NASM CNC Chapter 5: Energy Balance and Metabolism
NASM CNC Chapter 5: Energy Balance and Metabolism 5

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Chapter Goals:

  • Explain thermodynamics.
  • Define what an energy balance is.
  • Know the factors of energy out and energy in. 
  • Be able to explain the energy systems 


The metabolism is the chemical process occurring in living organisms that gives us energy. Metabolism is a key feature distinguishing life from non-life. Energy production by way of metabolism will give us a constant supply of energy needed for sustaining human life at rest, in physical activity, and also in recovery times.

The sun is the provider of the main source of energy needed for life on earth. The solar energy it gives off will go to plants and be used as chemical energy in the process called photosynthesis. The plant will then make it into animals, where we will use it.

ATP is the compound of energy that we use. It drives muscle contractions, transmits nerve impulses, and the majority of the other amazing chemical reactions that occur in the human body. 

TDEE stands for the total daily expenditure, and this is a calculation of the estimated calorie needs for us. This number of calories someone needs in a day will depend upon the person’s age, their thermic effect of feeding, resting metabolic rate, thermogenesis not from exercise, sex, height, weight, and their level of physical activity. The base calculation we always use is simply 2,000 calories per day. This is what the majority of nutrition labels will be based upon.


Thermodynamics describes the relationship there is between energy and heat and their conversion from one to another. This energy is in the form of mechanical, electrical, or chemical. 

Human metabolism is the conversion of food into heat, work, and stored energy. Calories are consumed Into the body, then broken down into smaller pieces, and then they are stored or used for movement. 

The first law of thermodynamics is the law of the conservation of energy. This law simply states that energy can be transferred from one form to another, but it can never actually be destroyed. 2,500 calories consumed will always represent 2,500 calories and be used in some way by the body.

Maintaining Energy Balance

Energy balance is self-explanatory. It is the balance of the calories that we consume minus the calories we burn. 

Energy In represents the energy intake within a certain amount of time.

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Energy Out represents the energy we expend during a certain time. 

The rise in obesity and being overweight is the result of an energy balance that is favoring a surplus, thus people are storing more calories in their body. They are not expending enough to keep the balance.

Components of Energy Out

These are the calories we expend. We have four main components of this:

  • Resting metabolic rate: 60 – 75% 
  • Thermic effect of feeding: 10%
  • Thermic effect of activity thermogenesis: 15 – 30%
  • Non-exercising activity thermogenesis
  • Resting Metabolic Rate

This refers to the total amount of calories that are burned when the body is at a complete rest. Some people may refer to this as the basal metabolic rate. The basic functions all included here are breathing, circulation, organ function, and also basic neurological function. Everyone has a different RMR and some may be really high and some really low. The amount of lean body mass, thyroid production levels, and some other slight factors will affect this. 

Thermic Effect of Food

This is basically the increase in the expenditure of energy after someone takes in a meal. This accounts for the energy needed for digestion of is about 10% of our daily expenditure.

Thermic Effect of Activity

This is the most variable part of the expenditure as it depends upon what you do throughout the day. It can account for between 15 – 30$ of expenditure, but this can vary greatly. It includes physical work, muscular activity, and planned exercise events. 

Non-Exercise Activity Thermogenesis

This is seen as all of the things we do besides sleeping, exercising, or eating. Walking to and from events is a big typical part for people. It can also vary greatly depending on someone’s job. 


Sleep deprivation and the restriction of metabolism and start of weight gain that accompanies is not very well understood yet. The thought is that when you do not have enough sleep, your RMR the next day is reduced and you intake more energy than normal. An easy recipe for weight gain. It is important for us to have a regular sleep schedule that achieves more than 5 hours of sleep. 


Hormones regulate our metabolism through many different effects. We will discuss the main ones here.

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Thyroid Hormone

Thyroid hormones work to regulate our metabolism and control our essential growth and development steps.

The thyroid hormone works to stimulate our metabolism through increases in consumption of oxygen and the rate of breaking down ATP. 

The thyroid hormone works to stimulate both lipogenesis and lipolysis. 

It also works for stimulating all aspects of carb digestion.


