By Michael Ormsbee, Ph.D., Florida State University

Edited by Kate Findley, The Great Courses Daily

In the oxidative system, unlike your other two energy-producing systems, you burn energy through light to moderate activity—even just sitting. However, you still need to exercise to get the most bang for your buck in this system, as Michael Ormsbee, Ph.D., explains.

The Oxidative System

The third energy system, the oxidative system, relies on carbohydrates, fats, and, in some cases, protein to provide adenosine triphosphate (ATP). The oxidative system is your long and slow system, which kicks in after about 90 seconds to 2 minutes of activity and can last almost indefinitely, as long as the intensity of activity is low to moderate. It’s aerobic, unlike the other two energy systems, so it uses oxygen.

With sufficient oxygen—from slowing down to breathing a little during vigorous exercise— pyruvate will get transferred into acetyl-CoA (a molecule that participates in many biochemical reactions) rather than forming lactate in the glycolytic energy system. It is through the aerobic transformation of acetyl-CoA that glucose and fat continue to be metabolized with this third energy system.

Here, the acetyl-CoA now enters what is called the Krebs cycle. In the Krebs cycle, and further down the metabolic pathway, you begin to produce much more ATP than was possible in the first two energy systems. 

This production of ATP is completed in the electron transport chain where oxygen is present. Because oxygen must be present, this means that the system runs when you are working at a low to moderate level of activity, or simply sitting still. For example, if you are walking, you’ll be using the aerobic system to produce energy.

Making More Energy

The way the electron transport chain works is by shuttling electrons down the electron transport chain to create a concentration gradient that ultimately produces ATP using an enzyme called ATP synthase. If glucose is being used in this third system to make ATP, you can make almost 40 molecules of ATP. 

Compare this to the two to three molecules of ATP made in the glycolytic system. Here’s another intriguing aspect of metabolism: When fat is used in the oxidative system, you really make a lot of energy.

Fat burning or fat oxidation requires more oxygen per ATP produced compared to energy production from glucose or glycogen, meaning, once again, you need to be resting or working out at a slow to moderate intensity to utilize fat as a fuel. However, fat provides 12 times more ATP than glucose, just at a slower rate. 

Now it is easy to see why fat is thought of as an almost endless source of energy. Fat provides ample ATP, and even very lean people have lots of stored fat on their bodies.

Three Fuels, One Goal

The oxidative system is active for most of your day—while sitting at a computer, walking around, or even just watching TV. Again, carbohydrates and fats are the primary fuel sources used to provide ATP in the oxidative system, but this system can also metabolize some protein for energy production. 

This does not typically happen, though, unless someone has been exercising for a very long time—say, longer than two to three hours—or for someone who has not eaten in a long time. Keep in mind that you never only use fat or only use carbohydrates. 

The reality is that the fuel used to provide ATP in these energy systems is typically a combination of fuels. For example, while you’re at rest and just sitting around, about two-thirds of the ATP you make is from fat and one-third will be from carbohydrates.

Why Exercise Matters

The more fit you are, the better you are at using fat as a fuel and at saving your stored glucose until you really need it, like when you pick up the intensity of your exercise.

As you exercise, you can increase the number of mitochondria that you have in the cells of the muscles at work. This is called mitochondrial density, and you want it to be high. 

The mitochondria are the organelles within your cells that help you produce large amounts of ATP. The more you exercise, the better you are at making energy.

Another benefit is that you can increase the number of capillaries or small blood vessels you have to bring blood, oxygen, and other nutrients to working muscles. This is called capillary density, which you also want to be high. 

What’s more is that, with exercise training, you actually increase the activity of enzymes called lipases that increase fat breakdown. Exercise also increases the hormones that are used to help initiate this fat breakdown.

By regularly exercising, you increase the ability to use fuels effectively by your body. You decrease your reliance on carbohydrates and use more fat as a fuel, regardless of what you’re doing all day long. 

This means that at rest and during exercise, you are burning a greater amount of fat, and over time this will no doubt impact your body composition.

Making the Most of Fat

The fat used to make ATP can come from stored fat tissue, like around your waist and hips, but it can also come from dietary fat that you’ve eaten. You even have fat stored in your muscles to use for energy, and this type of fat is called intramuscular triglycerides or IMTG.

Interestingly, you’ll find IMTG in both very fit and out-of-shape people, but how these stored depots of fat are utilized is extremely different between the fit and the unfit. For the out-of-shape people, these intramuscular fat stores can actually do harm and alter proper cell signaling that should occur in response to various hormones.

However, in physically active people, the intramuscular fats tend to be used as an energy source, sparing glycogen and increasing fat use for activity.

So, while both fat and carbohydrates will be used in the oxidative system, the amount of fat used compared to how much carbohydrate is used is highly dependent on a number of key factors. These factors include things like your last meal, the last time you exercised, your level of fitness. and what hormones are circulating in your blood.

This article was edited by Kate Findley, Writer for The Great Courses Daily.