ISSA Chapter 6: Energy and Metabolism 5

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Chapter Goals:

  • Know the organelles of the human cell.
  • Find and explain the main energy systems in the human body.
  • Be able to discuss how the energy systems overlap.
  • Discuss metabolism and energy balance and the factors that affect each.

The human body requires a constant energy supply to function and move.

Energy starts by coming from the sun and, through many processes, makes its way to humans and animals through macronutrients in the form of plants and animals as food.

Metabolism is the chemical processes that happen in the body which support life, like converting food to energy.

Bioenergetics is the study of how energy is transformed in living organisms.


Cells will perform all functions of life. Anything from specialized functions, converting nutrients into energy and creating structures for the body.

Cells all have the same basic components. They have a membrane encompassing multiple organelles and genetic material.

Plasma Membrane

This membrane surrounds all organelles and the cytoplasm of the cell. There are two layers made of lipids and proteins.

It may also be known as the phospholipid bilayer, which is one glycerol, two fatty acids, and a phosphate group.

The membrane will be semipermeable; thus, some molecules can pass through it.


The nucleus has its own membrane and it holds the DNA.

DNA in the form of chromosomes will have the genetic blueprint for every unique cell in the body.

The cytoplasm is the viscous fluid inside the plasma membrane. It is also the site of cell reactions like gluconeogenesis, synthesis of fatty acids, activation of amino acids, and glycolysis.


These are small, spherical organelles made of protein and RNA.

They can be free-floating or connected to the endoplasmic reticulum organelle.

These small organelles are involved in protein synthesis and polypeptides.

Endoplasmic Reticulum

The ER is an organelle that forms a network of canals in the cytoplasm and it is continuous with the nuclear membrane.

There is a smooth endoplasmic reticulum, which has no ribosomes attached, and the rough endoplasmic reticulum, which does have ribosomes.

Golgi Apparatus

This organelle is located near the nucleus and endoplasmic reticulum. It creates vesicles, or transport bubbles, which will move proteins from inside the cell to the cell membrane to be released into the body.


These organelles help the digestive system of the cell.

There are around 50 different enzymes that break down materials the cell has absorbed.

These cells can digest and destroy elements within the cell that are no longer needed.


This is nicknamed the “powerhouse of the cell,” as this is essentially playing a major role in creating energy.

These small and complex organelles also have their own DNA.

About 40 percent of the space in cardiac muscle’s cytoplasm comprises mitochondria. In the liver, it is closer to 25 percent.

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Food as Energy

The foods that humans take in need to be broken down to be used at the cellular level for energy production.

We have three types of macronutrients required by the body. These are carbohydrates, proteins, and fats.

Macronutrients are called such as they are needed in large amounts throughout the day.

Carbohydrates are one of the main energy sources for energy within the body and are broken down into glucose or stored as glycogen in the liver and muscles for future use.

When carbs are not used or stored as glycogen, they will get stored as fat in the body.

Protein is not a primary substrate for energy metabolism unless the body is starving or when other macronutrients are insufficient to support energy demands.

Protein plays the most important role in the body. It is responsible for the following:

  • Anabolism, or the growth and maintenance of tissues.
  • Protein enzymes aid in biochemical processes.
  • Protein hormones relay nervous system messages.
  • Proteins help build connective tissues like tendons, ligaments, and cartilage.
  • Protein helps keep blood pH via hemoglobin.
  • Albumin and Globulin support the balance of fluids.
  • Antibodies are created to fight infection.
  • Transportation and storage of nutrients are made possible by proteins.

Fat meets the need for most energy required during rest and is sufficient in this energy supply process.

The respiratory quotient is a method to find the fuel mix being used for an activity. It is a way to measure how many fats, carbs, and proteins are being burned for activity.

The Respiratory quotient equals the volume of CO2 exhaled / volume of O2 inhaled.

Indirect calorimetry is a way to measure the energy expenditure by oxygen consumed divided by carbon dioxide produced.

Adenosine Triphosphate

Macronutrients are not used directly as a source of energy for the body. Instead, they need to be transferred through processes to reach the form of ATP.

ATP is the energy-carrying molecule used to fuel all body processes.

