In the fitness industry, we often talk about burning calories, which can be important for individuals who are exercising specifically to lose weight or for those who need to quantify how hard they’re working with exact metrics. This begs the question: What is a calorie and how does it relate to your personal fitness goals?

Simply put, a calorie is a measure of unit of energy; specifically, it is the amount of energy necessary to increase the temperature of 1 liter of water by 1 degree centigrade. According to the first law of thermodynamics (also known as the law of conservation of energy), energy is neither created nor destroyed, but is merely transferred from one form to another. This means that when you eat food that contains 100 calories, you will do one of two things with it: You will either expend the energy through activity (technically called kinetic energy) or save it for use at another time (referred to as potential energy).

When it comes to managing a healthy body weight or achieving specific weight-loss goals, it is important to monitor both the number of calories coming in through dietary intake and the amount of calories being burned through physical activity. In addition, it is necessary to understand how exercise physiologists measure the body’s metabolism and classify different categories of energy expenditure.

Here are eight things to know about how the caloric cost of physical activity:

• Energy is consumed through the macronutrients of fats, carbohydrates and proteins. Protein is primarily used to repair damaged tissue or build new tissue, but it can also provide fuel when necessary. Carbohydrates and proteins provide about 4 calories of energy per gram, while fat provides 9 calories per gram.
• To be used by the muscles to fuel contractions, fats and carbohydrates (and sometimes proteins) are converted to adenosine triphosphate (ATP), which is the chemical form of energy in the human body. Muscles store a minimal amount of ATP; once it is used, more ATP is produced through either aerobic or anaerobic metabolism. Glycolysis is the breakdown of carbohydrates to ATP, and lipolysis is the process of converting fats (lipids) to ATP. Gluconeogenesis is the process for how the hormone cortisol converts protein into energy; the downside to this system is that it means less protein is available for repairing the tissues damaged during exercise.
• Total daily energy expenditure (TDEE) is the amount of energy the body burns on a daily basis and is influenced by three specific components:
  • The thermic effect of food (TEF) is the energy used to break down macronutrients for digestion, absorption and removal, and can consume up to 10% of TDEE. (Note: High-fiber foods require more energy for metabolism, but this is not a significant source of energy expenditure for weight loss.)
  • The thermic effect of physical activity (TEPA) is the amount of energy burned during all physical activity, which includes everything from standing up from a seated position to the most intense high-intensity interval training workout (and everything in between). TEPA can range anywhere from 15 to 30% of TDEE.
  • Resting metabolic rate (RMR) is the minimal amount of energy required to support the body’s physiological functions. RMR is approximately 60-75% of TDEE and is influenced by an individual’s amount of lean muscle mass, age, gender and climate (extreme environments can increase energy consumption). During rest, various organs are responsible for consuming energy to support their functions. Here is a breakdown of the percentages of RMR used by different parts of the body:
    • Liver: 27% 
    • Brain: 19%
    • Skeletal Muscle: 18%
    • Kidneys: 10%
    • Heart: 7%
    • Other organs: 19%

• TEPA includes both exercise, which is a specific, planned physical activity, and non-exercise activity thermogenesis (NEAT), which includes the energy cost of normal daily functions like walking the dog, cleaning the house or walking up stairs. Increasing the amount of NEAT can help burn up to an additional 300 calories per day, which is about the equivalent of a 3-mile run.
• Aerobic metabolism requires oxygen and uses either fats or carbohydrates to produce energy during low-intensity activities, while anaerobic metabolism converts carbohydrates to ATP when energy is needed more quickly. During anaerobic glycolysis, one molecule of glycogen (how carbohydrate is stored in muscle or transported in blood) can provide two to three molecules of ATP. Aerobic glycolysis can yield up to 39 molecules of ATP from one molecule of glycogen. When energy is produced during aerobic lipolysis, one molecule of FFA yields approximately 129 molecules of ATP. At the start of physical activity, during extremely high-intensity activities that last a brief period of time, or during the transition from low to higher intensities, energy is supplied by ATP stored in muscle cells. This stored ATP can provide up to 20 seconds or so of energy.
• Using fat for energy requires the mobilization of stored fats so they can be broken down into free fatty acids (FFAs). Fats are stored in adipose tissue, muscle and organs as triglycerides. Except in the cases of those with extreme overweight or obesity, the average person has the potential to store up to 12,000 grams of fat, which is the equivalent of 108,000 calories of potential energy. (Note: At 3,500 calories per pound of fat, that is about 31 pounds of energy.)
• Lipoprotein lipase (LPL) is an enzyme located on the walls of blood vessels, in adipose tissue and the liver, and functions to move FFAs in muscle cells to be used for energy or to adipose cells where they are stored for use at a later time. Those interested in exercise for the purpose of burning fat should be aware that remaining in a seated, sedentary position for an extended period of time can reduce the amount of LPL in the body.
• You can estimate your RMR using the Mifflin-St. Joer equation, below. Consuming more calories than needed can lead to weight gain, while consuming fewer calories should, theoretically, result in weight loss:

How to estimate RMR in women:

(9.99 x bodyweight (in kilograms)) + (6.25 x height (in centimeters)) - (4.92 x age) - 161

How to estimate RMR in men:

(9.99 x bodyweight (in kilograms) + (6.25 x height (in centimeters)) - (4.92 x age) + 5

To calculate your TDEE, multiply your estimated RMR by one of the following activity factors:

Case Study:

36-year-old male: 5’ 9” (175 cm), 200 pounds (90.9 kg)

RMR = (9.99 x 90.9) + (6.25 x 175) - (4.92 x 36) + 5 = 908 + 1094 - 177 + 5 = 1,830

Sedentary: 1,830 calories

Low activity: 2,050 calories

Active: 2324 calories

Very active: 2,654 calories

If he is sedentary, he requires fewer than 2,000 calories a day; if he eats more than that he will gain weight.

If this individual can increase his activity to the Active level, which means he is burning 2,300 calories per day, but maintains his intake at 2,000 calories per day, he will create a caloric deficit that should lead to weight loss. 

If you don’t like doing a lot of math, there is good news: ACE has created a calculator to help you estimate your TDEE. Simply type in your info to determine the amount of calories you need to support your activity level. If weight loss is your objective, you will need to reduce your caloric intake to a level below that indicated by the calculator.

Having a better understanding of how to measure energy expenditure, along with knowing the different components of how the body uses the energy you consume through your diet, can help you determine the best activities, both exercise and non-exercise, to help you reach your personal fitness goals.