The aerobic energy system has three substrates with which it can derive energy (ATP); carbohydrates, fats, and protein. The two pathways we are primarily concerned with regarding exercise are carbohydrate (aerobic glycolysis) and fats (aerobic lipolysis). These two pathways are very economic in their ability to generate ATP, with glycolysis netting 30-32 molecules of ATP per molecule of glucose and lipolysis generating a whopping 106 molecules of ATP per molecule of fatty acid!
It is the ability of the body to bring in and process oxygen that determines the efficacy of the aerobic system. If we can readily bring in lots of oxygen and deliver that oxygen to the working muscle, then we will have a powerful aerobic system. However, if oxygen intake or delivery is insufficient then high-intensity exercise will engage the anaerobic systems to a greater degree. These anaerobic systems create byproducts that can accumulate in the body and cause fatigue and discomfort.
Aerobic glycolysis synthesizes ATP through the breakdown of carbohydrates in the form of glucose or glycogen. This happens primarily through the Krebs cycle, a multistep process that uses pyruvic acid generated during glycolysis to synthesize ATP. Glycolysis leaves us with carbon dioxide which is diffused into the blood then exhaled and hydrogen ions which are used in the electron transport chain in the creation of even more ATP.
Aerobic lipolysis utilizes the most energy-dense source of fuel in our body, fat. Fat is first mobilized from it’s stored form triglyceride and released into the blood as fatty acids. Fatty acids are made up of hydrogen and carbon atoms bound together. It is the hydrogen in these fatty acids that mitochondria (the powerhouse of the cell) use to synthesize ATP via the Electron Transport Chain (ETC). Through a series of steps, hydrogen atoms are stripped from carbon and shuttled to the ETC.
When comparing fat vs. carbohydrate as fuel sources, fat is much more efficient at re-phosphorylating ADP into ATP but it comes at a greater oxygen cost. Thus when exercising at a higher intensity carbohydrate is the preferred source of fuel. When the intensity and oxygen demand is lower, fat is a preferred source of fuel.
The real value in the aerobic system is how it works to support the entire organism. The aerobic system is working all the time at some level or another. And a powerful aerobic system not only reduces our reliance on the anaerobic system but improves the function of the anaerobic system. We already talked about how the aerobic system uses some of the byproducts of anaerobic metabolism to synthesize ATP but a well developed aerobic system will actually prolong the time it takes for those byproducts to start accumulating.
Another way the aerobic system assists anaerobic systems is in the phosphorylation of PCr. PCr or phosphocreatine is the fastest acting, most powerful of our energy pathways. Once PCr is exhausted it uses excess ATP generated by the aerobic systems to re-phosphorylate creatine into PCr. For CrossFitters, this means having that little burst of energy for a heavy clean or muscle up even when you are exhausted. For athletes, this means having the ability to make one big tackle or burst of speed when it’s needed. Without well developed aerobic systems, power athletes like football players wouldn’t have the energy they need to explode over and over again throughout a game.
The aerobic system is massively important for athletes of all levels. It helps us recover from intense bouts of exercise and generates excessive amounts of energy for us to utilize at all intensities. During maximal efforts, the aerobic system is a primary contributor to energy metabolism after only 60 seconds! This just goes to show the importance of a well developed aerobic system.