ATP is made up of adenine and three phosphate groups. The bond between the phosphate groups stores energy which is released when the bond is broken.

During exercise, ATP is used in the muscle cells to generate muscular contraction. ATP works with myosin in the sarcomere to contract and release the filaments.

Adenosine triphosphate loses a phosphate molecule and becomes adenosine diphosphate, which is essential to the flow of energy in living cells.

There are three distinct energy pathways through which the body can go through. They involve different chemical reactions to supply their energy.

Anaerobic Energy Production

The body stores only a limited amount of ATP within the muscle cells, which is available for immediate energy needs.

Stored ATP lasts for about 10 seconds of work only. Power events like swinging a golf club, throwing, or jumping all use these energy stores.

Once the energy is utilized, the cell creates more for immediate use. This process is anaerobic since it does not require oxygen.

ATP / CP Energy Pathway

Creatine Phosphate is also called phosphocreatine and is stored in the muscle cells.

CPT is readily available for use when the 10 seconds’ worth of stored ATP is gone.

CP helps recycle and add the phosphate group back to the new ADP that formed from the storage of 10 seconds’ worth of energy. It is a very fast system but also does not last long.

Anaerobic Glycolysis

For activities that go from 10 – 120 seconds, the body will need to enter this second pathway to produce lasting energy.

Anaerobic glycolysis uses one molecule of ATP to convert glucose to glucose phosphate. Glycogen may also be used here.

There is a by-product of this energy system, which is called lactic acid. It is used in the body in three ways: making ATP, making glucose in the liver, and as a signaling molecule.

Lactate can be used for energy by the mitochondria.

The anaerobic threshold is the point where the body switches from using oxygen to mainly anaerobic metabolism. This is where we see lactate start to build up in the body.

Aerobic Energy Production

When the cells exhaust the immediate energy of the ATP stores and glucose is mostly depleted, the aerobic energy system will kick in.

This is after 2 minutes of time. It includes aerobic glycolysis, fatty acid oxidation, and gluconeogenesis as a last resort.

The oxidative energy pathway primarily uses fat and carbs to produce the required energy.

Oxidative Energy Pathway

It isn’t important for fitness professionals to know every step of aerobic metabolism, but it is important to understand the general steps and outcomes of energy production.

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Aerobic metabolism produces large amounts of energy but requires glycolysis, the Krebs cycle, and the electron transport chain.

Aerobic energy production is more efficient, but it takes significantly more time to do.

Glycolysis is literally translated to the breakdown of glucose, and it may occur aerobically or anaerobically.

Beta-oxidation is the process by which triglycerides are broken down into fatty acids.


Amino acids and proteins are the last resort for the body. The body would prefer to keep as much protein as possible, as it is important.

Gluconeogenesis is limited by the availability of the enzymes needed to break down proteins.

Low blood sugar levels will stimulate the production of glucagon, which stimulates these enzymes, and the breakdown of protein.

The Energy System Overlap

All of the energy systems are interconnected and always working in the body, to some degree.

What matters is the intensity and duration of the activity, which will dictate the energy system that dominates energy production at any moment.

In lower intensity exercise, aerobic metabolism supplies the body with energy as the primary source.

During higher intensity exercise, the body relies on anaerobic and aerobic energy systems, with ratios changed.

People who are in peak physical condition may use more fatty acids as the primary substrates, but their bodies may use the protein more often via gluconeogenesis.

If someone were to do exercise for 60 seconds and then take a 2-minute rest before the next set, let’s look at what would happen energy-wise.

During the 60 seconds of work, the body will mostly rely on anaerobic energy systems, which are most prominent from 1 – 2 minutes.

During the 2 minute rest period, the energy demand is much lower, and the oxidative energy system will dominate at this time.

Reading this text on your computer will primarily have you use your oxidative energy system, as the demand is very low.

Steady-state exercise is an exercise that stays at a steady exertion level from start to finish. There is an aerobic plateau reached.

When exercise ceases, oxygen consumption remains high for a while before returning back to the resting metabolic state. This is called excess post-exercise oxygen consumption.

Metabolism and Energy Balance

Metabolism is a detailed chemical process in which aerobic and anaerobic metabolism occurs in the body’s cells.

The body requires a set amount of energy to survive and engage in physical activity.

