity increases since the body needs more oxygen and will, of course, expel more carbon dioxide, as a result – for more information on RER, click here. So, because we have quantified the amount of calories used with a given VO2 exchange, we can then extrapolate and find out how many calories a person uses at a given time or time period.

This is, however, not flawless as the RER is useful in predicting metabolic VO2 (oxygen and carbon dioxide) exchange, but does not accurately predict non-metabolic carbon dioxide (CO2) production from the bicarbonate system [6].



So, by now we should understand that a calorie is not a physical thing, but a unit of energy quantifiable by the expression and measure of heat. This heat then allows us to measure the caloric expenditure and amount of mammalian (human) metabolism and food via direct calorimetry. Calorimetry, being the measure of calories via the measure of heat production, allows us to quantify caloric amount. Finally, although direct calorimetry is the gold standard of calorimetry, it is indirect calorimetry, using VO2 and the respiratory exchange ratio, this is most often used to determine caloric expenditure in human metabolism; bomb calorimetry (direct calorimetry) is still used to determine caloric content of food.

Writer: Nicolas Verhoeven


[1] The Definition of a Calorie! (2001). Retrieved from Princeton Plasma Physics Lab website:

[2] Chapter 48: The Digestive System. (2016). In Biology (8th ed.). Retrieved from

[3] Nave, R. (n.d.). Calorimetry. Retrieved from

[4] Czura, A. W. (n.d.). Energy Production In A Cell [PDF]. Retrieved from

[5] Kaiyala, K. J. (2014). What does indirect calorimetry really tell us? Molecular Metabolism,3(4), 340-341. Retrieved from

[6] Brooks. (n.d.). Basics of Metabolism [PDF]. Retrieved from file:///C:/Users/Nicolas%20Verhoeven/Downloads/brooks_ch04.pdf

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