Physiology Basics: What is Metabolism?

You often hear the phrase “boosting your metabolism” in reference to weight loss, or feeling energized. We hear about people with “fast metabolism” as a reference to their ability to seemingly eat what they want and remain thin. I used to ask my general health students: Under these contextual uses, what does the word “metabolism” mean?

Most students reported that metabolism was a science term that defined our ability to be skinny, and/or burn fat.

What if I told you that metabolism means a lot of things in the body, including our ability to develop and even build up fat cells? What if metabolism also meant our ability to break down protein into amino acids? Or what if metabolism referenced our body’s ability to take glucose from the carbs we eat and build those glucose molecules into a bundled up polymer called glycogen?

“Nutrients from ingested foods are provided in the form of carbohydrates, fats, and proteins. These three basic fuels, or energy substrates, can ultimately be broken down to release the stored energy. Each cell contains chemical pathways that convert these substrates to energy that can then be used by the cell and other cells of the body, a process called bioenergetics. All of the chemical reactions in the body are collectively called metabolism (Kenney, Wilmore, & Costill, 2020, p. 54).”

Metabolism is an umbrella term that includes when your body breaks down substrates (i.e., catabolism), and also when your body builds up substrates (i.e., anabolism). For example, you can have a glucose molecule, which can be catabolized into two acetyl-CoA molecules from the one glucose, or it can be anabolized along with many other glucose molecules into a bundled storage form of glucose known as glycogen. Similarly, a fatty acid molecule can be catabolized to many acetyl-CoA molecules, or anabolized toward the growth of adipose (i.e., stored body fat.).

Understanding the grander definition for metabolism will help you to make sense of many sports nutrition principles, the processes of energy production and your body’s relationship with Calories and your ability to produce work.


Kenney, W. L., Wilmore, J. H., & Costill, D. L. (2020). Physiology of sport and exercise (7th ed.). Champaign, IL: Human Kinetics.