What is Metabolism? And Why You Should Care.

By Ryan Juel, Certified Personal Trainer

People are constantly talking about boosting their metabolism. What foods should be eaten and when for a higher metabolism; how you should drink water first thing in the morning to jumpstart your metabolism. The word gets thrown around constantly, but what are we actually talking about? What is the process that happens in our bodies? That’s exactly what I’m going to explore today.

An Overview

In short, metabolism is the chemical process inside a living organism in order to maintain life. Energy is essential in maintaining all process within the body. These processes include:

  • Chemical
  • Biological
  • Mechanical
  • Temperature regulation.

The breakdown of nutrients yields a high-energy compound called adenosine triphosphate (ATP) that provides energy to the body and therefore life is maintained. However, metabolism isn’t just about breaking down nutrients. The process of metabolism is divided into two categories, catabolism and anabolism. Catabolism is the breakdown of molecules to obtain energy while anabolism is the synthesis of compounds needed by the cells. Because our main goal is to understand how the nutrients we eat provide energy to our bodies, specifically to do mechanical work, I’ll focus on catabolism. However, be aware that there is another side to metabolism. Catabolism can be broken down into three general categories:

  • Fermentation
  • Aerobic (with oxygen)
  • Anaerobic (without oxygen).

Again, I’ll narrow this down and for our purposes, aerobic and anaerobic metabolism will be the focus.

In the process of preforming mechanical work, energy demands increase substantially. There is only enough ATP stored in muscles to provide approximately 4 seconds of a strength or power exercise such as a golf swing. Therefore, ATP must be readily synthesized to maintain the energy requirements of exercise. This energy production relies on the respiratory and cardiovascular systems, which provide oxygen and nutrients (from foods we consume) as well as remove waste products to maintain a cellular equilibrium. ATP releases energy when the enzyme ATPase catalyzes the hydrolysis of ATP to ADP (adenosine diphosphate), phosphate, and energy (produced in the breaking of the phosphate bond). This breaking of the phosphate bond helps facilitate skeletal muscle contraction (I’ll talk about the physiological steps involved in muscle contraction in the future). Now I’ll break it down a little bit further into the three main energy pathways.

Anaerobic Processes (without oxygen)

There are two anaerobic metabolic processes the phosphagen pathway and glycolysis. In the phophagen pathway, creatine phosphate (CP), which is stored in skeletal muscle, transfers its phosphate to ADP to create ATP with the assistance of a creatine kinase enzyme. Because this reaction is only one step it happens rapidly but it is short lived, as there is only a finite amount of CP stored in cells. Creatine most be rephosphorylated using ATP in the mitochondria of the cell before more energy can be provided to the muscle. Depending on the level of CP depletion in the muscle it can take 1-2 minutes for creatine to be rephosphorylated. The CP that is stored in the muscle provides approximately 5-10 seconds of sustained power exercise such as sprinting.

Glycolysis provides approximately 1.5 minutes of endurance exercises like a short dash or swimming a lap. Glycolysis is the breakdown of carbohydrate molecules (glycogen or glucose) and produces ATP rapidly. This process requires a series of steps catalyzed by enzymes just as the other processes described. Glycolysis produces ATP as well as pyruvate or lactate. Although glycolysis is an anaerobic process if there is sufficient oxygen quantities in the cell pyruvate can participate in the aerobic production of ATP as well. Lactate can also be resynthesized for ATP production however it accumulates in the muscle faster than it can be removed during exercise, which contributes to fatigue. As oxygen becomes available during rest lactate is oxidized and used for ATP production.

Aerobic Processes (with oxygen)

Aerobic production of ATP involves two very complex metabolic processes, the Krebs cycle and the electron transport chain (ETC). This process occurs within the mitochondria of the cell. This process can use fat, protein, and carbohydrates to produce ATP. I won’t go into the details of this complex process but it is important to know that aerobic metabolism requires steady concentrations of oxygen and allows the muscle to sustain high rates of ATP production for muscular energy over long periods of sustained endurance exercises. For aerobic exercise, carbohydrates and fat are the main sources of fuel although protein can be used as well. Usually, carbohydrates are used first and as the duration of the exercise increases there is a gradual shift to the use of fats as fuel.

Why you should care…

For these processes to work properly and allow us to do our best in any task, we have to provide our bodies with the right nutrients. If the body is not fed properly to support the energy demands of mechanical work our speed, agility, strength, balance, or any other aspect of desired performance will suffer.

So how do we fuel our bodies in order to meet our physical goals? What effects do proteins, carbohydrates, and fats have on our bodies in order to help us preform better? I’ll delve into these specifics over the next few weeks. 

A (Very) Quick Review

  • Food is metabolized to ATP.
  • ATP goes through hydrolysis to break the phosphate bond and produce energy.
  • The energy is used to contract muscles.
  • ATP must be continually synthesized to provide energy to our muscles during work, this is accomplished either anaerobically or aerobically.


Metabolism. 2015. In Merriam-Webster.com.

Retrieved March 20, 2015, from http://www.merriam-webster.com/dictionary/metabolism

Acsm’s Resources for the Personal Trainer, 4th Ed. Wolters Kluwer Health, 2014. Print.