Fuel Source and Glycolysis

Aerobic respiration is the primary energy system used during low-moderate intensity, long-duration exercise (Cleveland Clinic, 2023). Its main fuel sources are carbohydrates, fats, and proteins ( to a lesser degree). Carbohydrates are the most efficient source because they can be metabolised rapidly and require less oxygen per ATP produced compared to fats (BioNinja, 2024). This is why endurance athletes will prioritise carbohydrate availability before competition. 

The first stage of aerobic respiration is glycolysis, which occurs in the cytoplasm of body cells. During glycolysis, one molecule of glucose is broken down into two molecules of pyruvate, producing a net gain of 2 ATP and 2 NADH (Bailey, 2019). Although glycolysis does not require oxygen, its end product (pyruvate) must enter the mitochondria to continue aerobic energy production

During exercise, the rate of glycolysis increases in response to ATP demand. If oxygen supply is sufficient, pyruvate is transported into the mitochondria. If oxygen is limited, it is converted into lactate, linking glycolysis directly to anaerobic metabolism (Burns, 2023). This can be harmful for athletes as the lactate produces a build up of hydrogen ions, decreasing the pH levels which restricts muscle movement, and increases the rate of fatigue.