This is typically called the stress hormone, as they are produced in the adrenal glands as a response to stress. It leads us to increase our heart rate, blood pressure, and elevated glucose levels. 


This hormone is a key hormone for metabolism of all three macronutrients. It has a major role in the fat composition and the muscle mass for our bodies. It helps to control many pathways in glycolysis, glycogen synthesis, and lipid or cholesterol metabolism.

Components of Energy In

This is the number of calories we simply take into the body


We have a few different factors influencing our appetite:

  • Our activity level
  • Some social factors
  • Leptin and ghrelin production
  • The food currently in the stomach
  • Environmental Factors

This includes factors like cultural and psychological influencers, economic status, your level of education, the past experiences you have had, your usual portion size, and the palatability of food.

Any disturbance in these systems may lead to a negative effect on energy balance, and most often this is in the form of gaining weight. 

Psychological Factors

Distress of the psychological type will alter eating behaviors slightly. 

Here are some ways that people can practice mindful eating. These can help in times of stress and really any time.

  • Eat when you actually are hungry, not just bored.
  • Do not let your emotions control your eating habits.
  • Engage all six of your senses.
  • Make sure to eat in moderate portions.
  • Savor small bites of food while you eat.
  • Eat slower than normal.
  • Chew your food very thoroughly.
  • Do not skip any meals throughout the day.
  • Individual Variation
  • The factors that cause these many variations are:
  • Body composition
  • Low calorie diets
  • Age
  • Brown Fat
  • Food allergies
  • Processing of food
  • Caffeine
  • Hypothyroidism
  • Caffeine
  • Lifestyle habits

Food processing

This is defined as anything that alters the food from its original form, like drying, freezing, milling, canning, mixing, adding salt, sugar, fat, or any other additives. The processing of food changes the chemical and the physical nature of the food we take in.

Processing food may make it safer, tastier, or more stable, but it is not without consequences. 

Metabolic Energy Systems

The body needs energy to fuel itself and to function in all of the physical activity we do. The energy is utilized in the body by different systems that come into action at various times and we should understand these systems.

Energy Transfer

The carbs, protein, and fats get broken down to the simplest form in digestion. These macronutrients are used for biosynthesis, detoxification, thermogenesis, ion transport, and muscle contraction.

We have three main energy systems we use:

ATP – PC System

This is the first pathway that the body uses when starting physical activity of any kind. We also refer to this as the phosphagen system. This pathway uses energy released from the breakdown of phosphocreatine for immediate use and then it makes use of the phosphate molecule for making more ATP. It is held in the muscles in a pretty limited supply. The ATP – Pc system is our quickest way to resynthesize ATP, but it is limited to seconds. This system fatigues very fast compared to the other systems. The system that takes over next is the glycogen system. 

Anaerobic (Glycolytic) Energy System

This system provides energy for medium to high intensity burst of activity. This is usually from 10 seconds to around two minutes of time. glycolysis literally means the breakdown of glucose. The glycogen that is use is the glycogen found in the blood or the stored glycogen in the muscles. So, ATP is formed rapidly and without the need for oxygen.

End-product of Glycolysis

The end product is pyruvate. This is either transported and used for supporting more energy production through the Krebs cycle, or it is converted into lactic acid. 

Aerobic Energy System

This is the energy system that is used much more than the other two. This is because it is used for low intensity or simply endurance activity. This lasts from two hours to some hours. This system requires oxygen, unlike the other systems, and it takes a lot more time to overload. We keep a very large amount of energy for this system in reserves when compared to the things used in the other two systems. Some sports that make use of the system are things such as swimming, running, and cycling. The main fuel being used here would be the fats and carbs. As the body reaches max effort, the main macronutrient being used is the carbohydrates. 

The Krebs Cycle and Electron Transport Chain

The aerobic system includes both the electron transport chain and the Krebs cycle. Both of these use the blood glucose, glycogen stores, and plasma-free fatty acids or triglycerides in order to resynthesize ATP using the mitochondria. 

NASM CNC Chapter 5: Energy Balance and Metabolism 6
NASM CNC Chapter 5: Energy Balance and Metabolism 7
NASM CNC Chapter 5: Energy Balance and Metabolism 8

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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