During exercise, the body does not rely on a single energy source. Carbohydrates and fats are used simultaneously, but their relative contributions change depending on exercise intensity, duration, substrate availability, and training status.
During short, high-intensity sessions, the body relies more heavily on carbohydrates, particularly muscle glycogen and available glucose. As exercise continues, other energy sources become increasingly important, changing the dynamics of energy metabolism.
At the Beginning of Exercise, Carbohydrates Play an Important Role
When exercise begins, the demand for ATP increases rapidly. Carbohydrates can provide energy at a high rate, which is why their contribution tends to be greater when exercise intensity is high.
Muscle glycogen is one of the main carbohydrate sources used during exercise. Circulating glucose also contributes, while fat is used simultaneously, especially at lower intensities.
This is why duration and intensity need to be considered together. A 30-minute high-intensity workout may require a much greater contribution from carbohydrates than a longer session performed at a low intensity.
As Exercise Continues, Metabolism Changes
During prolonged exercise, substrate availability becomes increasingly important.
As the effort continues, glycogen stores are progressively used. At the same time, the contribution of fatty acids released from adipose tissue to energy production increases. This shift helps the body preserve some of its carbohydrate stores, which are more limited.
This does not mean that the body switches from carbohydrates to fat at a specific point. Both fuels continue to be used. What changes is their relative contribution.
During prolonged exercise at moderate intensity, for example, fat oxidation tends to increase over time, while the relative contribution of carbohydrates may decrease.
Training Also Changes This Response
Exercise duration is not the only determining factor. The body of an athlete trained for endurance can adapt the way it uses energy sources.
Endurance training increases the capacity to use fat as an energy source during exercise, which may reduce the relative dependence on carbohydrates at certain intensities. This adaptation helps preserve glycogen during prolonged efforts.
However, a greater capacity to oxidize fat does not mean that carbohydrates become less important.
When intensity increases, the demand for energy at a high rate also rises. In this situation, the relative contribution of carbohydrates tends to increase because fat oxidation alone cannot meet the required rate of ATP production.
In Long-Distance Events, Carbohydrate Availability Can Become a Key Factor
During an endurance event, athletes need to sustain a certain intensity for an extended period. In this context, it is not enough to consider only which fuel the body can use, but also how much of that fuel is available.
As exercise duration increases, maintaining carbohydrate availability can help preserve the ability to sustain the planned intensity. Carbohydrate intake during exercise can help maintain blood glucose levels, provide substrate for oxidation, and preserve some of the body's internal energy stores.
A 2024 meta-analysis combining 136 studies found that carbohydrate intake benefits endurance performance, with more pronounced effects as event duration increases.
For this reason, nutritional strategies should not be based solely on the type of event. Expected duration and intensity are important factors in determining energy demands.
Duration, Intensity, and Fuel Must Be Considered Together
Thinking about energy metabolism simply as a sequence of "carbohydrates first, fat later" oversimplifies what happens during exercise.
From the beginning of a session, carbohydrates and fats are used simultaneously. Duration changes their relative contribution, but intensity, training status, diet, and substrate availability also influence fuel selection.
For endurance athletes, understanding this interaction helps explain why a nutritional strategy that works for a short session may not be sufficient for a long training session or race.
Ultimately, exercise duration alone does not determine which fuel the body uses. It is the combination of how long, at what intensity, and with what energy reserves the exercise is performed that shapes the body's energy metabolism.
