During prolonged exercise, the main challenge for the body is to maintain energy production for several hours. To do this, metabolism continuously adapts by changing the energy sources used throughout the effort.
These adaptations involve the use of carbohydrates, fats and, to a lesser extent, proteins, along with hormonal and physiological changes.
Initial use of carbohydrates as the main energy source
At the beginning of exercise, the body primarily uses carbohydrates as its main source of energy. These carbohydrates come from muscle and liver glycogen, which are stored energy reserves in the body. Muscle glycogen is used directly by the working muscles, while liver glycogen helps maintain stable blood glucose levels. Carbohydrates are the most efficient source of energy, especially at moderate to high intensities.
Reduction of glycogen stores
Because these reserves are limited, their reduction over time leads the body to rely on other energy sources. After approximately 60 to 90 minutes of continuous exercise, glycogen levels begin to decrease more significantly, especially when there is no carbohydrate intake. This reduction is directly associated with decreased performance and increased fatigue.
Increase in fat oxidation
As glycogen levels decline, the body increases the use of fat as an energy source. Fat is an abundant source of energy, but its rate of energy release is slower compared to carbohydrates. As a result, when the body relies more on fat, the athlete may have more difficulty sustaining higher intensities.
Protein contribution in very prolonged exercise
During very long exercise sessions, especially when carbohydrate intake is insufficient, the body may begin to use proteins as a source of energy. This occurs through the breakdown of amino acids. Although this contribution is small, it can negatively affect muscle integrity and recovery.
Hormonal changes
Prolonged exercise also leads to important hormonal changes. Insulin levels decrease, reducing energy storage, while hormones such as glucagon and cortisol increase, stimulating the release of stored energy. These adaptations allow the body to continue supplying energy throughout the exercise.
Maintenance of blood glucose
The body constantly works to maintain blood glucose at adequate levels. During very long exercise without carbohydrate intake, blood glucose levels may drop. This affects not only physical performance but also cognitive function, impairing decision making, focus and perception of effort.
Impact of dehydration and electrolytes
Fluid and electrolyte losses during exercise also influence metabolism. Dehydration reduces the body’s ability to transport nutrients and dissipate heat, while sodium loss can impair fluid balance and muscle function. These factors contribute to performance decline over time.
Central and peripheral fatigue
Fatigue during prolonged exercise is multifactorial. Peripheral fatigue is related to reduced energy availability in the muscles, while central fatigue involves the central nervous system and is associated, among other factors, with carbohydrate availability. Both directly impact the athlete’s ability to sustain effort.
Conclusion
Over many hours of exercise, metabolism continuously adapts to maintain energy production. The body initially relies more on carbohydrates, but as these reserves decrease, it increases fat utilization and, in more extreme situations, may rely on proteins.
These changes are associated with decreased performance, increased fatigue and a greater difficulty in sustaining higher intensities.
For this reason, nutrition during exercise is not just supportive, but essential to maintain energy availability, preserve performance and delay fatigue. Adequate intake of carbohydrates, fluids and electrolytes helps sustain metabolism over time and supports performance for longer.
Referência:
Schierbauer J, et al. (2025). Effect of low to moderate exogenous carbohydrate supplementation on time to exhaustion during constant load intense cycling in healthy individuals. Nutrients.
Disponível em: https://pmc.ncbi.nlm.nih.gov/articles/PMC12166815/
