CHAPTER 6: Basics of Human Metabolism

In the complex tapestry of the human body, energy plays a pivotal role in every aspect of our lives. From the slightest flicker of a finger to the powerful movements of a professional athlete, energy is the driving force behind it all. But how does the body obtain this vital energy, and where does it come from? Throughout this chapter, we will unravel the mysteries of how our bodies harness the energy we need to thrive, which ultimately stems from the food we digest, absorb, and utilize within our cells. Once nutrients are digested and absorbed, the liver acts as a gatekeeper, screening and distributing them into general circulation throughout the body. These nutrients then arrive at their target cells, where they can follow various paths, including:
  • oxidation for energy production;
  • transformation and incorporation into structural components of the body; 
  • storage for future energy provision; and/or 
  • excretion and elimination from the body.
Our primary focus in this chapter will be on energy production. Contrary to what you might think, our bodies do not truly produce energy. Following the laws of thermodynamics, which dictate the fundamental rules governing the physical world, energy is neither created nor destroyed but rather transferred between entities. The human body adheres to these principles, converting energy stored within our food. The energy released when breaking bonds between carbon and hydrogen molecules in our food fuels the processes necessary to form ATP (adenosine triphosphate). We will use the term “energy transfer” to describe the process of converting carbohydrates, proteins, and fats into ATP. Our bodies follow a cyclical pattern: we eat, digest, absorb, circulate, store, transfer energy, utilize energy, and repeat. We will delve deeper into this process, particularly emphasizing ATP, which is essential for nearly every action in the body. From muscle movement to enzyme production and even the formation of more ATP, a constant supply of ATP is crucial. Although it might seem intuitive that our bodies prefer “new” nutrients for energy, the stored nutrients provide the majority of our daily needs. Most of the energy we require comes from nutrients we consumed and stored in the past, except for longer-duration exercise. During such activities, muscle cells tap into both stored and recently absorbed nutrients to fuel the ongoing activity. Our primary nutrient stores are triglycerides (found in fat and muscle cells) and glycogen (found in muscle and liver cells). These nutrient types provide energy to various tissues and cells that lack the capacity to store energy-producing nutrients, such as the brain and red blood cells. When our cells need energy for ATP production, stored glycogen and triglycerides are activated and broken down into glucose, fatty acids, and amino acids. These nutrients then participate in the energy transfer process or are distributed into the general circulation to help peripheral cells generate ATP. This highly coordinated process is vital for our survival, especially during low food intake. As we explore the fascinating world of energy transfer within the human body, we will better understand how to optimize our fitness and health. So, let’s embark on this enlightening journey together and uncover the secrets behind the intricate dance of energy in our bodies.

Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.

E. Châtelet
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