NASM CPT 7th Edition Chapter 8: Exercise Metabolism and Bioenergetics
NASM CPT 7th Edition Chapter 8: Exercise Metabolism and Bioenergetics 1

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Chapter Goals:

  • Give a summary of the first law of thermodynamics and its governing of energy use.
  • Be able to describe the energy systems used in the human body.
  • Find how the macronutrients are used in the body for energy.
  • Be able to integrate the concepts of balancing energy about managing body composition.
  • Find how one efficiently fuels the body based on the intensity of activities.

Introduction to Exercise Metabolism and Bioenergetics

The body requires a constant supply of energy for proper functioning. The energy requirements are often changing, and exercise demands the body to supply energy.

All the food items we eat will contain carbs, fats, and proteins, which our cells need to make energy and properly work. 

The energy within these food sources is used through many chemical reactions to get it to the needed form of ATP, or adenosine triphosphate. ATP is the main form of energy used in the body.

Bioenergetics and Metabolism

The metabolism of energy, or bioenergetics, studies how energy is transformed in many different biological reactions.

Energy is needed for sustaining life, supporting exercise, and promoting recovery from physical activities and structured programmed exercise. 

Metabolism is defined as the chemical reactions occurring inside the body that allows the body to maintain life.

Exercise metabolism is how these energy and chemical reactions work to make physiological changes based on the demands placed. 

The first law of thermodynamics states that energy may not be created or destroyed, just recycled or converted from one form to another. For us, this is the concept of energy balance. 

The balance of energy is something that will determine weight changes, whether it be losing or gaining weight. 

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Fuel for Energy Metabolism

Dietary food will supply our body with energy, but it does not directly do this on its own. It needs to be broken down into smaller by-products which are known as substrates.

These proteins, carbs, and fats are the main substrates, but they and come in different forms. 

The energy being used in the body is a complicated process, like when we use fats stored in the body, which can come from fats from the diet or excess carbs. All of these come with their own differences in the reactions needed.

Glucose is the main energy source we use and prefer, especially regarding brain functioning and higher intensity activities. It is possible to make it from other components, but a large majority of daily glucose needs to come from carbohydrate based foods. 

Glucose is stored in a branched structure in the body for easy use as glycogen. 

Glucose makes a very little contribution during low intensity and restful situations. 

Fats are the main sources of energy during these low intensity times. During activity, when things are ramping up in intensity, we see a shift in starting to use glucose for energy mainly.

Fat always requires oxygen to be used for energy, whereas carb sources do not require the use of oxygen. 

The fats we use for energy will be free fatty acids, also known to us as lipids. 

Triglycerides are the form in which these fats will exist in your food, but in the body, when we are going to use them, they will be the free fatty acid forms. 

The third main source of energy is protein, and it is going to be in the amino acid form that we refer to them in the chapter. There are a total of 20 amino acids that assemble the body’s proteins. 9 of these are known as essential.

Proteins are not the preferred energy source for the body and are used as more of a last resort for us. They are really only utilized when there is a dire need, and the carbs and fats are mostly used. 

Using these amino acids requires the breakdown of the valuable muscle in the body. This is often seen when in a negative energy balance. 

Ketone bodies are used to refer to the molecules of acetone, acetoacetic acid, and beta-hydroxybutyric acid. These can be made similarly to glucose and are a by-product of the use of fatty acids or the conversion of ketogenic amino acids. 

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Ketone levels will increase in the body in these ways:

  • By restricting overall calories to very low levels
  • By following very low-carbohydrate diets
  • By consuming exogenous ketones
  • When there is a lack of insulin produced (type 1 diabetes) or substantial insulin resistance (type 2 diabetes)

Energy and Mechanical Work

To do mechanical work, we must fuel our bodies, which will then go through the processes to provide energy.

Around 40 percent of the energy released during metabolism will be utilized for cellular work. An example of cellular work could be creating more molecules. 

Energy Systems

ATP is the main molecule in the human body that provides us with energy. It comprises one nitrogenous base, a sugar molecule, and three phosphate groups.

When the chemical bonds are broken, this is what releases energy for us to function.

The three main pathways for metabolism are the ATP-PC system, the glycolytic system, and the oxidative system.

Depending on the intensity we are working at, these systems will shift in their priority: supplying the most ATP for the body. It is not a linear fashion that the systems turn on and off, but instead, like dimming switches where one will be the primary and the others still are somewhat active.

The first 10 – 15 seconds of activity has the ATP-PC system providing the majority of the energy because it is the quickest and for the power moves. Another name for this is the phosphagen system or the phosphocreatine system.

This phosphocreatine system does not need oxygen to work and instead relies on creatine that we store in the muscles.

We use the Phosphocreatine system during high-intensity, short term work, mainly for strength and power exercises.

Glycolysis is the other anaerobic system; this means that it does not require the use of oxygen. This system breaks down glucose to make energy.

The glycolytic system is used as the primary system for 30 – 60 seconds of activity, with the possibility to extend this by some training over time.

The oxidative system is the system that uses oxygen to make energy for us, and it also uses fats in the body or through the diet to make energy. It is present for activities that are endurance.

Energy During Exercise

The intensity and duration of the activity can define any exercise form. And these two things are inversely related. The intensity is lower if the duration is longer, and vice versa.

Carbs will provide the most energy for making ATP at higher intensities than fats. Lower intensities will have fats being the main source of energy.

Estimating Fuel Contribution During Activity

Even if the intensity is low, if the activity goes on for long enough, it is possible that carbs have to start taking over more and more of the work with glycolysis. 

These changes in fuel source change the buildup of carbon dioxide, which then needs to be exhaled, and this causes the body to increase the respiration rates.

The Fat-Burning zone is a myth; it depends on many more variables than just training at a particular intensity.

Daily Energy Needs

When the daily food intake matches the energy requirements, people are thought to be in their energy balance, which keeps the person’s weight constant.

It is important to consider this balance and shift the food intake to meet the person’s goals. 

Total Daily Expenditure

This is seen as the total number of calories someone spends a day. 

1 calorie is the energy required to raise the temperature of 1 kg of water by one degree Celsius.

The number of calories expended each day will vary greatly.

The resting metabolic rate is the number of calories that we use to function at rest. This represents about 60 – 75% of the total daily energy expenditure.

The thermic effect of food is the number of calories that we need to break down the components of a meal. This represents about 10% of the total daily expenditure.

The last part of the total daily expenditure is activity thermogenesis, representing 15 – 30% of the total daily expenditure. This is simply the calories burned during the day due to simple activity or exercise.

NASM CPT 7th Edition Chapter 8: Exercise Metabolism and Bioenergetics 2
NASM CPT 7th Edition Chapter 8: Exercise Metabolism and Bioenergetics 3
NASM CPT 7th Edition Chapter 8: Exercise Metabolism and Bioenergetics 4

Tyler Read

Tyler Read, BSc, CPT. Tyler holds a B.S. in Kinesiology from Sonoma State University and is a certified personal trainer (CPT) with NASM (National Academy of sports medicine), and has over 15 years of experience working as a personal trainer. He is a published author of running start, and a frequent contributing author on Healthline and Eat this, not that.

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