By Michael Ormsbee, PhD, Florida State University

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

How many calories do you burn each day? Professor Ormsbee explains the science of calorimetry, which is much more precise than looking up your activities on a calorie chart.

Calculating Energy Expenditure

The energy balance equation consists of two parts. One concerns “energy in,” or how many calories you consume. But how do you measure the “energy out,” or the energy expenditure portion?

You may be aware of the main things you do each day that require you to use energy and burn calories, but do you know how to actually measure or quantify how much energy you expend in a typical day? Luckily, we have ways to scientifically measure the energy that we expend.

Direct calorimetry is the most accurate way to measure your energy expenditure, but it is very expensive and highly impractical. This method is usually reserved for people who volunteer as research participants in large-scale clinical exercise and nutrition research studies. 

Direct Calorimetry

To give foods a calorie count, scientists burn them in a bomb calorimeter and measure how much heat is given off. For the sake of clarity, when it comes to measuring energy expenditure, you don’t actually go into a bomb calorimeter and ignite. It is similar in many ways, though.

For direct calorimetry, you are housed in a well-insulated chamber, called a metabolic chamber, that is surrounded by a known amount of water for a set amount of time. Let’s choose 24 hours for this example. Once you enter the chamber, you would not leave it for the entire 24 hours. 

As you know, you are always producing heat to some extent. The heat that you give off in the metabolic chamber—from your basic metabolic requirements—will dissipate into the room and heat up the environment of the chamber you are in.

Think about a time when you’ve been in a small room with lots of people. The room gets warm quickly, especially if there is no airflow. 

The change in the temperature of the insulated chamber will ultimately change the temperature of the known volume of water that surrounds that chamber. Then, the change in temperature of the water is used to calculate energy expenditure or the calories that are burned. However, due to the impracticality and expensiveness of direct calorimetry, indirect calorimetry is more often used in lab settings.

Indirect Calorimetry

Indirect calorimetry measures within ± 1% of the direct method and does not require a special chamber. Indirect calorimetry measures your breath—specifically the oxygen that you use and the carbon dioxide that you exhale.

How does this work? During metabolism, oxygen is used and carbon dioxide is produced. Oxygen is needed to produce ATP during rest and during slow-to-moderate intensity exercise. 

At the same time, as you break down stored fuels in your body for energy, you produce a lot of carbon dioxide. Carbohydrates, fats, and protein all have different amounts of oxygen that is needed to fully break them down for energy, meaning that different amounts of carbon dioxide and oxygen in your breath can indicate the type of fuel that you are burning.

On average, about five calories are burned for every one liter of oxygen that you consume. Therefore, putting this all together, we actually know how many calories you are burning and from which macronutrient. This is done through the respiratory exchange ratio (RER), which is calculated by dividing the amount of carbon dioxide produced or exhaled by the amount of oxygen that you consume or inhale. 

Variations in RER

RER values range between 0.7 and 1.0. According to Professor Ormsbee, even though things like hyperventilation or excessive acid buffering in the cells can skew this ratio a bit, it does give an excellent estimation of fuel use.

For example, if you are burning mostly carbohydrates, your RER would be 1.0—top of the range. An RER of 1.0 is equivalent to about five calories per liter of oxygen that you consume. This would occur while exercising very hard. 

If you are burning mostly fat, your RER would be lower at 0.7, which is equivalent to about 4.7 calories per liter oxygen consumed. This would occur while resting and while lying around.

However, most people tend to eat a mix of fuels from carbohydrates, proteins, and fats. As a result, people don’t usually burn just carbohydrates or just fats. The RER for this scenario would be right in the middle of 0.7 and 1.0, with a value of about 0.85 or 4.86 calories per liter of oxygen indicating that you have a mixed use of fuels. 

Because protein is not used nearly as much as carbohydrates and fats for energy production, it is not pinpointed on the RER scale. We can also calculate exactly how much fat or carbohydrate you oxidize using standard scientific equations. We’ll explore the implications for your health of measuring energy expenditure in tomorrow’s article.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.

Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.