Author: Michael Bernstein, MS, IFNCP
Date: April 2025
Understanding the body’s energy systems is essential for planning nutrition to help optimize athletic performance. ATP (adenosine triphosphate) is basically the energy currency that powers muscle contraction and all exercise. It is primarily produced by mitochondria which is an organelle in most cells in the human body. There are three energy systems which includes the phosphagen system, the anaerobic system, and the aerobic system. These energy systems produce ATP at different rates. The phosphagen energy system is an immediate energy system and uses cells ATP stores as well as creatine phosphate (CP). The phosphagen system is also known as the ATP-PC system, where the PC stands for phosphocreatine which basically refers to creatine phosphate. The popular ergogenic supplement creatine monohydrate increases creatine phosphate (CP) which enhances athletic performance by suppling energy to the ATP-PC system. The phosphagen/ATP-PC system can only provide energy for 5-15 seconds, and after this the body relies on the anaerobic and aerobic systems. The anaerobic system can’t produce ATP as fast as the phosphagen/ATP-PC system, however, it can produce ATP faster than the aerobic system. It can also produce a slightly greater amount of ATP than the aerobic system. The aerobic system can produce unlimited ATP, but it’s much slower compared to the anaerobic and phosphagen/ATP-PC systems. Exercise that involves faster and higher intensity movements requires a greater demand for ATP. If the energy systems can’t provide sufficient ATP, then fatigue sets in. The exercise intensity and duration determines which energy systems are used and in which proportions. For example, 10 seconds of all-out activity uses mainly the phosphagen/ATP-PC system. Examples include short, intense weightlifting and short sprints. Long sprints, higher repetition weightlifting, or 1-3 minutes of all-out activity uses mostly the anaerobic system but also the phosphagen/ATP-PC system. 30 minutes or hours of steady-state aerobic activity, such as low to moderate intensity distance running, cycling, etc., relies mainly on the aerobic energy system. Highly trained endurance runners have increased numbers and sizes of mitochondria in their muscles which produce ATP more efficiently, enhancing their aerobic capacity. The aerobic energy system is also associated with slow-twitch muscle fibers found in high amounts in endurance runners whereas the anaerobic energy system is associated with fast-twitch muscle fibers found in high amounts in weightlifters and sprinters (Fink & Mikesky, 2021; Plotkin et al., 2021).
Another important point is that the anaerobic system uses anaerobic glycolysis and the aerobic system uses aerobic glycolysis, both of which use carbohydrates to produce energy. Anaerobic means without oxygen and anaerobic glycolysis does not use oxygen to produce ATP whereas aerobic glycolysis does use oxygen to produce ATP. The end product of anaerobic glycolysis is pyruvate which is converted to lactate. The aerobic energy system involves multiple metabolic pathways. First is aerobic glycolysis, in which the end product is pyruvate, but instead of being converted to lactate, it is converted to acetyl CoA, which is used in the citric acid cycle (aka the krebs cycle). In the citric acid cycle, without getting too detailed, a series of reactions take place, and the end products enter the last aerobic pathway known as the electron transport chain (ETC). The ETC produces most of the ATP. Both the citric acid cycle and the ETC take place within the mitochondria of cells. It should be mentioned that fats can be used for energy, however, they use a process called beta-oxidation, and then enter the citric acid cycle and then the ETC. Proteins are broken down by a process called deamination. They are not usually used for energy, but if they are, they can enter the citric acid cycle and the ETC (Fink & Mikesky, 2021).
References:
Fink, H. H., & Mikesky, A. E. (2021). Practical Applications in Sports Nutrition (6th ed.). Jones & Bartlett Learning.
Plotkin, D. L., Roberts, M. D., Haun, C. T., & Schoenfeld, B. J. (2021). Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives. Sports (Basel, Switzerland), 9(9), 127. https://doi.org/10.3390/sports9090127
* The content provided by this article is for informational purposes only. These statements are not intended to diagnose, treat, cure, or prevent any disease. This article is not intended to provide personal medical advice, which should be obtained from a medical professional.