Energy comes from the external source of food taken in the diet.

Food breakdown, and nutrient breakdown, result in calories being taken in.

A calorie is the amount of energy needed to raise the temperature of 1 kg of water by 1 degree celsius at a pressure of 1 atmosphere.

Carbohydrates yield 4 calories per gram, proteins yield 4 calories per gram, and fats yield 9 calories per gram.

Energy balance is the state achieved when energy intake is equal to energy expenditure. This can be positive or negative, also.

A positive energy balance is when there is more energy consumed than expended.

A negative energy balance is when there is more energy expended than consumed.

Total daily energy expenditure is the accumulated calorie burn made up of resting metabolic rate, thermic effect of food, physical activity, and physical growth.

The resting metabolic rate is the energy expenditure of metabolic and physical processes when the body is resting.

The thermic effect of food is the energy expenditure associated with food digestion and absorption.

The resting metabolic rate is the rate of expending energy when at rest. It accounts for around 70 percent of the body’s total daily expenditure.

RMR will be directly related to body size and sex. Determining the exact number for this is nearly impossible, but formulas can get somewhat close for accurate guesswork.

Caloric Expenditure

The Harris-Benedict equation is what is used for total daily calorie expenditure (DCE).

The DCE is the total number of calories someone spends, including resting metabolic rate, activity level factor, and thermic effect of food.

The activity level factor is a multiplier that reflects the level of activity.

Thermic Effect of Food

The TEF is the energy associated with food breakdown and absorption.

This will make up around 10 percent of the total daily expenditure. It can also be referred to as diet-induced thermogenesis.

Overall, macronutrient composition will also affect the TEF.

Physical Activity

Physical activity is second only to the resting metabolic rate in terms of contributions to total energy expenditure.

Physical activity will make up around 20% of TDEE, but this can be changed significantly based on exercise.

Physical activity can be broken up into two categories: exercise activity thermogenesis and non-exercise activity thermogenesis.

Exercise activity thermogenesis, or EAT, involves planned, structured, and repetitive movement with the goal being to improve or maintain physical fitness.

EAT will be around 5 percent of the TDEE on average, with some variance based on how much exercise is done.

Then we have non-exercise activity thermogenesis, which is also called NEAT.

NEAT involves all physical activity and movement besides what is done during exercise. The more someone moves and needs to do small things throughout the day, the more energy they expend.

NEAT accounts for around 15 percent of the TDEE.


The body is growing, changing, and repairing. We have millions of cells dying every day, and millions of cells are created to replace them.

Especially in the case of youth, their bodies are growing rapidly and even more, cells are created every day.

The energy costs of creating cells can be important for a personal trainer to consider, especially when working with youth or pregnant clients.

Lifestyle and Metabolism

People’s lifestyle includes their dietary patterns, activities, opinions, and behaviors.

Eating patterns with an excess of certain macronutrients will cause the body to use that nutrient over others in energy production.

Weight Management

Weight management involves your physiological processes and techniques to achieve or maintain a specific body weight.

Fitness and nutrition programs vary depending on the needs and goals of the clients.

Energy balance is complex and involves factors beyond simply food and exercise.

Body Types

Body types are also called somatotypes. These help to classify someone’s body structure.

An ectomorph is a long, lean body with little body fat and muscle mass. The shoulders and hips are narrow.

An endomorph is a body type with lots of body fat and muscle mass with large upper arms and thighs.

A mesomorph is an athletic, muscular build with broad shoulders and a healthy body weight.

For training, ectomorphs have difficulty gaining weight, endomorphs have an easy time gaining weight, and mesomorphs can gain or lose weight easily if they decide.

Ectomorphs will likely need a higher carb and protein intake to keep their body weight and muscle mass.

Endomorphs will need a diet lower in carbs and higher in protein to prevent excess fat storage and support muscle mass.

Mesomorphs need a more balanced diet that is focused on daily calorie expenditure for energy balance.

ISSA Chapter 6: Energy and Metabolism 6
ISSA Chapter 6: Energy and Metabolism 7
ISSA Chapter 6: Energy and Metabolism 8

Tyler Read - Certified Personal Trainer with PTPioneer

Tyler Read

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